diff --git a/.eslintrc.js b/.eslintrc.js
index 2b21ed9160..6d8aa70f17 100644
--- a/.eslintrc.js
+++ b/.eslintrc.js
@@ -7,7 +7,6 @@ module.exports = {
     'accessor-pairs': 'error',
     'array-bracket-spacing': ['error', 'never'],
     'array-callback-return': 'off',
-    'arrow-body-style': 'error',
     'arrow-parens': 'error',
     'arrow-spacing': 'error',
     'block-scoped-var': 'off',
@@ -43,7 +42,7 @@ module.exports = {
     'func-call-spacing': 'error',
     'func-name-matching': 'error',
     'func-names': 'off',
-    'func-style': ['error', 'declaration'],
+    'func-style': ['error', 'declaration', {allowArrowFunctions: true}],
     'generator-star-spacing': 'error',
     'global-require': 'off',
     'guard-for-in': 'off',
diff --git a/examples/webhook-signing/express.js b/examples/webhook-signing/express.js
index f011e631ca..9317c86f7d 100644
--- a/examples/webhook-signing/express.js
+++ b/examples/webhook-signing/express.js
@@ -37,6 +37,6 @@ app.post(
   }
 );
 
-app.listen(3000, function() {
+app.listen(3000, () => {
   console.log('Example app listening on port 3000!');
 });
diff --git a/lib/Error.js b/lib/Error.js
index cc7a0d18df..6e5bd43e67 100644
--- a/lib/Error.js
+++ b/lib/Error.js
@@ -1,6 +1,6 @@
 'use strict';
 
-var utils = require('./utils');
+const utils = require('./utils');
 
 module.exports = _Error;
 
@@ -8,7 +8,7 @@ module.exports = _Error;
  * Generic Error klass to wrap any errors returned by stripe-node
  */
 function _Error(raw) {
-  this.populate.apply(this, arguments);
+  this.populate(...arguments);
   this.stack = new Error(this.message).stack;
 }
 
@@ -27,9 +27,9 @@ _Error.extend = utils.protoExtend;
  * Create subclass of internal Error klass
  * (Specifically for errors returned from Stripe's REST API)
  */
-var StripeError = (_Error.StripeError = _Error.extend({
+const StripeError = (_Error.StripeError = _Error.extend({
   type: 'StripeError',
-  populate: function(raw) {
+  populate(raw) {
     // Move from prototype def (so it appears in stringified obj)
     this.type = this.type;
 
@@ -49,7 +49,7 @@ var StripeError = (_Error.StripeError = _Error.extend({
 /**
  * Helper factory which takes raw stripe errors and outputs wrapping instances
  */
-StripeError.generate = function(rawStripeError) {
+StripeError.generate = (rawStripeError) => {
   switch (rawStripeError.type) {
     case 'card_error':
       return new _Error.StripeCardError(rawStripeError);
diff --git a/lib/MultipartDataGenerator.js b/lib/MultipartDataGenerator.js
index dbdc0e22ba..76b8cb2962 100644
--- a/lib/MultipartDataGenerator.js
+++ b/lib/MultipartDataGenerator.js
@@ -1,21 +1,21 @@
 'use strict';
 
-var Buffer = require('safe-buffer').Buffer;
-var utils = require('./utils');
+const Buffer = require('safe-buffer').Buffer;
+const utils = require('./utils');
 
 // Method for formatting HTTP body for the multipart/form-data specification
 // Mostly taken from Fermata.js
 // https://github.com/natevw/fermata/blob/5d9732a33d776ce925013a265935facd1626cc88/fermata.js#L315-L343
 function multipartDataGenerator(method, data, headers) {
-  var segno = (
+  const segno = (
     Math.round(Math.random() * 1e16) + Math.round(Math.random() * 1e16)
   ).toString();
-  headers['Content-Type'] = 'multipart/form-data; boundary=' + segno;
-  var buffer = Buffer.alloc(0);
+  headers['Content-Type'] = `multipart/form-data; boundary=${segno}`;
+  let buffer = Buffer.alloc(0);
 
   function push(l) {
-    var prevBuffer = buffer;
-    var newBuffer = l instanceof Buffer ? l : Buffer.from(l);
+    const prevBuffer = buffer;
+    const newBuffer = l instanceof Buffer ? l : Buffer.from(l);
     buffer = Buffer.alloc(prevBuffer.length + newBuffer.length + 2);
     prevBuffer.copy(buffer);
     newBuffer.copy(buffer, prevBuffer.length);
@@ -23,29 +23,28 @@ function multipartDataGenerator(method, data, headers) {
   }
 
   function q(s) {
-    return '"' + s.replace(/"|"/g, '%22').replace(/\r\n|\r|\n/g, ' ') + '"';
+    return `"${s.replace(/"|"/g, '%22').replace(/\r\n|\r|\n/g, ' ')}"`;
   }
 
-  for (var k in utils.flattenAndStringify(data)) {
-    var v = data[k];
-    push('--' + segno);
+  for (const k in utils.flattenAndStringify(data)) {
+    const v = data[k];
+    push(`--${segno}`);
     if (v.hasOwnProperty('data')) {
       push(
-        'Content-Disposition: form-data; name=' +
-          q(k) +
-          '; filename=' +
-          q(v.name || 'blob')
+        `Content-Disposition: form-data; name=${q(k)}; filename=${q(
+          v.name || 'blob'
+        )}`
       );
-      push('Content-Type: ' + (v.type || 'application/octet-stream'));
+      push(`Content-Type: ${v.type || 'application/octet-stream'}`);
       push('');
       push(v.data);
     } else {
-      push('Content-Disposition: form-data; name=' + q(k));
+      push(`Content-Disposition: form-data; name=${q(k)}`);
       push('');
       push(v);
     }
   }
-  push('--' + segno + '--');
+  push(`--${segno}--`);
 
   return buffer;
 }
diff --git a/lib/ResourceNamespace.js b/lib/ResourceNamespace.js
index 2f0c053669..0d76787d81 100644
--- a/lib/ResourceNamespace.js
+++ b/lib/ResourceNamespace.js
@@ -4,10 +4,10 @@
 // It also works recursively, so you could do i.e. `stripe.billing.invoicing.pay`.
 
 function ResourceNamespace(stripe, resources) {
-  for (var name in resources) {
-    var camelCaseName = name[0].toLowerCase() + name.substring(1);
+  for (const name in resources) {
+    const camelCaseName = name[0].toLowerCase() + name.substring(1);
 
-    var resource = new resources[name](stripe);
+    const resource = new resources[name](stripe);
 
     this[camelCaseName] = resource;
   }
diff --git a/lib/StripeMethod.basic.js b/lib/StripeMethod.basic.js
index a863c38151..1db6b8efbf 100644
--- a/lib/StripeMethod.basic.js
+++ b/lib/StripeMethod.basic.js
@@ -1,8 +1,8 @@
 'use strict';
 
-var isPlainObject = require('lodash.isplainobject');
-var stripeMethod = require('./StripeMethod');
-var utils = require('./utils');
+const isPlainObject = require('lodash.isplainobject');
+const stripeMethod = require('./StripeMethod');
+const utils = require('./utils');
 
 module.exports = {
   create: stripeMethod({
@@ -33,12 +33,12 @@ module.exports = {
     urlParams: ['id'],
   }),
 
-  setMetadata: function(id, key, value, auth, cb) {
-    var self = this;
-    var data = key;
-    var isObject = isPlainObject(key);
+  setMetadata(id, key, value, auth, cb) {
+    const self = this;
+    const data = key;
+    const isObject = isPlainObject(key);
     // We assume null for an empty object
-    var isNull = data === null || (isObject && !Object.keys(data).length);
+    const isNull = data === null || (isObject && !Object.keys(data).length);
 
     // Allow optional passing of auth & cb:
     if ((isNull || isObject) && typeof value == 'string') {
@@ -50,85 +50,78 @@ module.exports = {
       auth = null;
     }
 
-    var urlData = this.createUrlData();
-    var path = this.createFullPath('/' + id, urlData);
+    const urlData = this.createUrlData();
+    const path = this.createFullPath(`/${id}`, urlData);
 
     return utils.callbackifyPromiseWithTimeout(
-      new Promise(
-        function(resolve, reject) {
-          if (isNull) {
-            // Reset metadata:
-            sendMetadata(null, auth);
-          } else if (!isObject) {
-            // Set individual metadata property:
-            var metadata = {};
-            metadata[key] = value;
-            sendMetadata(metadata, auth);
-          } else {
-            // Set entire metadata object after resetting it:
-            this._request(
-              'POST',
-              null,
-              path,
-              {metadata: null},
-              auth,
-              {},
-              function(err, response) {
-                if (err) {
-                  return reject(err);
-                }
-                sendMetadata(data, auth);
+      new Promise((resolve, reject) => {
+        if (isNull) {
+          // Reset metadata:
+          sendMetadata(null, auth);
+        } else if (!isObject) {
+          // Set individual metadata property:
+          const metadata = {};
+          metadata[key] = value;
+          sendMetadata(metadata, auth);
+        } else {
+          // Set entire metadata object after resetting it:
+          this._request(
+            'POST',
+            null,
+            path,
+            {metadata: null},
+            auth,
+            {},
+            (err, response) => {
+              if (err) {
+                return reject(err);
               }
-            );
-          }
+              sendMetadata(data, auth);
+            }
+          );
+        }
 
-          function sendMetadata(metadata, auth) {
-            self._request(
-              'POST',
-              null,
-              path,
-              {metadata: metadata},
-              auth,
-              {},
-              function(err, response) {
-                if (err) {
-                  reject(err);
-                } else {
-                  resolve(response.metadata);
-                }
+        function sendMetadata(metadata, auth) {
+          self._request(
+            'POST',
+            null,
+            path,
+            {metadata},
+            auth,
+            {},
+            (err, response) => {
+              if (err) {
+                reject(err);
+              } else {
+                resolve(response.metadata);
               }
-            );
-          }
-        }.bind(this)
-      ),
+            }
+          );
+        }
+      }),
       cb
     );
   },
 
-  getMetadata: function(id, auth, cb) {
+  getMetadata(id, auth, cb) {
     if (!cb && typeof auth == 'function') {
       cb = auth;
       auth = null;
     }
 
-    var urlData = this.createUrlData();
-    var path = this.createFullPath('/' + id, urlData);
+    const urlData = this.createUrlData();
+    const path = this.createFullPath(`/${id}`, urlData);
 
     return utils.callbackifyPromiseWithTimeout(
-      new Promise(
-        function(resolve, reject) {
-          this._request('GET', null, path, {}, auth, {}, function(
-            err,
-            response
-          ) {
-            if (err) {
-              reject(err);
-            } else {
-              resolve(response.metadata);
-            }
-          });
-        }.bind(this)
-      ),
+      new Promise((resolve, reject) => {
+        this._request('GET', null, path, {}, auth, {}, (err, response) => {
+          if (err) {
+            reject(err);
+          } else {
+            resolve(response.metadata);
+          }
+        });
+      }),
       cb
     );
   },
diff --git a/lib/StripeMethod.js b/lib/StripeMethod.js
index 461cee3756..99f18426a7 100644
--- a/lib/StripeMethod.js
+++ b/lib/StripeMethod.js
@@ -1,8 +1,8 @@
 'use strict';
 
-var utils = require('./utils');
-var makeRequest = require('./makeRequest');
-var makeAutoPaginationMethods = require('./autoPagination')
+const utils = require('./utils');
+const makeRequest = require('./makeRequest');
+const makeAutoPaginationMethods = require('./autoPagination')
   .makeAutoPaginationMethods;
 
 /**
@@ -21,18 +21,18 @@ var makeAutoPaginationMethods = require('./autoPagination')
  */
 function stripeMethod(spec) {
   return function() {
-    var self = this;
-    var args = [].slice.call(arguments);
+    const self = this;
+    const args = [].slice.call(arguments);
 
-    var callback = typeof args[args.length - 1] == 'function' && args.pop();
+    const callback = typeof args[args.length - 1] == 'function' && args.pop();
 
-    var requestPromise = utils.callbackifyPromiseWithTimeout(
+    const requestPromise = utils.callbackifyPromiseWithTimeout(
       makeRequest(self, args, spec, {}),
       callback
     );
 
     if (spec.methodType === 'list') {
-      var autoPaginationMethods = makeAutoPaginationMethods(
+      const autoPaginationMethods = makeAutoPaginationMethods(
         self,
         args,
         spec,
diff --git a/lib/StripeResource.js b/lib/StripeResource.js
index e6f39bdd18..a252123451 100644
--- a/lib/StripeResource.js
+++ b/lib/StripeResource.js
@@ -1,17 +1,17 @@
 'use strict';
 
-var http = require('http');
-var https = require('https');
-var path = require('path');
-var uuid = require('uuid/v4');
+const http = require('http');
+const https = require('https');
+const path = require('path');
+const uuid = require('uuid/v4');
 
-var utils = require('./utils');
-var Error = require('./Error');
+const utils = require('./utils');
+const Error = require('./Error');
 
-var hasOwn = {}.hasOwnProperty;
+const hasOwn = {}.hasOwnProperty;
 
-var defaultHttpAgent = new http.Agent({keepAlive: true});
-var defaultHttpsAgent = new https.Agent({keepAlive: true});
+const defaultHttpAgent = new http.Agent({keepAlive: true});
+const defaultHttpsAgent = new https.Agent({keepAlive: true});
 
 // Provide extension mechanism for Stripe Resource Sub-Classes
 StripeResource.extend = utils.protoExtend;
@@ -41,7 +41,7 @@ function StripeResource(stripe, urlData) {
     }, this);
   }
 
-  this.initialize.apply(this, arguments);
+  this.initialize(...arguments);
 }
 
 StripeResource.prototype = {
@@ -50,7 +50,7 @@ StripeResource.prototype = {
   // Methods that don't use the API's default '/v1' path can override it with this setting.
   basePath: null,
 
-  initialize: function() {},
+  initialize() {},
 
   // Function to override the default data processor. This allows full control
   // over how a StripeResource's request data will get converted into an HTTP
@@ -62,7 +62,7 @@ StripeResource.prototype = {
   // be thrown, and they will be passed to the callback/promise.
   validateRequest: null,
 
-  createFullPath: function(commandPath, urlData) {
+  createFullPath(commandPath, urlData) {
     return path
       .join(
         this.basePath(urlData),
@@ -75,17 +75,16 @@ StripeResource.prototype = {
   // Creates a relative resource path with symbols left in (unlike
   // createFullPath which takes some data to replace them with). For example it
   // might produce: /invoices/{id}
-  createResourcePathWithSymbols: function(pathWithSymbols) {
-    return (
-      '/' +
-      path.join(this.resourcePath, pathWithSymbols || '').replace(/\\/g, '/')
-    ); // ugly workaround for Windows
+  createResourcePathWithSymbols(pathWithSymbols) {
+    return `/${path
+      .join(this.resourcePath, pathWithSymbols || '')
+      .replace(/\\/g, '/')}`; // ugly workaround for Windows
   },
 
-  createUrlData: function() {
-    var urlData = {};
+  createUrlData() {
+    const urlData = {};
     // Merge in baseData
-    for (var i in this._urlData) {
+    for (const i in this._urlData) {
       if (hasOwn.call(this._urlData, i)) {
         urlData[i] = this._urlData[i];
       }
@@ -96,10 +95,10 @@ StripeResource.prototype = {
   // DEPRECATED: Here for backcompat in case users relied on this.
   wrapTimeout: utils.callbackifyPromiseWithTimeout,
 
-  _timeoutHandler: function(timeout, req, callback) {
-    var self = this;
-    return function() {
-      var timeoutErr = new Error('ETIMEDOUT');
+  _timeoutHandler(timeout, req, callback) {
+    const self = this;
+    return () => {
+      const timeoutErr = new Error('ETIMEDOUT');
       timeoutErr.code = 'ETIMEDOUT';
 
       req._isAborted = true;
@@ -108,8 +107,7 @@ StripeResource.prototype = {
       callback.call(
         self,
         new Error.StripeConnectionError({
-          message:
-            'Request aborted due to timeout being reached (' + timeout + 'ms)',
+          message: `Request aborted due to timeout being reached (${timeout}ms)`,
           detail: timeoutErr,
         }),
         null
@@ -117,26 +115,26 @@ StripeResource.prototype = {
     };
   },
 
-  _responseHandler: function(req, callback) {
-    var self = this;
-    return function(res) {
-      var response = '';
+  _responseHandler(req, callback) {
+    const self = this;
+    return (res) => {
+      let response = '';
 
       res.setEncoding('utf8');
-      res.on('data', function(chunk) {
+      res.on('data', (chunk) => {
         response += chunk;
       });
-      res.on('end', function() {
-        var headers = res.headers || {};
+      res.on('end', () => {
+        const headers = res.headers || {};
         // NOTE: Stripe responds with lowercase header names/keys.
 
         // For convenience, make Request-Id easily accessible on
         // lastResponse.
         res.requestId = headers['request-id'];
 
-        var requestDurationMs = Date.now() - req._requestStart;
+        const requestDurationMs = Date.now() - req._requestStart;
 
-        var responseEvent = utils.removeEmpty({
+        const responseEvent = utils.removeEmpty({
           api_version: headers['stripe-version'],
           account: headers['stripe-account'],
           idempotency_key: headers['idempotency-key'],
@@ -153,7 +151,7 @@ StripeResource.prototype = {
           response = JSON.parse(response);
 
           if (response.error) {
-            var err;
+            let err;
 
             // Convert OAuth error responses into a standard format
             // so that the rest of the error logic can be shared
@@ -184,7 +182,7 @@ StripeResource.prototype = {
             self,
             new Error.StripeAPIError({
               message: 'Invalid JSON received from the Stripe API',
-              response: response,
+              response,
               exception: e,
               requestId: headers['request-id'],
             }),
@@ -205,18 +203,15 @@ StripeResource.prototype = {
     };
   },
 
-  _generateConnectionErrorMessage: function(requestRetries) {
-    return (
-      'An error occurred with our connection to Stripe.' +
-      (requestRetries > 0
-        ? ' Request was retried ' + requestRetries + ' times.'
-        : '')
-    );
+  _generateConnectionErrorMessage(requestRetries) {
+    return `An error occurred with our connection to Stripe.${
+      requestRetries > 0 ? ` Request was retried ${requestRetries} times.` : ''
+    }`;
   },
 
-  _errorHandler: function(req, requestRetries, callback) {
-    var self = this;
-    return function(error) {
+  _errorHandler(req, requestRetries, callback) {
+    const self = this;
+    return (error) => {
       if (req._isAborted) {
         // already handled
         return;
@@ -232,7 +227,7 @@ StripeResource.prototype = {
     };
   },
 
-  _shouldRetry: function(res, numRetries) {
+  _shouldRetry(res, numRetries) {
     // Do not retry if we are out of retries.
     if (numRetries >= this._stripe.getMaxNetworkRetries()) {
       return false;
@@ -257,14 +252,14 @@ StripeResource.prototype = {
     return false;
   },
 
-  _getSleepTimeInMS: function(numRetries) {
-    var initialNetworkRetryDelay = this._stripe.getInitialNetworkRetryDelay();
-    var maxNetworkRetryDelay = this._stripe.getMaxNetworkRetryDelay();
+  _getSleepTimeInMS(numRetries) {
+    const initialNetworkRetryDelay = this._stripe.getInitialNetworkRetryDelay();
+    const maxNetworkRetryDelay = this._stripe.getMaxNetworkRetryDelay();
 
     // Apply exponential backoff with initialNetworkRetryDelay on the
     // number of numRetries so far as inputs. Do not allow the number to exceed
     // maxNetworkRetryDelay.
-    var sleepSeconds = Math.min(
+    let sleepSeconds = Math.min(
       initialNetworkRetryDelay * Math.pow(numRetries - 1, 2),
       maxNetworkRetryDelay
     );
@@ -279,17 +274,18 @@ StripeResource.prototype = {
     return sleepSeconds * 1000;
   },
 
-  _defaultHeaders: function(auth, contentLength, apiVersion) {
-    var userAgentString =
-      'Stripe/v1 NodeBindings/' + this._stripe.getConstant('PACKAGE_VERSION');
+  _defaultHeaders(auth, contentLength, apiVersion) {
+    let userAgentString = `Stripe/v1 NodeBindings/${this._stripe.getConstant(
+      'PACKAGE_VERSION'
+    )}`;
 
     if (this._stripe._appInfo) {
-      userAgentString += ' ' + this._stripe.getAppInfoAsString();
+      userAgentString += ` ${this._stripe.getAppInfoAsString()}`;
     }
 
-    var headers = {
+    const headers = {
       // Use specified auth token or use default from this stripe instance:
-      Authorization: auth ? 'Bearer ' + auth : this._stripe.getApiField('auth'),
+      Authorization: auth ? `Bearer ${auth}` : this._stripe.getApiField('auth'),
       Accept: 'application/json',
       'Content-Type': 'application/x-www-form-urlencoded',
       'Content-Length': contentLength,
@@ -303,19 +299,19 @@ StripeResource.prototype = {
     return headers;
   },
 
-  _addTelemetryHeader: function(headers) {
+  _addTelemetryHeader(headers) {
     if (
       this._stripe.getTelemetryEnabled() &&
       this._stripe._prevRequestMetrics.length > 0
     ) {
-      var metrics = this._stripe._prevRequestMetrics.shift();
+      const metrics = this._stripe._prevRequestMetrics.shift();
       headers['X-Stripe-Client-Telemetry'] = JSON.stringify({
         last_request_metrics: metrics,
       });
     }
   },
 
-  _recordRequestMetrics: function(requestId, requestDurationMs) {
+  _recordRequestMetrics(requestId, requestDurationMs) {
     if (this._stripe.getTelemetryEnabled() && requestId) {
       if (
         this._stripe._prevRequestMetrics.length >
@@ -333,23 +329,24 @@ StripeResource.prototype = {
     }
   },
 
-  _request: function(method, host, path, data, auth, options, callback) {
-    var self = this;
-    var requestData;
+  _request(method, host, path, data, auth, options, callback) {
+    const self = this;
+    let requestData;
 
     function makeRequestWithData(error, data) {
-      var apiVersion;
-      var headers;
-
       if (error) {
         return callback(error);
       }
 
-      apiVersion = self._stripe.getApiField('version');
+      const apiVersion = self._stripe.getApiField('version');
       requestData = data;
-      headers = self._defaultHeaders(auth, requestData.length, apiVersion);
+      const headers = self._defaultHeaders(
+        auth,
+        requestData.length,
+        apiVersion
+      );
 
-      self._stripe.getClientUserAgent(function(cua) {
+      self._stripe.getClientUserAgent((cua) => {
         headers['X-Stripe-Client-User-Agent'] = cua;
 
         if (options.headers) {
@@ -386,20 +383,21 @@ StripeResource.prototype = {
     }
 
     function makeRequest(apiVersion, headers, numRetries) {
-      var timeout = self._stripe.getApiField('timeout');
-      var isInsecureConnection = self._stripe.getApiField('protocol') == 'http';
-      var agent = self._stripe.getApiField('agent');
+      const timeout = self._stripe.getApiField('timeout');
+      const isInsecureConnection =
+        self._stripe.getApiField('protocol') == 'http';
+      let agent = self._stripe.getApiField('agent');
       if (agent == null) {
         agent = isInsecureConnection ? defaultHttpAgent : defaultHttpsAgent;
       }
 
-      var req = (isInsecureConnection ? http : https).request({
+      const req = (isInsecureConnection ? http : https).request({
         host: host || self._stripe.getApiField('host'),
         port: self._stripe.getApiField('port'),
-        path: path,
-        method: method,
-        agent: agent,
-        headers: headers,
+        path,
+        method,
+        agent,
+        headers,
         ciphers: 'DEFAULT:!aNULL:!eNULL:!LOW:!EXPORT:!SSLv2:!MD5',
       });
 
@@ -411,15 +409,15 @@ StripeResource.prototype = {
         }
       }
 
-      var requestEvent = utils.removeEmpty({
+      const requestEvent = utils.removeEmpty({
         api_version: apiVersion,
         account: headers['Stripe-Account'],
         idempotency_key: headers['Idempotency-Key'],
-        method: method,
-        path: path,
+        method,
+        path,
       });
 
-      var requestRetries = numRetries || 0;
+      const requestRetries = numRetries || 0;
 
       req._requestEvent = requestEvent;
 
@@ -429,7 +427,7 @@ StripeResource.prototype = {
 
       req.setTimeout(timeout, self._timeoutHandler(timeout, req, callback));
 
-      req.on('response', function(res) {
+      req.on('response', (res) => {
         if (self._shouldRetry(res, requestRetries)) {
           return retryRequest(makeRequest, apiVersion, headers, requestRetries);
         } else {
@@ -437,7 +435,7 @@ StripeResource.prototype = {
         }
       });
 
-      req.on('error', function(error) {
+      req.on('error', (error) => {
         if (self._shouldRetry(null, requestRetries)) {
           return retryRequest(makeRequest, apiVersion, headers, requestRetries);
         } else {
@@ -445,16 +443,13 @@ StripeResource.prototype = {
         }
       });
 
-      req.on('socket', function(socket) {
+      req.on('socket', (socket) => {
         if (socket.connecting) {
-          socket.on(
-            isInsecureConnection ? 'connect' : 'secureConnect',
-            function() {
-              // Send payload; we're safe:
-              req.write(requestData);
-              req.end();
-            }
-          );
+          socket.on(isInsecureConnection ? 'connect' : 'secureConnect', () => {
+            // Send payload; we're safe:
+            req.write(requestData);
+            req.end();
+          });
         } else {
           // we're already connected
           req.write(requestData);
diff --git a/lib/Webhooks.js b/lib/Webhooks.js
index 2543761b87..cfe16f3e84 100644
--- a/lib/Webhooks.js
+++ b/lib/Webhooks.js
@@ -1,15 +1,15 @@
 'use strict';
 
-var Buffer = require('safe-buffer').Buffer;
-var crypto = require('crypto');
+const Buffer = require('safe-buffer').Buffer;
+const crypto = require('crypto');
 
-var utils = require('./utils');
-var Error = require('./Error');
+const utils = require('./utils');
+const Error = require('./Error');
 
-var Webhook = {
+const Webhook = {
   DEFAULT_TOLERANCE: 300, // 5 minutes
 
-  constructEvent: function(payload, header, secret, tolerance) {
+  constructEvent(payload, header, secret, tolerance) {
     this.signature.verifyHeader(
       payload,
       header,
@@ -17,33 +17,33 @@ var Webhook = {
       tolerance || Webhook.DEFAULT_TOLERANCE
     );
 
-    var jsonPayload = JSON.parse(payload);
+    const jsonPayload = JSON.parse(payload);
     return jsonPayload;
   },
 };
 
-var signature = {
+const signature = {
   EXPECTED_SCHEME: 'v1',
 
-  _computeSignature: function(payload, secret) {
+  _computeSignature: (payload, secret) => {
     return crypto
       .createHmac('sha256', secret)
       .update(payload, 'utf8')
       .digest('hex');
   },
 
-  verifyHeader: function(payload, header, secret, tolerance) {
+  verifyHeader(payload, header, secret, tolerance) {
     payload = Buffer.isBuffer(payload) ? payload.toString('utf8') : payload;
     header = Buffer.isBuffer(header) ? header.toString('utf8') : header;
 
-    var details = parseHeader(header, this.EXPECTED_SCHEME);
+    const details = parseHeader(header, this.EXPECTED_SCHEME);
 
     if (!details || details.timestamp === -1) {
       throw new Error.StripeSignatureVerificationError({
         message: 'Unable to extract timestamp and signatures from header',
         detail: {
-          header: header,
-          payload: payload,
+          header,
+          payload,
         },
       });
     }
@@ -52,18 +52,18 @@ var signature = {
       throw new Error.StripeSignatureVerificationError({
         message: 'No signatures found with expected scheme',
         detail: {
-          header: header,
-          payload: payload,
+          header,
+          payload,
         },
       });
     }
 
-    var expectedSignature = this._computeSignature(
-      details.timestamp + '.' + payload,
+    const expectedSignature = this._computeSignature(
+      `${details.timestamp}.${payload}`,
       secret
     );
 
-    var signatureFound = !!details.signatures.filter(
+    const signatureFound = !!details.signatures.filter(
       utils.secureCompare.bind(utils, expectedSignature)
     ).length;
 
@@ -74,20 +74,20 @@ var signature = {
           ' Are you passing the raw request body you received from Stripe?' +
           ' https://github.com/stripe/stripe-node#webhook-signing',
         detail: {
-          header: header,
-          payload: payload,
+          header,
+          payload,
         },
       });
     }
 
-    var timestampAge = Math.floor(Date.now() / 1000) - details.timestamp;
+    const timestampAge = Math.floor(Date.now() / 1000) - details.timestamp;
 
     if (tolerance > 0 && timestampAge > tolerance) {
       throw new Error.StripeSignatureVerificationError({
         message: 'Timestamp outside the tolerance zone',
         detail: {
-          header: header,
-          payload: payload,
+          header,
+          payload,
         },
       });
     }
@@ -102,8 +102,8 @@ function parseHeader(header, scheme) {
   }
 
   return header.split(',').reduce(
-    function(accum, item) {
-      var kv = item.split('=');
+    (accum, item) => {
+      const kv = item.split('=');
 
       if (kv[0] === 't') {
         accum.timestamp = kv[1];
diff --git a/lib/autoPagination.js b/lib/autoPagination.js
index f54d5504dd..c3f052169b 100644
--- a/lib/autoPagination.js
+++ b/lib/autoPagination.js
@@ -1,12 +1,12 @@
 'use strict';
 
-var makeRequest = require('./makeRequest');
-var utils = require('./utils');
+const makeRequest = require('./makeRequest');
+const utils = require('./utils');
 
 function makeAutoPaginationMethods(self, requestArgs, spec, firstPagePromise) {
-  var promiseCache = {currentPromise: null};
-  var listPromise = firstPagePromise;
-  var i = 0;
+  const promiseCache = {currentPromise: null};
+  let listPromise = firstPagePromise;
+  let i = 0;
 
   function iterate(listResult) {
     if (
@@ -22,13 +22,13 @@ function makeAutoPaginationMethods(self, requestArgs, spec, firstPagePromise) {
     }
 
     if (i < listResult.data.length) {
-      var value = listResult.data[i];
+      const value = listResult.data[i];
       i += 1;
-      return {value: value, done: false};
+      return {value, done: false};
     } else if (listResult.has_more) {
       // Reset counter, request next page, and recurse.
       i = 0;
-      var lastId = getLastId(listResult);
+      const lastId = getLastId(listResult);
       listPromise = makeRequest(self, requestArgs, spec, {
         starting_after: lastId,
       });
@@ -38,7 +38,7 @@ function makeAutoPaginationMethods(self, requestArgs, spec, firstPagePromise) {
   }
 
   function asyncIteratorNext() {
-    return memoizedPromise(promiseCache, function(resolve, reject) {
+    return memoizedPromise(promiseCache, (resolve, reject) => {
       return listPromise
         .then(iterate)
         .then(resolve)
@@ -46,20 +46,20 @@ function makeAutoPaginationMethods(self, requestArgs, spec, firstPagePromise) {
     });
   }
 
-  var autoPagingEach = makeAutoPagingEach(asyncIteratorNext);
-  var autoPagingToArray = makeAutoPagingToArray(autoPagingEach);
+  const autoPagingEach = makeAutoPagingEach(asyncIteratorNext);
+  const autoPagingToArray = makeAutoPagingToArray(autoPagingEach);
 
-  var autoPaginationMethods = {
-    autoPagingEach: autoPagingEach,
-    autoPagingToArray: autoPagingToArray,
+  const autoPaginationMethods = {
+    autoPagingEach,
+    autoPagingToArray,
 
     // Async iterator functions:
     next: asyncIteratorNext,
-    return: function() {
+    return: () => {
       // This is required for `break`.
       return {};
     },
-    [getAsyncIteratorSymbol()]: function() {
+    [getAsyncIteratorSymbol()]: () => {
       return autoPaginationMethods;
     },
   };
@@ -86,11 +86,10 @@ function getDoneCallback(args) {
   if (args.length < 2) {
     return undefined;
   }
-  var onDone = args[1];
+  const onDone = args[1];
   if (typeof onDone !== 'function') {
     throw Error(
-      'The second argument to autoPagingEach, if present, must be a callback function; receieved ' +
-        typeof onDone
+      `The second argument to autoPagingEach, if present, must be a callback function; receieved ${typeof onDone}`
     );
   }
   return onDone;
@@ -111,11 +110,10 @@ function getItemCallback(args) {
   if (args.length === 0) {
     return undefined;
   }
-  var onItem = args[0];
+  const onItem = args[0];
   if (typeof onItem !== 'function') {
     throw Error(
-      'The first argument to autoPagingEach, if present, must be a callback function; receieved ' +
-        typeof onItem
+      `The first argument to autoPagingEach, if present, must be a callback function; receieved ${typeof onItem}`
     );
   }
 
@@ -126,8 +124,7 @@ function getItemCallback(args) {
 
   if (onItem.length > 2) {
     throw Error(
-      'The `onItem` callback function passed to autoPagingEach must accept at most two arguments; got ' +
-        onItem
+      `The \`onItem\` callback function passed to autoPagingEach must accept at most two arguments; got ${onItem}`
     );
   }
 
@@ -136,15 +133,15 @@ function getItemCallback(args) {
   // 2. `.autoPagingEach(async (item) => { await doSomething(item); return false; });`
   // 3. `.autoPagingEach((item) => doSomething(item).then(() => false));`
   return function _onItem(item, next) {
-    var shouldContinue = onItem(item);
+    const shouldContinue = onItem(item);
     next(shouldContinue);
   };
 }
 
 function getLastId(listResult) {
-  var lastIdx = listResult.data.length - 1;
-  var lastItem = listResult.data[lastIdx];
-  var lastId = lastItem && lastItem.id;
+  const lastIdx = listResult.data.length - 1;
+  const lastItem = listResult.data[lastIdx];
+  const lastId = lastItem && lastItem.id;
   if (!lastId) {
     throw Error(
       'Unexpected: No `id` found on the last item while auto-paging a list.'
@@ -162,7 +159,7 @@ function memoizedPromise(promiseCache, cb) {
   if (promiseCache.currentPromise) {
     return promiseCache.currentPromise;
   }
-  promiseCache.currentPromise = new Promise(cb).then(function(ret) {
+  promiseCache.currentPromise = new Promise(cb).then((ret) => {
     promiseCache.currentPromise = undefined;
     return ret;
   });
@@ -171,14 +168,14 @@ function memoizedPromise(promiseCache, cb) {
 
 function makeAutoPagingEach(asyncIteratorNext) {
   return function autoPagingEach(/* onItem?, onDone? */) {
-    var args = [].slice.call(arguments);
-    var onItem = getItemCallback(args);
-    var onDone = getDoneCallback(args);
+    const args = [].slice.call(arguments);
+    const onItem = getItemCallback(args);
+    const onDone = getDoneCallback(args);
     if (args.length > 2) {
       throw Error('autoPagingEach takes up to two arguments; received:', args);
     }
 
-    var autoPagePromise = wrapAsyncIteratorWithCallback(
+    const autoPagePromise = wrapAsyncIteratorWithCallback(
       asyncIteratorNext,
       onItem
     );
@@ -188,7 +185,7 @@ function makeAutoPagingEach(asyncIteratorNext) {
 
 function makeAutoPagingToArray(autoPagingEach) {
   return function autoPagingToArray(opts, onDone) {
-    var limit = opts && opts.limit;
+    const limit = opts && opts.limit;
     if (!limit) {
       throw Error(
         'You must pass a `limit` option to autoPagingToArray, eg; `autoPagingToArray({limit: 1000});`.'
@@ -199,15 +196,15 @@ function makeAutoPagingToArray(autoPagingEach) {
         'You cannot specify a limit of more than 10,000 items to fetch in `autoPagingToArray`; use `autoPagingEach` to iterate through longer lists.'
       );
     }
-    var promise = new Promise(function(resolve, reject) {
-      var items = [];
-      autoPagingEach(function(item) {
+    const promise = new Promise((resolve, reject) => {
+      const items = [];
+      autoPagingEach((item) => {
         items.push(item);
         if (items.length >= limit) {
           return false;
         }
       })
-        .then(function() {
+        .then(() => {
           resolve(items);
         })
         .catch(reject);
@@ -217,20 +214,20 @@ function makeAutoPagingToArray(autoPagingEach) {
 }
 
 function wrapAsyncIteratorWithCallback(asyncIteratorNext, onItem) {
-  return new Promise(function(resolve, reject) {
+  return new Promise((resolve, reject) => {
     function handleIteration(iterResult) {
       if (iterResult.done) {
         resolve();
         return;
       }
 
-      var item = iterResult.value;
-      return new Promise(function(next) {
+      const item = iterResult.value;
+      return new Promise((next) => {
         // Bit confusing, perhaps; we pass a `resolve` fn
         // to the user, so they can decide when and if to continue.
         // They can return false, or a promise which resolves to false, to break.
         onItem(item, next);
-      }).then(function(shouldContinue) {
+      }).then((shouldContinue) => {
         if (shouldContinue === false) {
           return handleIteration({done: true});
         } else {
diff --git a/lib/makeRequest.js b/lib/makeRequest.js
index 1f5ea60664..cb49d4c2ed 100644
--- a/lib/makeRequest.js
+++ b/lib/makeRequest.js
@@ -1,50 +1,40 @@
 'use strict';
 
-var utils = require('./utils');
-var OPTIONAL_REGEX = /^optional!/;
+const utils = require('./utils');
+const OPTIONAL_REGEX = /^optional!/;
 
 function getRequestOpts(self, requestArgs, spec, overrideData) {
   // Extract spec values with defaults.
-  var commandPath =
+  const commandPath =
     typeof spec.path == 'function'
       ? spec.path
       : utils.makeURLInterpolator(spec.path || '');
-  var requestMethod = (spec.method || 'GET').toUpperCase();
-  var urlParams = spec.urlParams || [];
-  var encode =
-    spec.encode ||
-    function(data) {
-      return data;
-    };
-  var host = spec.host;
+  const requestMethod = (spec.method || 'GET').toUpperCase();
+  const urlParams = spec.urlParams || [];
+  const encode = spec.encode || ((data) => data);
+  const host = spec.host;
 
   // Don't mutate args externally.
-  var args = [].slice.call(requestArgs);
+  const args = [].slice.call(requestArgs);
 
   // Generate and validate url params.
-  var urlData = self.createUrlData();
-  for (var i = 0, l = urlParams.length; i < l; ++i) {
+  const urlData = self.createUrlData();
+  for (let i = 0, l = urlParams.length; i < l; ++i) {
     var path;
 
     // Note that we shift the args array after every iteration so this just
     // grabs the "next" argument for use as a URL parameter.
-    var arg = args[0];
+    const arg = args[0];
 
-    var param = urlParams[i];
+    let param = urlParams[i];
 
-    var isOptional = OPTIONAL_REGEX.test(param);
+    const isOptional = OPTIONAL_REGEX.test(param);
     param = param.replace(OPTIONAL_REGEX, '');
 
     if (param == 'id' && typeof arg !== 'string') {
       path = self.createResourcePathWithSymbols(spec.path);
       throw new Error(
-        'Stripe: "id" must be a string, but got: ' +
-          typeof arg +
-          ' (on API request to `' +
-          requestMethod +
-          ' ' +
-          path +
-          '`)'
+        `Stripe: "id" must be a string, but got: ${typeof arg} (on API request to \`${requestMethod} ${path}\`)`
       );
     }
 
@@ -56,15 +46,9 @@ function getRequestOpts(self, requestArgs, spec, overrideData) {
 
       path = self.createResourcePathWithSymbols(spec.path);
       throw new Error(
-        'Stripe: Argument "' +
-          urlParams[i] +
-          '" required, but got: ' +
-          arg +
-          ' (on API request to `' +
-          requestMethod +
-          ' ' +
-          path +
-          '`)'
+        `Stripe: Argument "${
+          urlParams[i]
+        }" required, but got: ${arg} (on API request to \`${requestMethod} ${path}\`)`
       );
     }
 
@@ -72,45 +56,37 @@ function getRequestOpts(self, requestArgs, spec, overrideData) {
   }
 
   // Pull request data and options (headers, auth) from args.
-  var dataFromArgs = utils.getDataFromArgs(args);
-  var data = encode(Object.assign({}, dataFromArgs, overrideData));
-  var options = utils.getOptionsFromArgs(args);
+  const dataFromArgs = utils.getDataFromArgs(args);
+  const data = encode(Object.assign({}, dataFromArgs, overrideData));
+  const options = utils.getOptionsFromArgs(args);
 
   // Validate that there are no more args.
   if (args.length) {
     path = self.createResourcePathWithSymbols(spec.path);
     throw new Error(
-      'Stripe: Unknown arguments (' +
-        args +
-        '). Did you mean to pass an options ' +
-        'object? See https://github.com/stripe/stripe-node/wiki/Passing-Options.' +
-        ' (on API request to ' +
-        requestMethod +
-        ' `' +
-        path +
-        '`)'
+      `Stripe: Unknown arguments (${args}). Did you mean to pass an options object? See https://github.com/stripe/stripe-node/wiki/Passing-Options. (on API request to ${requestMethod} \`${path}\`)`
     );
   }
 
-  var requestPath = self.createFullPath(commandPath, urlData);
-  var headers = Object.assign(options.headers, spec.headers);
+  const requestPath = self.createFullPath(commandPath, urlData);
+  const headers = Object.assign(options.headers, spec.headers);
 
   if (spec.validator) {
-    spec.validator(data, {headers: headers});
+    spec.validator(data, {headers});
   }
 
   return {
-    requestMethod: requestMethod,
-    requestPath: requestPath,
-    data: data,
+    requestMethod,
+    requestPath,
+    data,
     auth: options.auth,
-    headers: headers,
-    host: host,
+    headers,
+    host,
   };
 }
 
 function makeRequest(self, requestArgs, spec, overrideData) {
-  return new Promise(function(resolve, reject) {
+  return new Promise((resolve, reject) => {
     try {
       var opts = getRequestOpts(self, requestArgs, spec, overrideData);
     } catch (err) {
diff --git a/lib/resources/Accounts.js b/lib/resources/Accounts.js
index 7e8b4b8781..75e25acf1d 100644
--- a/lib/resources/Accounts.js
+++ b/lib/resources/Accounts.js
@@ -1,7 +1,7 @@
 'use strict';
 
-var StripeResource = require('../StripeResource');
-var stripeMethod = StripeResource.method;
+const StripeResource = require('../StripeResource');
+const stripeMethod = StripeResource.method;
 
 module.exports = StripeResource.extend({
   // Since path can either be `account` or `accounts`, support both through stripeMethod path
@@ -36,7 +36,7 @@ module.exports = StripeResource.extend({
     urlParams: ['id'],
   }),
 
-  retrieve: function(id) {
+  retrieve(id) {
     // No longer allow an api key to be passed as the first string to this function due to ambiguity between
     // old account ids and api keys. To request the account for an api key, send null as the id
     if (typeof id === 'string') {
diff --git a/lib/resources/ApplicationFeeRefunds.js b/lib/resources/ApplicationFeeRefunds.js
index e73a018cd4..616d1d60fe 100644
--- a/lib/resources/ApplicationFeeRefunds.js
+++ b/lib/resources/ApplicationFeeRefunds.js
@@ -1,6 +1,6 @@
 'use strict';
 
-var StripeResource = require('../StripeResource');
+const StripeResource = require('../StripeResource');
 
 /**
  * ApplicationFeeRefunds is a unique resource in that, upon instantiation,
diff --git a/lib/resources/ApplicationFees.js b/lib/resources/ApplicationFees.js
index ec41ed3171..d3261bf216 100644
--- a/lib/resources/ApplicationFees.js
+++ b/lib/resources/ApplicationFees.js
@@ -1,7 +1,7 @@
 'use strict';
 
-var StripeResource = require('../StripeResource');
-var stripeMethod = StripeResource.method;
+const StripeResource = require('../StripeResource');
+const stripeMethod = StripeResource.method;
 
 module.exports = StripeResource.extend({
   path: 'application_fees',
diff --git a/lib/resources/Balance.js b/lib/resources/Balance.js
index 7e8adb9116..1a351806dd 100644
--- a/lib/resources/Balance.js
+++ b/lib/resources/Balance.js
@@ -1,7 +1,7 @@
 'use strict';
 
-var StripeResource = require('../StripeResource');
-var stripeMethod = StripeResource.method;
+const StripeResource = require('../StripeResource');
+const stripeMethod = StripeResource.method;
 
 module.exports = StripeResource.extend({
   path: 'balance',
diff --git a/lib/resources/BitcoinReceivers.js b/lib/resources/BitcoinReceivers.js
index 5e430d3632..2642e1e9b3 100644
--- a/lib/resources/BitcoinReceivers.js
+++ b/lib/resources/BitcoinReceivers.js
@@ -1,7 +1,7 @@
 'use strict';
 
-var StripeResource = require('../StripeResource');
-var stripeMethod = StripeResource.method;
+const StripeResource = require('../StripeResource');
+const stripeMethod = StripeResource.method;
 
 module.exports = StripeResource.extend({
   path: 'bitcoin/receivers',
diff --git a/lib/resources/ChargeRefunds.js b/lib/resources/ChargeRefunds.js
index f6de00306d..693a79b0db 100644
--- a/lib/resources/ChargeRefunds.js
+++ b/lib/resources/ChargeRefunds.js
@@ -1,6 +1,6 @@
 'use strict';
 
-var StripeResource = require('../StripeResource');
+const StripeResource = require('../StripeResource');
 
 /**
  * ChargeRefunds is a unique resource in that, upon instantiation,
diff --git a/lib/resources/Charges.js b/lib/resources/Charges.js
index 8f326b7a44..4702bc5bf5 100644
--- a/lib/resources/Charges.js
+++ b/lib/resources/Charges.js
@@ -1,7 +1,7 @@
 'use strict';
 
-var StripeResource = require('../StripeResource');
-var stripeMethod = StripeResource.method;
+const StripeResource = require('../StripeResource');
+const stripeMethod = StripeResource.method;
 
 module.exports = StripeResource.extend({
   path: 'charges',
@@ -68,11 +68,11 @@ module.exports = StripeResource.extend({
     urlParams: ['chargeId', 'refundId'],
   }),
 
-  markAsSafe: function(chargeId) {
+  markAsSafe(chargeId) {
     return this.update(chargeId, {fraud_details: {user_report: 'safe'}});
   },
 
-  markAsFraudulent: function(chargeId) {
+  markAsFraudulent(chargeId) {
     return this.update(chargeId, {fraud_details: {user_report: 'fraudulent'}});
   },
 });
diff --git a/lib/resources/Checkout/Sessions.js b/lib/resources/Checkout/Sessions.js
index 936377c45a..dd47cf7add 100644
--- a/lib/resources/Checkout/Sessions.js
+++ b/lib/resources/Checkout/Sessions.js
@@ -1,6 +1,6 @@
 'use strict';
 
-var StripeResource = require('../../StripeResource');
+const StripeResource = require('../../StripeResource');
 
 module.exports = StripeResource.extend({
   path: 'checkout/sessions',
diff --git a/lib/resources/CountrySpecs.js b/lib/resources/CountrySpecs.js
index 690b77cd12..b6d5477afe 100644
--- a/lib/resources/CountrySpecs.js
+++ b/lib/resources/CountrySpecs.js
@@ -1,6 +1,6 @@
 'use strict';
 
-var StripeResource = require('../StripeResource');
+const StripeResource = require('../StripeResource');
 
 module.exports = StripeResource.extend({
   path: 'country_specs',
diff --git a/lib/resources/CreditNotes.js b/lib/resources/CreditNotes.js
index ba7d15f45c..5baf5b53f3 100644
--- a/lib/resources/CreditNotes.js
+++ b/lib/resources/CreditNotes.js
@@ -1,7 +1,7 @@
 'use strict';
 
-var StripeResource = require('../StripeResource');
-var stripeMethod = StripeResource.method;
+const StripeResource = require('../StripeResource');
+const stripeMethod = StripeResource.method;
 
 module.exports = StripeResource.extend({
   path: 'credit_notes',
diff --git a/lib/resources/CustomerCards.js b/lib/resources/CustomerCards.js
index 059c403de2..194d85bec0 100644
--- a/lib/resources/CustomerCards.js
+++ b/lib/resources/CustomerCards.js
@@ -1,6 +1,6 @@
 'use strict';
 
-var StripeResource = require('../StripeResource');
+const StripeResource = require('../StripeResource');
 
 /**
  * CustomerCard is a unique resource in that, upon instantiation,
diff --git a/lib/resources/CustomerSubscriptions.js b/lib/resources/CustomerSubscriptions.js
index 3082d84049..6cde760a2f 100644
--- a/lib/resources/CustomerSubscriptions.js
+++ b/lib/resources/CustomerSubscriptions.js
@@ -1,7 +1,7 @@
 'use strict';
 
-var StripeResource = require('../StripeResource');
-var stripeMethod = StripeResource.method;
+const StripeResource = require('../StripeResource');
+const stripeMethod = StripeResource.method;
 
 /**
  * CustomerSubscription is a unique resource in that, upon instantiation,
diff --git a/lib/resources/Customers.js b/lib/resources/Customers.js
index d70f3302b9..7cce50e42c 100644
--- a/lib/resources/Customers.js
+++ b/lib/resources/Customers.js
@@ -1,8 +1,8 @@
 'use strict';
 
-var StripeResource = require('../StripeResource');
-var utils = require('../utils');
-var stripeMethod = StripeResource.method;
+const StripeResource = require('../StripeResource');
+const utils = require('../utils');
+const stripeMethod = StripeResource.method;
 
 module.exports = StripeResource.extend({
   path: 'customers',
@@ -63,21 +63,21 @@ module.exports = StripeResource.extend({
     urlParams: ['customerId', 'subscriptionId'],
   }),
 
-  updateSubscription: function(customerId, subscriptionId) {
+  updateSubscription(customerId, subscriptionId) {
     if (typeof subscriptionId == 'string') {
-      return this._newstyleUpdateSubscription.apply(this, arguments);
+      return this._newstyleUpdateSubscription(...arguments);
     } else {
-      return this._legacyUpdateSubscription.apply(this, arguments);
+      return this._legacyUpdateSubscription(...arguments);
     }
   },
 
-  cancelSubscription: function(customerId, subscriptionId) {
+  cancelSubscription(customerId, subscriptionId) {
     // This is a hack, but it lets us maximize our overloading.
     // Precarious assumption: If it's not an auth key it _could_ be a sub id:
     if (typeof subscriptionId == 'string' && !utils.isAuthKey(subscriptionId)) {
-      return this._newstyleCancelSubscription.apply(this, arguments);
+      return this._newstyleCancelSubscription(...arguments);
     } else {
-      return this._legacyCancelSubscription.apply(this, arguments);
+      return this._legacyCancelSubscription(...arguments);
     }
   },
 
diff --git a/lib/resources/Disputes.js b/lib/resources/Disputes.js
index 4a60c73eab..83c5526c25 100644
--- a/lib/resources/Disputes.js
+++ b/lib/resources/Disputes.js
@@ -1,7 +1,7 @@
 'use strict';
 
-var StripeResource = require('../StripeResource');
-var stripeMethod = StripeResource.method;
+const StripeResource = require('../StripeResource');
+const stripeMethod = StripeResource.method;
 
 module.exports = StripeResource.extend({
   path: 'disputes',
diff --git a/lib/resources/EphemeralKeys.js b/lib/resources/EphemeralKeys.js
index 7280a69fda..96cee89368 100644
--- a/lib/resources/EphemeralKeys.js
+++ b/lib/resources/EphemeralKeys.js
@@ -1,12 +1,12 @@
 'use strict';
 
-var StripeResource = require('../StripeResource');
-var stripeMethod = StripeResource.method;
+const StripeResource = require('../StripeResource');
+const stripeMethod = StripeResource.method;
 
 module.exports = StripeResource.extend({
   create: stripeMethod({
     method: 'POST',
-    validator: function(data, options) {
+    validator: (data, options) => {
       if (!options.headers || !options.headers['Stripe-Version']) {
         throw new Error(
           'stripe_version must be specified to create an ephemeral key'
diff --git a/lib/resources/ExchangeRates.js b/lib/resources/ExchangeRates.js
index b97e82eae8..7a314cadca 100644
--- a/lib/resources/ExchangeRates.js
+++ b/lib/resources/ExchangeRates.js
@@ -1,6 +1,6 @@
 'use strict';
 
-var StripeResource = require('../StripeResource');
+const StripeResource = require('../StripeResource');
 
 module.exports = StripeResource.extend({
   path: 'exchange_rates',
diff --git a/lib/resources/FileLinks.js b/lib/resources/FileLinks.js
index a07235a5a5..8c264d061a 100644
--- a/lib/resources/FileLinks.js
+++ b/lib/resources/FileLinks.js
@@ -1,6 +1,6 @@
 'use strict';
 
-var StripeResource = require('../StripeResource');
+const StripeResource = require('../StripeResource');
 
 module.exports = StripeResource.extend({
   path: 'file_links',
diff --git a/lib/resources/Files.js b/lib/resources/Files.js
index 4fd258fea4..b400d002fe 100644
--- a/lib/resources/Files.js
+++ b/lib/resources/Files.js
@@ -1,14 +1,14 @@
 'use strict';
 
-var Buffer = require('safe-buffer').Buffer;
-var utils = require('../utils');
-var StripeResource = require('../StripeResource');
-var stripeMethod = StripeResource.method;
-var multipartDataGenerator = require('../MultipartDataGenerator');
-var Error = require('../Error');
+const Buffer = require('safe-buffer').Buffer;
+const utils = require('../utils');
+const StripeResource = require('../StripeResource');
+const stripeMethod = StripeResource.method;
+const multipartDataGenerator = require('../MultipartDataGenerator');
+const Error = require('../Error');
 
 module.exports = StripeResource.extend({
-  requestDataProcessor: function(method, data, headers, callback) {
+  requestDataProcessor(method, data, headers, callback) {
     data = data || {};
 
     if (method === 'POST') {
@@ -18,43 +18,43 @@ module.exports = StripeResource.extend({
     }
 
     function getProcessorForSourceType(data) {
-      var isStream = utils.checkForStream(data);
+      const isStream = utils.checkForStream(data);
       if (isStream) {
         return streamProcessor(multipartDataGenerator);
       } else {
-        var buffer = multipartDataGenerator(method, data, headers);
+        const buffer = multipartDataGenerator(method, data, headers);
         return callback(null, buffer);
       }
     }
 
     function streamProcessor(fn) {
-      var bufferArray = [];
+      const bufferArray = [];
       data.file.data
-        .on('data', function(line) {
+        .on('data', (line) => {
           bufferArray.push(line);
         })
-        .on('end', function() {
-          var bufferData = Object.assign({}, data);
+        .on('end', () => {
+          const bufferData = Object.assign({}, data);
           bufferData.file.data = Buffer.concat(bufferArray);
-          var buffer = fn(method, bufferData, headers);
+          const buffer = fn(method, bufferData, headers);
           callback(null, buffer);
         })
-        .on('error', function(err) {
-          var errorHandler = streamError(callback);
+        .on('error', (err) => {
+          const errorHandler = streamError(callback);
           errorHandler(err);
         });
     }
 
     function streamError(callback) {
-      var StreamProcessingError = Error.extend({
+      const StreamProcessingError = Error.extend({
         type: 'StreamProcessingError',
-        populate: function(raw) {
+        populate(raw) {
           this.type = this.type;
           this.message = raw.message;
           this.detail = raw.detail;
         },
       });
-      return function(error) {
+      return (error) => {
         callback(
           new StreamProcessingError({
             message:
diff --git a/lib/resources/Invoices.js b/lib/resources/Invoices.js
index 86e0c9c819..39f1dea8f3 100644
--- a/lib/resources/Invoices.js
+++ b/lib/resources/Invoices.js
@@ -1,8 +1,8 @@
 'use strict';
 
-var StripeResource = require('../StripeResource');
-var stripeMethod = StripeResource.method;
-var utils = require('../utils');
+const StripeResource = require('../StripeResource');
+const stripeMethod = StripeResource.method;
+const utils = require('../utils');
 
 module.exports = StripeResource.extend({
   path: 'invoices',
@@ -34,9 +34,9 @@ module.exports = StripeResource.extend({
 
   retrieveUpcoming: stripeMethod({
     method: 'GET',
-    path: function(urlData) {
-      var url = 'upcoming?';
-      var hasParam = false;
+    path(urlData) {
+      let url = 'upcoming?';
+      let hasParam = false;
 
       // If you pass just a hash with the relevant parameters, including customer id inside.
       if (
@@ -53,7 +53,7 @@ module.exports = StripeResource.extend({
         urlData.invoiceOptionsOrCustomerId &&
         typeof urlData.invoiceOptionsOrCustomerId === 'string'
       ) {
-        url = url + 'customer=' + urlData.invoiceOptionsOrCustomerId;
+        url = `${url}customer=${urlData.invoiceOptionsOrCustomerId}`;
         hasParam = true;
       }
 
@@ -62,12 +62,9 @@ module.exports = StripeResource.extend({
         urlData.invoiceOptionsOrSubscriptionId &&
         typeof urlData.invoiceOptionsOrSubscriptionId === 'string'
       ) {
-        return (
-          url +
-          (hasParam ? '&' : '') +
-          'subscription=' +
+        return `${url + (hasParam ? '&' : '')}subscription=${
           urlData.invoiceOptionsOrSubscriptionId
-        );
+        }`;
       } else if (
         urlData.invoiceOptionsOrSubscriptionId &&
         typeof urlData.invoiceOptionsOrSubscriptionId === 'object'
diff --git a/lib/resources/IssuerFraudRecords.js b/lib/resources/IssuerFraudRecords.js
index 76c1e1e4d0..97d38fd133 100644
--- a/lib/resources/IssuerFraudRecords.js
+++ b/lib/resources/IssuerFraudRecords.js
@@ -1,6 +1,6 @@
 'use strict';
 
-var StripeResource = require('../StripeResource');
+const StripeResource = require('../StripeResource');
 
 module.exports = StripeResource.extend({
   path: 'issuer_fraud_records',
diff --git a/lib/resources/Issuing/Authorizations.js b/lib/resources/Issuing/Authorizations.js
index a17deec892..85756a6935 100644
--- a/lib/resources/Issuing/Authorizations.js
+++ b/lib/resources/Issuing/Authorizations.js
@@ -1,7 +1,7 @@
 'use strict';
 
-var StripeResource = require('../../StripeResource');
-var stripeMethod = StripeResource.method;
+const StripeResource = require('../../StripeResource');
+const stripeMethod = StripeResource.method;
 
 module.exports = StripeResource.extend({
   path: 'issuing/authorizations',
diff --git a/lib/resources/Issuing/Cardholders.js b/lib/resources/Issuing/Cardholders.js
index 0e6e5a8bfe..58dd2630b9 100644
--- a/lib/resources/Issuing/Cardholders.js
+++ b/lib/resources/Issuing/Cardholders.js
@@ -1,6 +1,6 @@
 'use strict';
 
-var StripeResource = require('../../StripeResource');
+const StripeResource = require('../../StripeResource');
 
 module.exports = StripeResource.extend({
   path: 'issuing/cardholders',
diff --git a/lib/resources/Issuing/Cards.js b/lib/resources/Issuing/Cards.js
index 5f5664b88f..f8026ab5fc 100644
--- a/lib/resources/Issuing/Cards.js
+++ b/lib/resources/Issuing/Cards.js
@@ -1,7 +1,7 @@
 'use strict';
 
-var StripeResource = require('../../StripeResource');
-var stripeMethod = StripeResource.method;
+const StripeResource = require('../../StripeResource');
+const stripeMethod = StripeResource.method;
 
 module.exports = StripeResource.extend({
   path: 'issuing/cards',
diff --git a/lib/resources/Issuing/Disputes.js b/lib/resources/Issuing/Disputes.js
index 096ba8eb43..a26015a3ca 100644
--- a/lib/resources/Issuing/Disputes.js
+++ b/lib/resources/Issuing/Disputes.js
@@ -1,6 +1,6 @@
 'use strict';
 
-var StripeResource = require('../../StripeResource');
+const StripeResource = require('../../StripeResource');
 
 module.exports = StripeResource.extend({
   path: 'issuing/disputes',
diff --git a/lib/resources/Issuing/Transactions.js b/lib/resources/Issuing/Transactions.js
index f7f004bbd8..789292ff2a 100644
--- a/lib/resources/Issuing/Transactions.js
+++ b/lib/resources/Issuing/Transactions.js
@@ -1,6 +1,6 @@
 'use strict';
 
-var StripeResource = require('../../StripeResource');
+const StripeResource = require('../../StripeResource');
 
 module.exports = StripeResource.extend({
   path: 'issuing/transactions',
diff --git a/lib/resources/LoginLinks.js b/lib/resources/LoginLinks.js
index ac82bc71c9..b76439e000 100644
--- a/lib/resources/LoginLinks.js
+++ b/lib/resources/LoginLinks.js
@@ -1,6 +1,6 @@
 'use strict';
 
-var StripeResource = require('../StripeResource');
+const StripeResource = require('../StripeResource');
 
 module.exports = StripeResource.extend({
   path: 'accounts/{accountId}/login_links',
diff --git a/lib/resources/OAuth.js b/lib/resources/OAuth.js
index 7258b8c371..70ce4a2358 100644
--- a/lib/resources/OAuth.js
+++ b/lib/resources/OAuth.js
@@ -1,23 +1,23 @@
 'use strict';
 
-var StripeResource = require('../StripeResource');
-var stripeMethod = StripeResource.method;
-var utils = require('../utils');
+const StripeResource = require('../StripeResource');
+const stripeMethod = StripeResource.method;
+const utils = require('../utils');
 
-var oAuthHost = 'connect.stripe.com';
+const oAuthHost = 'connect.stripe.com';
 
 module.exports = StripeResource.extend({
   basePath: '/',
 
-  authorizeUrl: function(params, options) {
+  authorizeUrl(params, options) {
     params = params || {};
     options = options || {};
 
-    var path = 'oauth/authorize';
+    let path = 'oauth/authorize';
 
     // For Express accounts, the path changes
     if (options.express) {
-      path = 'express/' + path;
+      path = `express/${path}`;
     }
 
     if (!params.response_type) {
@@ -32,14 +32,7 @@ module.exports = StripeResource.extend({
       params.scope = 'read_write';
     }
 
-    return (
-      'https://' +
-      oAuthHost +
-      '/' +
-      path +
-      '?' +
-      utils.stringifyRequestData(params)
-    );
+    return `https://${oAuthHost}/${path}?${utils.stringifyRequestData(params)}`;
   },
 
   token: stripeMethod({
@@ -48,7 +41,7 @@ module.exports = StripeResource.extend({
     host: oAuthHost,
   }),
 
-  deauthorize: function(spec) {
+  deauthorize(spec) {
     if (!spec.client_id) {
       spec.client_id = this._stripe.getClientId();
     }
diff --git a/lib/resources/OrderReturns.js b/lib/resources/OrderReturns.js
index b0e1be62e3..058c702c87 100644
--- a/lib/resources/OrderReturns.js
+++ b/lib/resources/OrderReturns.js
@@ -1,6 +1,6 @@
 'use strict';
 
-var StripeResource = require('../StripeResource');
+const StripeResource = require('../StripeResource');
 
 module.exports = StripeResource.extend({
   path: 'order_returns',
diff --git a/lib/resources/Orders.js b/lib/resources/Orders.js
index c59ea450aa..371c3ec486 100644
--- a/lib/resources/Orders.js
+++ b/lib/resources/Orders.js
@@ -1,7 +1,7 @@
 'use strict';
 
-var StripeResource = require('../StripeResource');
-var stripeMethod = StripeResource.method;
+const StripeResource = require('../StripeResource');
+const stripeMethod = StripeResource.method;
 
 module.exports = StripeResource.extend({
   path: 'orders',
diff --git a/lib/resources/PaymentIntents.js b/lib/resources/PaymentIntents.js
index 86128201da..7093b21ee7 100644
--- a/lib/resources/PaymentIntents.js
+++ b/lib/resources/PaymentIntents.js
@@ -1,7 +1,7 @@
 'use strict';
 
-var StripeResource = require('../StripeResource');
-var stripeMethod = StripeResource.method;
+const StripeResource = require('../StripeResource');
+const stripeMethod = StripeResource.method;
 
 module.exports = StripeResource.extend({
   path: 'payment_intents',
diff --git a/lib/resources/PaymentMethods.js b/lib/resources/PaymentMethods.js
index 55c1495e32..cc6201749e 100644
--- a/lib/resources/PaymentMethods.js
+++ b/lib/resources/PaymentMethods.js
@@ -1,7 +1,7 @@
 'use strict';
 
-var StripeResource = require('../StripeResource');
-var stripeMethod = StripeResource.method;
+const StripeResource = require('../StripeResource');
+const stripeMethod = StripeResource.method;
 
 module.exports = StripeResource.extend({
   path: 'payment_methods',
diff --git a/lib/resources/Payouts.js b/lib/resources/Payouts.js
index 9c41679c12..8cd32d9c9c 100644
--- a/lib/resources/Payouts.js
+++ b/lib/resources/Payouts.js
@@ -1,7 +1,7 @@
 'use strict';
 
-var StripeResource = require('../StripeResource');
-var stripeMethod = StripeResource.method;
+const StripeResource = require('../StripeResource');
+const stripeMethod = StripeResource.method;
 
 module.exports = StripeResource.extend({
   path: 'payouts',
diff --git a/lib/resources/Persons.js b/lib/resources/Persons.js
index 6429dd0d26..8de5bacd0b 100644
--- a/lib/resources/Persons.js
+++ b/lib/resources/Persons.js
@@ -1,6 +1,6 @@
 'use strict';
 
-var StripeResource = require('../StripeResource');
+const StripeResource = require('../StripeResource');
 
 /**
  * Persons is a unique resource in that, upon instantiation,
diff --git a/lib/resources/Products.js b/lib/resources/Products.js
index 3777c567ca..02da55d5da 100644
--- a/lib/resources/Products.js
+++ b/lib/resources/Products.js
@@ -1,6 +1,6 @@
 'use strict';
 
-var StripeResource = require('../StripeResource');
+const StripeResource = require('../StripeResource');
 
 module.exports = StripeResource.extend({
   path: 'products',
diff --git a/lib/resources/Radar/ValueListItems.js b/lib/resources/Radar/ValueListItems.js
index d6776fd764..055ea00004 100644
--- a/lib/resources/Radar/ValueListItems.js
+++ b/lib/resources/Radar/ValueListItems.js
@@ -1,6 +1,6 @@
 'use strict';
 
-var StripeResource = require('../../StripeResource');
+const StripeResource = require('../../StripeResource');
 
 module.exports = StripeResource.extend({
   path: 'radar/value_list_items',
diff --git a/lib/resources/Radar/ValueLists.js b/lib/resources/Radar/ValueLists.js
index fdec19aba0..63d4134239 100644
--- a/lib/resources/Radar/ValueLists.js
+++ b/lib/resources/Radar/ValueLists.js
@@ -1,6 +1,6 @@
 'use strict';
 
-var StripeResource = require('../../StripeResource');
+const StripeResource = require('../../StripeResource');
 
 module.exports = StripeResource.extend({
   path: 'radar/value_lists',
diff --git a/lib/resources/RecipientCards.js b/lib/resources/RecipientCards.js
index 2986003798..0dc675bdcc 100644
--- a/lib/resources/RecipientCards.js
+++ b/lib/resources/RecipientCards.js
@@ -1,6 +1,6 @@
 'use strict';
 
-var StripeResource = require('../StripeResource');
+const StripeResource = require('../StripeResource');
 
 /**
  * RecipientCard is similar to CustomerCard in that, upon instantiation, it
diff --git a/lib/resources/Recipients.js b/lib/resources/Recipients.js
index b4a97cf19c..e9adaec076 100644
--- a/lib/resources/Recipients.js
+++ b/lib/resources/Recipients.js
@@ -1,7 +1,7 @@
 'use strict';
 
-var StripeResource = require('../StripeResource');
-var stripeMethod = StripeResource.method;
+const StripeResource = require('../StripeResource');
+const stripeMethod = StripeResource.method;
 
 module.exports = StripeResource.extend({
   path: 'recipients',
diff --git a/lib/resources/Refunds.js b/lib/resources/Refunds.js
index a1351a27d2..76373c92c0 100644
--- a/lib/resources/Refunds.js
+++ b/lib/resources/Refunds.js
@@ -1,6 +1,6 @@
 'use strict';
 
-var StripeResource = require('../StripeResource');
+const StripeResource = require('../StripeResource');
 
 module.exports = StripeResource.extend({
   path: 'refunds',
diff --git a/lib/resources/Reporting/ReportRuns.js b/lib/resources/Reporting/ReportRuns.js
index 46d8a989d6..9cf3cbc6df 100644
--- a/lib/resources/Reporting/ReportRuns.js
+++ b/lib/resources/Reporting/ReportRuns.js
@@ -1,6 +1,6 @@
 'use strict';
 
-var StripeResource = require('../../StripeResource');
+const StripeResource = require('../../StripeResource');
 
 module.exports = StripeResource.extend({
   path: 'reporting/report_runs',
diff --git a/lib/resources/Reporting/ReportTypes.js b/lib/resources/Reporting/ReportTypes.js
index 576c2aad4b..ff3dee3167 100644
--- a/lib/resources/Reporting/ReportTypes.js
+++ b/lib/resources/Reporting/ReportTypes.js
@@ -1,6 +1,6 @@
 'use strict';
 
-var StripeResource = require('../../StripeResource');
+const StripeResource = require('../../StripeResource');
 
 module.exports = StripeResource.extend({
   path: 'reporting/report_types',
diff --git a/lib/resources/Reviews.js b/lib/resources/Reviews.js
index 6d3fb2c4f1..40a150512d 100644
--- a/lib/resources/Reviews.js
+++ b/lib/resources/Reviews.js
@@ -1,7 +1,7 @@
 'use strict';
 
-var StripeResource = require('../StripeResource');
-var stripeMethod = StripeResource.method;
+const StripeResource = require('../StripeResource');
+const stripeMethod = StripeResource.method;
 
 module.exports = StripeResource.extend({
   path: 'reviews',
diff --git a/lib/resources/SKUs.js b/lib/resources/SKUs.js
index 36f7445dfd..9fd01cf439 100644
--- a/lib/resources/SKUs.js
+++ b/lib/resources/SKUs.js
@@ -1,6 +1,6 @@
 'use strict';
 
-var StripeResource = require('../StripeResource');
+const StripeResource = require('../StripeResource');
 
 module.exports = StripeResource.extend({
   path: 'skus',
diff --git a/lib/resources/Sigma/ScheduledQueryRuns.js b/lib/resources/Sigma/ScheduledQueryRuns.js
index bc80b40040..0714497062 100644
--- a/lib/resources/Sigma/ScheduledQueryRuns.js
+++ b/lib/resources/Sigma/ScheduledQueryRuns.js
@@ -1,6 +1,6 @@
 'use strict';
 
-var StripeResource = require('../../StripeResource');
+const StripeResource = require('../../StripeResource');
 
 module.exports = StripeResource.extend({
   path: 'sigma/scheduled_query_runs',
diff --git a/lib/resources/Sources.js b/lib/resources/Sources.js
index d0e103509e..78e7a87675 100644
--- a/lib/resources/Sources.js
+++ b/lib/resources/Sources.js
@@ -1,7 +1,7 @@
 'use strict';
 
-var StripeResource = require('../StripeResource');
-var stripeMethod = StripeResource.method;
+const StripeResource = require('../StripeResource');
+const stripeMethod = StripeResource.method;
 
 module.exports = StripeResource.extend({
   path: 'sources',
diff --git a/lib/resources/SubscriptionItems.js b/lib/resources/SubscriptionItems.js
index fad2894f7c..4c159e0ced 100644
--- a/lib/resources/SubscriptionItems.js
+++ b/lib/resources/SubscriptionItems.js
@@ -1,6 +1,6 @@
 'use strict';
 
-var StripeResource = require('../StripeResource');
+const StripeResource = require('../StripeResource');
 
 module.exports = StripeResource.extend({
   path: 'subscription_items',
diff --git a/lib/resources/SubscriptionScheduleRevisions.js b/lib/resources/SubscriptionScheduleRevisions.js
index 591f64fc62..8998b7007a 100644
--- a/lib/resources/SubscriptionScheduleRevisions.js
+++ b/lib/resources/SubscriptionScheduleRevisions.js
@@ -1,6 +1,6 @@
 'use strict';
 
-var StripeResource = require('../StripeResource');
+const StripeResource = require('../StripeResource');
 
 /**
  * SubscriptionScheduleRevisions is a unique resource in that, upon instantiation,
diff --git a/lib/resources/SubscriptionSchedules.js b/lib/resources/SubscriptionSchedules.js
index 6045fd093f..f135a0ef2a 100644
--- a/lib/resources/SubscriptionSchedules.js
+++ b/lib/resources/SubscriptionSchedules.js
@@ -1,7 +1,7 @@
 'use strict';
 
-var StripeResource = require('../StripeResource');
-var stripeMethod = StripeResource.method;
+const StripeResource = require('../StripeResource');
+const stripeMethod = StripeResource.method;
 
 module.exports = StripeResource.extend({
   path: 'subscription_schedules',
diff --git a/lib/resources/Subscriptions.js b/lib/resources/Subscriptions.js
index 4deb1b9262..5bb1d5f784 100644
--- a/lib/resources/Subscriptions.js
+++ b/lib/resources/Subscriptions.js
@@ -1,7 +1,7 @@
 'use strict';
 
-var StripeResource = require('../StripeResource');
-var stripeMethod = StripeResource.method;
+const StripeResource = require('../StripeResource');
+const stripeMethod = StripeResource.method;
 
 module.exports = StripeResource.extend({
   path: 'subscriptions',
diff --git a/lib/resources/TaxIds.js b/lib/resources/TaxIds.js
index bc8d860452..301782eda2 100644
--- a/lib/resources/TaxIds.js
+++ b/lib/resources/TaxIds.js
@@ -1,6 +1,6 @@
 'use strict';
 
-var StripeResource = require('../StripeResource');
+const StripeResource = require('../StripeResource');
 
 /**
  * TaxIds is a unique resource in that, upon instantiation,
diff --git a/lib/resources/Terminal/ConnectionTokens.js b/lib/resources/Terminal/ConnectionTokens.js
index 3b52fc93d1..76195dda70 100644
--- a/lib/resources/Terminal/ConnectionTokens.js
+++ b/lib/resources/Terminal/ConnectionTokens.js
@@ -1,6 +1,6 @@
 'use strict';
 
-var StripeResource = require('../../StripeResource');
+const StripeResource = require('../../StripeResource');
 
 module.exports = StripeResource.extend({
   path: 'terminal/connection_tokens',
diff --git a/lib/resources/Terminal/Locations.js b/lib/resources/Terminal/Locations.js
index 7066d48e2b..0748bbd0cb 100644
--- a/lib/resources/Terminal/Locations.js
+++ b/lib/resources/Terminal/Locations.js
@@ -1,6 +1,6 @@
 'use strict';
 
-var StripeResource = require('../../StripeResource');
+const StripeResource = require('../../StripeResource');
 
 module.exports = StripeResource.extend({
   path: 'terminal/locations',
diff --git a/lib/resources/Terminal/Readers.js b/lib/resources/Terminal/Readers.js
index 4263dbf21c..b5dddc3929 100644
--- a/lib/resources/Terminal/Readers.js
+++ b/lib/resources/Terminal/Readers.js
@@ -1,6 +1,6 @@
 'use strict';
 
-var StripeResource = require('../../StripeResource');
+const StripeResource = require('../../StripeResource');
 
 module.exports = StripeResource.extend({
   path: 'terminal/readers',
diff --git a/lib/resources/ThreeDSecure.js b/lib/resources/ThreeDSecure.js
index 5756019b49..96248d1ea9 100644
--- a/lib/resources/ThreeDSecure.js
+++ b/lib/resources/ThreeDSecure.js
@@ -1,6 +1,6 @@
 'use strict';
 
-var StripeResource = require('../StripeResource');
+const StripeResource = require('../StripeResource');
 
 module.exports = StripeResource.extend({
   path: '3d_secure',
diff --git a/lib/resources/Topups.js b/lib/resources/Topups.js
index 41d755b48b..ef57d1ff3f 100644
--- a/lib/resources/Topups.js
+++ b/lib/resources/Topups.js
@@ -1,7 +1,7 @@
 'use strict';
 
-var StripeResource = require('../StripeResource');
-var stripeMethod = StripeResource.method;
+const StripeResource = require('../StripeResource');
+const stripeMethod = StripeResource.method;
 
 module.exports = StripeResource.extend({
   path: 'topups',
diff --git a/lib/resources/TransferReversals.js b/lib/resources/TransferReversals.js
index 6794fde77f..4eb4603796 100644
--- a/lib/resources/TransferReversals.js
+++ b/lib/resources/TransferReversals.js
@@ -1,6 +1,6 @@
 'use strict';
 
-var StripeResource = require('../StripeResource');
+const StripeResource = require('../StripeResource');
 
 /**
  * TransferReversals is a unique resource in that, upon instantiation,
diff --git a/lib/resources/Transfers.js b/lib/resources/Transfers.js
index 04f4dc2386..2a0cb92ddf 100644
--- a/lib/resources/Transfers.js
+++ b/lib/resources/Transfers.js
@@ -1,7 +1,7 @@
 'use strict';
 
-var StripeResource = require('../StripeResource');
-var stripeMethod = StripeResource.method;
+const StripeResource = require('../StripeResource');
+const stripeMethod = StripeResource.method;
 
 module.exports = StripeResource.extend({
   path: 'transfers',
diff --git a/lib/resources/UsageRecordSummaries.js b/lib/resources/UsageRecordSummaries.js
index f0f438f33e..8a59a43ef6 100644
--- a/lib/resources/UsageRecordSummaries.js
+++ b/lib/resources/UsageRecordSummaries.js
@@ -1,7 +1,7 @@
 'use strict';
 
-var StripeResource = require('../StripeResource');
-var stripeMethod = StripeResource.method;
+const StripeResource = require('../StripeResource');
+const stripeMethod = StripeResource.method;
 
 module.exports = StripeResource.extend({
   path: 'subscription_items',
diff --git a/lib/resources/UsageRecords.js b/lib/resources/UsageRecords.js
index 697539aa14..26829eccd3 100644
--- a/lib/resources/UsageRecords.js
+++ b/lib/resources/UsageRecords.js
@@ -1,7 +1,7 @@
 'use strict';
 
-var StripeResource = require('../StripeResource');
-var stripeMethod = StripeResource.method;
+const StripeResource = require('../StripeResource');
+const stripeMethod = StripeResource.method;
 
 module.exports = StripeResource.extend({
   path: 'subscription_items',
diff --git a/lib/stripe.js b/lib/stripe.js
index 46ed677b63..2ce637fa17 100644
--- a/lib/stripe.js
+++ b/lib/stripe.js
@@ -24,14 +24,14 @@ Stripe.USER_AGENT_SERIALIZED = null;
 Stripe.MAX_NETWORK_RETRY_DELAY_SEC = 2;
 Stripe.INITIAL_NETWORK_RETRY_DELAY_SEC = 0.5;
 
-var APP_INFO_PROPERTIES = ['name', 'version', 'url', 'partner_id'];
+const APP_INFO_PROPERTIES = ['name', 'version', 'url', 'partner_id'];
 
-var EventEmitter = require('events').EventEmitter;
-var utils = require('./utils');
+const EventEmitter = require('events').EventEmitter;
+const utils = require('./utils');
 
-var resourceNamespace = require('./ResourceNamespace');
+const resourceNamespace = require('./ResourceNamespace');
 
-var resources = {
+const resources = {
   // Support Accounts for consistency, Account for backwards compat
   Account: require('./resources/Accounts'),
   Accounts: require('./resources/Accounts'),
@@ -168,7 +168,7 @@ Stripe.errors = require('./Error');
 Stripe.webhooks = require('./Webhooks');
 
 Stripe.prototype = {
-  setHost: function(host, port, protocol) {
+  setHost(host, port, protocol) {
     this._setApiField('host', host);
     if (port) {
       this.setPort(port);
@@ -178,34 +178,34 @@ Stripe.prototype = {
     }
   },
 
-  setProtocol: function(protocol) {
+  setProtocol(protocol) {
     this._setApiField('protocol', protocol.toLowerCase());
   },
 
-  setPort: function(port) {
+  setPort(port) {
     this._setApiField('port', port);
   },
 
-  setApiVersion: function(version) {
+  setApiVersion(version) {
     if (version) {
       this._setApiField('version', version);
     }
   },
 
-  setApiKey: function(key) {
+  setApiKey(key) {
     if (key) {
-      this._setApiField('auth', 'Bearer ' + key);
+      this._setApiField('auth', `Bearer ${key}`);
     }
   },
 
-  setTimeout: function(timeout) {
+  setTimeout(timeout) {
     this._setApiField(
       'timeout',
       timeout == null ? Stripe.DEFAULT_TIMEOUT : timeout
     );
   },
 
-  setAppInfo: function(info) {
+  setAppInfo(info) {
     if (info && typeof info !== 'object') {
       throw new Error('AppInfo must be an object.');
     }
@@ -216,7 +216,7 @@ Stripe.prototype = {
 
     info = info || {};
 
-    var appInfo = APP_INFO_PROPERTIES.reduce(function(accum, prop) {
+    const appInfo = APP_INFO_PROPERTIES.reduce((accum, prop) => {
       if (typeof info[prop] == 'string') {
         accum = accum || {};
 
@@ -232,35 +232,35 @@ Stripe.prototype = {
     this._appInfo = appInfo;
   },
 
-  setHttpAgent: function(agent) {
+  setHttpAgent(agent) {
     this._setApiField('agent', agent);
   },
 
-  _setApiField: function(key, value) {
+  _setApiField(key, value) {
     this._api[key] = value;
   },
 
-  getApiField: function(key) {
+  getApiField(key) {
     return this._api[key];
   },
 
-  setClientId: function(clientId) {
+  setClientId(clientId) {
     this._clientId = clientId;
   },
 
-  getClientId: function() {
+  getClientId() {
     return this._clientId;
   },
 
-  getConstant: function(c) {
+  getConstant: (c) => {
     return Stripe[c];
   },
 
-  getMaxNetworkRetries: function() {
+  getMaxNetworkRetries() {
     return this.getApiField('maxNetworkRetries');
   },
 
-  setMaxNetworkRetries: function(maxNetworkRetries) {
+  setMaxNetworkRetries(maxNetworkRetries) {
     if (
       (maxNetworkRetries && typeof maxNetworkRetries !== 'number') ||
       arguments.length < 1
@@ -271,21 +271,21 @@ Stripe.prototype = {
     this._setApiField('maxNetworkRetries', maxNetworkRetries);
   },
 
-  getMaxNetworkRetryDelay: function() {
+  getMaxNetworkRetryDelay() {
     return this.getConstant('MAX_NETWORK_RETRY_DELAY_SEC');
   },
 
-  getInitialNetworkRetryDelay: function() {
+  getInitialNetworkRetryDelay() {
     return this.getConstant('INITIAL_NETWORK_RETRY_DELAY_SEC');
   },
 
   // Gets a JSON version of a User-Agent and uses a cached version for a slight
   // speed advantage.
-  getClientUserAgent: function(cb) {
+  getClientUserAgent(cb) {
     if (Stripe.USER_AGENT_SERIALIZED) {
       return cb(Stripe.USER_AGENT_SERIALIZED);
     }
-    this.getClientUserAgentSeeded(Stripe.USER_AGENT, function(cua) {
+    this.getClientUserAgentSeeded(Stripe.USER_AGENT, (cua) => {
       Stripe.USER_AGENT_SERIALIZED = cua;
       cb(Stripe.USER_AGENT_SERIALIZED);
     });
@@ -293,12 +293,12 @@ Stripe.prototype = {
 
   // Gets a JSON version of a User-Agent by encoding a seeded object and
   // fetching a uname from the system.
-  getClientUserAgentSeeded: function(seed, cb) {
-    var self = this;
+  getClientUserAgentSeeded(seed, cb) {
+    const self = this;
 
-    utils.safeExec('uname -a', function(err, uname) {
-      var userAgent = {};
-      for (var field in seed) {
+    utils.safeExec('uname -a', (err, uname) => {
+      const userAgent = {};
+      for (const field in seed) {
         userAgent[field] = encodeURIComponent(seed[field]);
       }
 
@@ -313,34 +313,34 @@ Stripe.prototype = {
     });
   },
 
-  getAppInfoAsString: function() {
+  getAppInfoAsString() {
     if (!this._appInfo) {
       return '';
     }
 
-    var formatted = this._appInfo.name;
+    let formatted = this._appInfo.name;
 
     if (this._appInfo.version) {
-      formatted += '/' + this._appInfo.version;
+      formatted += `/${this._appInfo.version}`;
     }
 
     if (this._appInfo.url) {
-      formatted += ' (' + this._appInfo.url + ')';
+      formatted += ` (${this._appInfo.url})`;
     }
 
     return formatted;
   },
 
-  setTelemetryEnabled: function(enableTelemetry) {
+  setTelemetryEnabled(enableTelemetry) {
     this._enableTelemetry = enableTelemetry;
   },
 
-  getTelemetryEnabled: function() {
+  getTelemetryEnabled() {
     return this._enableTelemetry;
   },
 
-  _prepResources: function() {
-    for (var name in resources) {
+  _prepResources() {
+    for (const name in resources) {
       this[utils.pascalToCamelCase(name)] = new resources[name](this);
     }
   },
diff --git a/lib/utils.js b/lib/utils.js
index 9cbfe9b918..ae8ddfd778 100644
--- a/lib/utils.js
+++ b/lib/utils.js
@@ -1,46 +1,39 @@
 'use strict';
 
-var Buffer = require('safe-buffer').Buffer;
-var EventEmitter = require('events').EventEmitter;
-var exec = require('child_process').exec;
-var qs = require('qs');
-var crypto = require('crypto');
+const Buffer = require('safe-buffer').Buffer;
+const EventEmitter = require('events').EventEmitter;
+const exec = require('child_process').exec;
+const qs = require('qs');
+const crypto = require('crypto');
 
-var hasOwn = {}.hasOwnProperty;
-var isPlainObject = require('lodash.isplainobject');
+const hasOwn = {}.hasOwnProperty;
+const isPlainObject = require('lodash.isplainobject');
 
-var OPTIONS_KEYS = [
+const OPTIONS_KEYS = [
   'api_key',
   'idempotency_key',
   'stripe_account',
   'stripe_version',
 ];
 
-var utils = (module.exports = {
-  isAuthKey: function(key) {
+const utils = (module.exports = {
+  isAuthKey: (key) => {
     return typeof key == 'string' && /^(?:[a-z]{2}_)?[A-z0-9]{32}$/.test(key);
   },
 
-  isOptionsHash: function(o) {
-    return (
-      isPlainObject(o) &&
-      OPTIONS_KEYS.some(function(key) {
-        return hasOwn.call(o, key);
-      })
-    );
+  isOptionsHash: (o) => {
+    return isPlainObject(o) && OPTIONS_KEYS.some((key) => hasOwn.call(o, key));
   },
 
   /**
    * Stringifies an Object, accommodating nested objects
    * (forming the conventional key 'parent[child]=value')
    */
-  stringifyRequestData: function(data) {
+  stringifyRequestData: (data) => {
     return (
       qs
         .stringify(data, {
-          serializeDate: function(d) {
-            return Math.floor(d.getTime() / 1000);
-          },
+          serializeDate: (d) => Math.floor(d.getTime() / 1000),
         })
         // Don't use strict form encoding by changing the square bracket control
         // characters back to their literals. This is fine by the server, and
@@ -56,21 +49,19 @@ var utils = (module.exports = {
    *   var fn = makeURLInterpolator('some/url/{param1}/{param2}');
    *   fn({ param1: 123, param2: 456 }); // => 'some/url/123/456'
    */
-  makeURLInterpolator: (function() {
-    var rc = {
+  makeURLInterpolator: (() => {
+    const rc = {
       '\n': '\\n',
       '"': '\\"',
       '\u2028': '\\u2028',
       '\u2029': '\\u2029',
     };
-    return function makeURLInterpolator(str) {
-      var cleanString = str.replace(/["\n\r\u2028\u2029]/g, function($0) {
-        return rc[$0];
-      });
-      return function(outputs) {
-        return cleanString.replace(/\{([\s\S]+?)\}/g, function($0, $1) {
-          return encodeURIComponent(outputs[$1] || '');
-        });
+    return (str) => {
+      const cleanString = str.replace(/["\n\r\u2028\u2029]/g, ($0) => rc[$0]);
+      return (outputs) => {
+        return cleanString.replace(/\{([\s\S]+?)\}/g, ($0, $1) =>
+          encodeURIComponent(outputs[$1] || '')
+        );
       };
     };
   })(),
@@ -78,7 +69,7 @@ var utils = (module.exports = {
   /**
    * Return the data argument from a list of arguments
    */
-  getDataFromArgs: function(args) {
+  getDataFromArgs: (args) => {
     if (args.length < 1 || !isPlainObject(args[0])) {
       return {};
     }
@@ -87,11 +78,11 @@ var utils = (module.exports = {
       return args.shift();
     }
 
-    var argKeys = Object.keys(args[0]);
+    const argKeys = Object.keys(args[0]);
 
-    var optionKeysInArgs = argKeys.filter(function(key) {
-      return OPTIONS_KEYS.indexOf(key) > -1;
-    });
+    const optionKeysInArgs = argKeys.filter((key) =>
+      OPTIONS_KEYS.includes(key)
+    );
 
     // In some cases options may be the provided as the first argument.
     // Here we're detecting a case where there are two distinct arguments
@@ -102,10 +93,9 @@ var utils = (module.exports = {
       optionKeysInArgs.length !== argKeys.length
     ) {
       emitWarning(
-        'Options found in arguments (' +
-          optionKeysInArgs.join(', ') +
-          '). Did you mean to pass an options ' +
-          'object? See https://github.com/stripe/stripe-node/wiki/Passing-Options.'
+        `Options found in arguments (${optionKeysInArgs.join(
+          ', '
+        )}). Did you mean to pass an options object? See https://github.com/stripe/stripe-node/wiki/Passing-Options.`
       );
     }
 
@@ -115,25 +105,25 @@ var utils = (module.exports = {
   /**
    * Return the options hash from a list of arguments
    */
-  getOptionsFromArgs: function(args) {
-    var opts = {
+  getOptionsFromArgs: (args) => {
+    const opts = {
       auth: null,
       headers: {},
     };
     if (args.length > 0) {
-      var arg = args[args.length - 1];
+      const arg = args[args.length - 1];
       if (utils.isAuthKey(arg)) {
         opts.auth = args.pop();
       } else if (utils.isOptionsHash(arg)) {
-        var params = args.pop();
+        const params = args.pop();
 
-        var extraKeys = Object.keys(params).filter(function(key) {
-          return OPTIONS_KEYS.indexOf(key) == -1;
-        });
+        const extraKeys = Object.keys(params).filter(
+          (key) => !OPTIONS_KEYS.includes(key)
+        );
 
         if (extraKeys.length) {
           emitWarning(
-            'Invalid options found (' + extraKeys.join(', ') + '); ignoring.'
+            `Invalid options found (${extraKeys.join(', ')}); ignoring.`
           );
         }
 
@@ -157,12 +147,12 @@ var utils = (module.exports = {
   /**
    * Provide simple "Class" extension mechanism
    */
-  protoExtend: function(sub) {
-    var Super = this;
-    var Constructor = hasOwn.call(sub, 'constructor')
+  protoExtend(sub) {
+    const Super = this;
+    const Constructor = hasOwn.call(sub, 'constructor')
       ? sub.constructor
-      : function() {
-          Super.apply(this, arguments);
+      : function(...args) {
+          Super.apply(this, args);
         };
 
     // This initialization logic is somewhat sensitive to be compatible with
@@ -180,7 +170,7 @@ var utils = (module.exports = {
   /**
    * Secure compare, from https://github.com/freewil/scmp
    */
-  secureCompare: function(a, b) {
+  secureCompare: (a, b) => {
     a = Buffer.from(a);
     b = Buffer.from(b);
 
@@ -196,10 +186,10 @@ var utils = (module.exports = {
       return crypto.timingSafeEqual(a, b);
     }
 
-    var len = a.length;
-    var result = 0;
+    const len = a.length;
+    let result = 0;
 
-    for (var i = 0; i < len; ++i) {
+    for (let i = 0; i < len; ++i) {
       result |= a[i] ^ b[i];
     }
     return result === 0;
@@ -208,12 +198,12 @@ var utils = (module.exports = {
   /**
    * Remove empty values from an object
    */
-  removeEmpty: function(obj) {
+  removeEmpty: (obj) => {
     if (typeof obj !== 'object') {
       throw new Error('Argument must be an object');
     }
 
-    Object.keys(obj).forEach(function(key) {
+    Object.keys(obj).forEach((key) => {
       if (obj[key] === null || obj[key] === undefined) {
         delete obj[key];
       }
@@ -226,24 +216,24 @@ var utils = (module.exports = {
    * Determine if file data is a derivative of EventEmitter class.
    * https://nodejs.org/api/events.html#events_events
    */
-  checkForStream: function(obj) {
+  checkForStream: (obj) => {
     if (obj.file && obj.file.data) {
       return obj.file.data instanceof EventEmitter;
     }
     return false;
   },
 
-  callbackifyPromiseWithTimeout: function(promise, callback) {
+  callbackifyPromiseWithTimeout: (promise, callback) => {
     if (callback) {
       // Ensure callback is called outside of promise stack.
       return promise.then(
-        function(res) {
-          setTimeout(function() {
+        (res) => {
+          setTimeout(() => {
             callback(null, res);
           }, 0);
         },
-        function(err) {
-          setTimeout(function() {
+        (err) => {
+          setTimeout(() => {
             callback(err, null);
           }, 0);
         }
@@ -256,7 +246,7 @@ var utils = (module.exports = {
   /**
    * Allow for special capitalization cases (such as OAuth)
    */
-  pascalToCamelCase: function(name) {
+  pascalToCamelCase: (name) => {
     if (name === 'OAuth') {
       return 'oauth';
     } else {
@@ -264,7 +254,7 @@ var utils = (module.exports = {
     }
   },
 
-  emitWarning: emitWarning,
+  emitWarning,
 
   /**
    * Node's built in `exec` function sometimes throws outright,
@@ -273,7 +263,7 @@ var utils = (module.exports = {
    *
    * This unifies that interface.
    */
-  safeExec: function safeExec(cmd, cb) {
+  safeExec: (cmd, cb) => {
     try {
       utils._exec(cmd, cb);
     } catch (e) {
@@ -284,20 +274,20 @@ var utils = (module.exports = {
   // For mocking in tests.
   _exec: exec,
 
-  isObject: function isObject(obj) {
-    var type = typeof obj;
+  isObject: (obj) => {
+    const type = typeof obj;
     return (type === 'function' || type === 'object') && !!obj;
   },
 
   // For use in multipart requests
-  flattenAndStringify: function flattenAndStringify(data) {
-    var result = {};
+  flattenAndStringify: (data) => {
+    const result = {};
 
-    function step(obj, prevKey) {
-      Object.keys(obj).forEach(function(key) {
-        var value = obj[key];
+    const step = (obj, prevKey) => {
+      Object.keys(obj).forEach((key) => {
+        const value = obj[key];
 
-        var newKey = prevKey ? `${prevKey}[${key}]` : key;
+        const newKey = prevKey ? `${prevKey}[${key}]` : key;
 
         if (utils.isObject(value)) {
           if (!Buffer.isBuffer(value) && !value.hasOwnProperty('data')) {
@@ -312,7 +302,7 @@ var utils = (module.exports = {
           result[newKey] = String(value);
         }
       });
-    }
+    };
 
     step(data);
 
@@ -323,7 +313,7 @@ var utils = (module.exports = {
 function emitWarning(warning) {
   if (typeof process.emitWarning !== 'function') {
     return console.warn(
-      'Stripe: ' + warning
+      `Stripe: ${warning}`
     ); /* eslint-disable-line no-console */
   }
 
diff --git a/package.json b/package.json
index b13849700c..b9dbceb2c1 100644
--- a/package.json
+++ b/package.json
@@ -22,7 +22,7 @@
   },
   "bugs:": "https://github.com/stripe/stripe-node/issues",
   "engines": {
-    "node": "6 || >=8"
+    "node": "6 || >=8.1"
   },
   "main": "lib/stripe.js",
   "devDependencies": {
@@ -50,10 +50,11 @@
   "scripts": {
     "clean": "rm -rf ./.nyc_output ./node_modules/.cache ./coverage",
     "mocha": "nyc mocha",
+    "mocha-only": "mocha",
     "test": "npm run lint && npm run mocha",
     "lint": "eslint --ext .js,.jsx .",
     "fix": "yarn lint --fix",
     "report": "nyc -r text -r lcov report",
     "coveralls": "cat coverage/lcov.info | ./node_modules/coveralls/bin/coveralls.js"
   }
-}
+}
\ No newline at end of file
diff --git a/test/Error.spec.js b/test/Error.spec.js
index 54c3bc5703..91b7ad9bc6 100644
--- a/test/Error.spec.js
+++ b/test/Error.spec.js
@@ -2,19 +2,19 @@
 
 require('../testUtils');
 
-var Error = require('../lib/Error');
-var expect = require('chai').expect;
+const Error = require('../lib/Error');
+const expect = require('chai').expect;
 
-describe('Error', function() {
-  it('Populates with type and message params', function() {
-    var e = new Error('FooError', 'Foo happened');
+describe('Error', () => {
+  it('Populates with type and message params', () => {
+    const e = new Error('FooError', 'Foo happened');
     expect(e).to.have.property('type', 'FooError');
     expect(e).to.have.property('message', 'Foo happened');
     expect(e).to.have.property('stack');
   });
 
-  describe('StripeError', function() {
-    it('Generates specific instance depending on error-type', function() {
+  describe('StripeError', () => {
+    it('Generates specific instance depending on error-type', () => {
       expect(Error.StripeError.generate({type: 'card_error'})).to.be.instanceOf(
         Error.StripeCardError
       );
@@ -29,25 +29,25 @@ describe('Error', function() {
       ).to.be.instanceOf(Error.StripeIdempotencyError);
     });
 
-    it('Pulls in headers', function() {
-      var headers = {'Request-Id': '123'};
-      var e = Error.StripeError.generate({
+    it('Pulls in headers', () => {
+      const headers = {'Request-Id': '123'};
+      const e = Error.StripeError.generate({
         type: 'card_error',
-        headers: headers,
+        headers,
       });
       expect(e).to.have.property('headers', headers);
     });
 
-    it('Pulls in request IDs', function() {
-      var e = Error.StripeError.generate({
+    it('Pulls in request IDs', () => {
+      const e = Error.StripeError.generate({
         type: 'card_error',
         requestId: 'foo',
       });
       expect(e).to.have.property('requestId', 'foo');
     });
 
-    it('Pulls in HTTP status code', function() {
-      var e = Error.StripeError.generate({
+    it('Pulls in HTTP status code', () => {
+      const e = Error.StripeError.generate({
         type: 'card_error',
         statusCode: 400,
       });
diff --git a/test/StripeResource.spec.js b/test/StripeResource.spec.js
index 9b30bbb779..9290bcc304 100644
--- a/test/StripeResource.spec.js
+++ b/test/StripeResource.spec.js
@@ -1,56 +1,56 @@
 'use strict';
 
-var utils = require('../testUtils');
-var uuid = require('uuid/v4');
+const utils = require('../testUtils');
+const uuid = require('uuid/v4');
 
-var nock = require('nock');
+const nock = require('nock');
 
-var stripe = require('../testUtils').getSpyableStripe();
-var expect = require('chai').expect;
+const stripe = require('../testUtils').getSpyableStripe();
+const expect = require('chai').expect;
 
-describe('StripeResource', function() {
-  describe('createResourcePathWithSymbols', function() {
-    it('Generates a path', function() {
+describe('StripeResource', () => {
+  describe('createResourcePathWithSymbols', () => {
+    it('Generates a path', () => {
       stripe.invoices.create({});
-      var path = stripe.invoices.createResourcePathWithSymbols('{id}');
+      const path = stripe.invoices.createResourcePathWithSymbols('{id}');
       expect(path).to.equal('/invoices/{id}');
     });
   });
 
-  describe('_defaultHeaders', function() {
-    it('sets the Authorization header with Bearer auth using the global API key', function() {
-      var headers = stripe.invoices._defaultHeaders(null, 0, null);
+  describe('_defaultHeaders', () => {
+    it('sets the Authorization header with Bearer auth using the global API key', () => {
+      const headers = stripe.invoices._defaultHeaders(null, 0, null);
       expect(headers.Authorization).to.equal('Bearer fakeAuthToken');
     });
-    it('sets the Authorization header with Bearer auth using the specified API key', function() {
-      var headers = stripe.invoices._defaultHeaders(
+    it('sets the Authorization header with Bearer auth using the specified API key', () => {
+      const headers = stripe.invoices._defaultHeaders(
         'anotherFakeAuthToken',
         0,
         null
       );
       expect(headers.Authorization).to.equal('Bearer anotherFakeAuthToken');
     });
-    it('sets the Stripe-Version header if an API version is provided', function() {
-      var headers = stripe.invoices._defaultHeaders(null, 0, '1970-01-01');
+    it('sets the Stripe-Version header if an API version is provided', () => {
+      const headers = stripe.invoices._defaultHeaders(null, 0, '1970-01-01');
       expect(headers['Stripe-Version']).to.equal('1970-01-01');
     });
-    it('does not the set the Stripe-Version header if no API version is provided', function() {
-      var headers = stripe.invoices._defaultHeaders(null, 0, null);
+    it('does not the set the Stripe-Version header if no API version is provided', () => {
+      const headers = stripe.invoices._defaultHeaders(null, 0, null);
       expect(headers).to.not.include.keys('Stripe-Version');
     });
   });
 
-  describe('Parameter encoding', function() {
+  describe('Parameter encoding', () => {
     // Use a real instance of stripe as we're mocking the http.request responses.
-    var realStripe = require('../lib/stripe')(utils.getUserStripeKey());
+    const realStripe = require('../lib/stripe')(utils.getUserStripeKey());
 
-    after(function() {
+    after(() => {
       nock.cleanAll();
     });
 
-    describe('_request', function() {
-      it('encodes the query string in GET requests', function(done) {
-        var options = {
+    describe('_request', () => {
+      it('encodes the query string in GET requests', (done) => {
+        const options = {
           host: stripe.getConstant('DEFAULT_HOST'),
           path: '/v1/invoices/upcoming',
           data: {
@@ -61,22 +61,23 @@ describe('StripeResource', function() {
           },
         };
 
-        const scope = nock('https://' + options.host)
+        const scope = nock(`https://${options.host}`)
           .get(options.path)
           .query(Object.assign({customer: 'cus_123'}, options.data))
           .reply(200, '{}');
 
-        realStripe.invoices.retrieveUpcoming('cus_123', options.data, function(
-          err,
-          response
-        ) {
-          done();
-          scope.done();
-        });
+        realStripe.invoices.retrieveUpcoming(
+          'cus_123',
+          options.data,
+          (err, response) => {
+            done();
+            scope.done();
+          }
+        );
       });
 
-      it('encodes the body in POST requests', function(done) {
-        var options = {
+      it('encodes the body in POST requests', (done) => {
+        const options = {
           host: stripe.getConstant('DEFAULT_HOST'),
           path: '/v1/subscriptions/sub_123',
           data: {
@@ -87,31 +88,30 @@ describe('StripeResource', function() {
             'customer=cus_123&items[0][plan]=foo&items[0][quantity]=2&items[1][id]=si_123&items[1][deleted]=true',
         };
 
-        const scope = nock('https://' + options.host)
+        const scope = nock(`https://${options.host}`)
           .post(options.path, options.body)
           .reply(200, '{}');
 
-        realStripe.subscriptions.update('sub_123', options.data, function(
-          err,
-          response
-        ) {
-          done();
-          scope.done();
-        });
+        realStripe.subscriptions.update(
+          'sub_123',
+          options.data,
+          (err, response) => {
+            done();
+            scope.done();
+          }
+        );
       });
     });
   });
 
-  describe('Retry Network Requests', function() {
+  describe('Retry Network Requests', () => {
     // Use a real instance of stripe as we're mocking the http.request responses.
-    var realStripe = require('../lib/stripe')(utils.getUserStripeKey());
+    const realStripe = require('../lib/stripe')(utils.getUserStripeKey());
 
     // Override the sleep timer to speed up tests
-    realStripe.charges._getSleepTimeInMS = function() {
-      return 0;
-    };
+    realStripe.charges._getSleepTimeInMS = () => 0;
 
-    var options = {
+    const options = {
       host: stripe.getConstant('DEFAULT_HOST'),
       path: '/v1/charges',
       data: {
@@ -123,30 +123,30 @@ describe('StripeResource', function() {
       params: 'amount=1000&currency=usd&source=tok_visa&description=test',
     };
 
-    afterEach(function() {
+    afterEach(() => {
       realStripe.setMaxNetworkRetries(0);
       stripe.setMaxNetworkRetries(0);
     });
 
-    after(function() {
+    after(() => {
       nock.cleanAll();
     });
 
-    describe('_request', function() {
-      it('throws an error on connection failure', function(done) {
+    describe('_request', () => {
+      it('throws an error on connection failure', (done) => {
         // Mock the connection error.
-        nock('https://' + options.host)
+        nock(`https://${options.host}`)
           .post(options.path, options.params)
           .replyWithError('bad stuff');
 
-        realStripe.charges.create(options.data, function(err) {
+        realStripe.charges.create(options.data, (err) => {
           expect(err.detail.message).to.deep.equal('bad stuff');
           done();
         });
       });
 
-      it('should retry the request if max retries are set', function(done) {
-        nock('https://' + options.host)
+      it('should retry the request if max retries are set', (done) => {
+        nock(`https://${options.host}`)
           .post(options.path, options.params)
           .replyWithError('bad stuff')
           .post(options.path, options.params)
@@ -154,8 +154,8 @@ describe('StripeResource', function() {
 
         realStripe.setMaxNetworkRetries(1);
 
-        realStripe.charges.create(options.data, function(err) {
-          var errorMessage = realStripe.invoices._generateConnectionErrorMessage(
+        realStripe.charges.create(options.data, (err) => {
+          const errorMessage = realStripe.invoices._generateConnectionErrorMessage(
             1
           );
           expect(err.message).to.equal(errorMessage);
@@ -164,8 +164,8 @@ describe('StripeResource', function() {
         });
       });
 
-      it('should stop retrying after a successful retry', function(done) {
-        nock('https://' + options.host)
+      it('should stop retrying after a successful retry', (done) => {
+        nock(`https://${options.host}`)
           .post(options.path, options.params)
           .replyWithError('bad stuff')
           .post(options.path, options.params)
@@ -177,14 +177,14 @@ describe('StripeResource', function() {
 
         realStripe.setMaxNetworkRetries(2);
 
-        realStripe.charges.create(options.data, function(err, charge) {
+        realStripe.charges.create(options.data, (err, charge) => {
           expect(charge.id).to.equal('ch_123');
           done();
         });
       });
 
-      it('should retry on a 409 error', function(done) {
-        nock('https://' + options.host)
+      it('should retry on a 409 error', (done) => {
+        nock(`https://${options.host}`)
           .post(options.path, options.params)
           .reply(409, {
             error: {
@@ -200,14 +200,14 @@ describe('StripeResource', function() {
 
         realStripe.setMaxNetworkRetries(1);
 
-        realStripe.charges.create(options.data, function(err, charge) {
+        realStripe.charges.create(options.data, (err, charge) => {
           expect(charge.id).to.equal('ch_123');
           done();
         });
       });
 
-      it('should not retry on a 400 error', function(done) {
-        nock('https://' + options.host)
+      it('should not retry on a 400 error', (done) => {
+        nock(`https://${options.host}`)
           .post(options.path, options.params)
           .reply(400, {
             error: {
@@ -217,14 +217,14 @@ describe('StripeResource', function() {
 
         realStripe.setMaxNetworkRetries(1);
 
-        realStripe.charges.create(options.data, function(err) {
+        realStripe.charges.create(options.data, (err) => {
           expect(err.type).to.equal('StripeCardError');
           done();
         });
       });
 
-      it('should not retry on a 500 error when the method is POST', function(done) {
-        nock('https://' + options.host)
+      it('should not retry on a 500 error when the method is POST', (done) => {
+        nock(`https://${options.host}`)
           .post(options.path, options.params)
           .reply(500, {
             error: {
@@ -234,13 +234,13 @@ describe('StripeResource', function() {
 
         realStripe.setMaxNetworkRetries(1);
 
-        realStripe.charges.create(options.data, function(err) {
+        realStripe.charges.create(options.data, (err) => {
           expect(err.type).to.equal('StripeAPIError');
           done();
         });
       });
 
-      it('should handle OAuth errors gracefully', function(done) {
+      it('should handle OAuth errors gracefully', (done) => {
         nock('https://connect.stripe.com')
           .post('/oauth/token')
           .reply(400, {
@@ -251,14 +251,14 @@ describe('StripeResource', function() {
 
         realStripe.setMaxNetworkRetries(1);
 
-        realStripe.oauth.token(options.data, function(err) {
+        realStripe.oauth.token(options.data, (err) => {
           expect(err.type).to.equal('StripeInvalidGrantError');
           done();
         });
       });
 
-      it('should retry on a 503 error when the method is POST', function(done) {
-        nock('https://' + options.host)
+      it('should retry on a 503 error when the method is POST', (done) => {
+        nock(`https://${options.host}`)
           .post(options.path, options.params)
           .reply(503, {
             error: {
@@ -274,21 +274,21 @@ describe('StripeResource', function() {
 
         realStripe.setMaxNetworkRetries(1);
 
-        realStripe.charges.create(options.data, function(err, charge) {
+        realStripe.charges.create(options.data, (err, charge) => {
           expect(charge.id).to.equal('ch_123');
           done();
         });
       });
 
-      it('should retry on a 500 error when the method is GET', function(done) {
-        nock('https://' + options.host)
-          .get(options.path + '/ch_123')
+      it('should retry on a 500 error when the method is GET', (done) => {
+        nock(`https://${options.host}`)
+          .get(`${options.path}/ch_123`)
           .reply(500, {
             error: {
               type: 'api_error',
             },
           })
-          .get(options.path + '/ch_123')
+          .get(`${options.path}/ch_123`)
           .reply(200, {
             id: 'ch_123',
             object: 'charge',
@@ -297,17 +297,17 @@ describe('StripeResource', function() {
 
         realStripe.setMaxNetworkRetries(1);
 
-        realStripe.charges.retrieve('ch_123', function(err, charge) {
+        realStripe.charges.retrieve('ch_123', (err, charge) => {
           expect(charge.id).to.equal('ch_123');
           done();
         });
       });
 
-      it('should add an idempotency key for retries using the POST method', function(done) {
-        var headers;
+      it('should add an idempotency key for retries using the POST method', (done) => {
+        let headers;
 
         // Fail the first request but succeed on the 2nd.
-        nock('https://' + options.host)
+        nock(`https://${options.host}`)
           .post(options.path, options.params)
           .replyWithError('bad stuff')
           .post(options.path, options.params)
@@ -326,19 +326,19 @@ describe('StripeResource', function() {
 
         realStripe.setMaxNetworkRetries(1);
 
-        realStripe.charges.create(options.data, function() {
+        realStripe.charges.create(options.data, () => {
           expect(headers).to.have.property('idempotency-key');
           done();
         });
       });
 
-      it('should not add idempotency key for retries using the GET method', function(done) {
-        var headers;
+      it('should not add idempotency key for retries using the GET method', (done) => {
+        let headers;
 
-        nock('https://' + options.host)
-          .get(options.path + '/ch_123')
+        nock(`https://${options.host}`)
+          .get(`${options.path}/ch_123`)
           .replyWithError('bad stuff')
-          .get(options.path + '/ch_123')
+          .get(`${options.path}/ch_123`)
           .reply(function(uri, requestBody, cb) {
             headers = this.req.headers;
 
@@ -354,17 +354,17 @@ describe('StripeResource', function() {
 
         realStripe.setMaxNetworkRetries(1);
 
-        realStripe.charges.retrieve('ch_123', function() {
+        realStripe.charges.retrieve('ch_123', () => {
           expect(headers).to.not.have.property('idempotency-key');
           done();
         });
       });
 
-      it('should reuse the given idempotency key provided for retries', function(done) {
-        var key = uuid();
-        var headers;
+      it('should reuse the given idempotency key provided for retries', (done) => {
+        const key = uuid();
+        let headers;
 
-        nock('https://' + options.host)
+        nock(`https://${options.host}`)
           .post(options.path, options.params)
           .replyWithError('bad stuff')
           .post(options.path, options.params)
@@ -383,21 +383,17 @@ describe('StripeResource', function() {
 
         realStripe.setMaxNetworkRetries(1);
 
-        realStripe.charges.create(
-          options.data,
-          {idempotency_key: key},
-          function() {
-            expect(headers['idempotency-key']).to.equal(key);
-            done();
-          }
-        );
+        realStripe.charges.create(options.data, {idempotency_key: key}, () => {
+          expect(headers['idempotency-key']).to.equal(key);
+          done();
+        });
       });
     });
 
-    describe('_shouldRetry', function() {
-      it("should return false if we've reached maximum retries", function() {
+    describe('_shouldRetry', () => {
+      it("should return false if we've reached maximum retries", () => {
         stripe.setMaxNetworkRetries(1);
-        var res = stripe.invoices._shouldRetry(
+        const res = stripe.invoices._shouldRetry(
           {
             statusCode: 409,
           },
@@ -407,9 +403,9 @@ describe('StripeResource', function() {
         expect(res).to.equal(false);
       });
 
-      it('should return true if we have more retries available', function() {
+      it('should return true if we have more retries available', () => {
         stripe.setMaxNetworkRetries(1);
-        var res = stripe.invoices._shouldRetry(
+        const res = stripe.invoices._shouldRetry(
           {
             statusCode: 409,
           },
@@ -419,9 +415,9 @@ describe('StripeResource', function() {
         expect(res).to.equal(true);
       });
 
-      it('should return true if the error code is either 409 or 503', function() {
+      it('should return true if the error code is either 409 or 503', () => {
         stripe.setMaxNetworkRetries(1);
-        var res = stripe.invoices._shouldRetry(
+        let res = stripe.invoices._shouldRetry(
           {
             statusCode: 409,
           },
@@ -440,11 +436,11 @@ describe('StripeResource', function() {
         expect(res).to.equal(true);
       });
 
-      it('should return false if the status is 200', function() {
+      it('should return false if the status is 200', () => {
         stripe.setMaxNetworkRetries(2);
 
         // mocking that we're on our 2nd request
-        var res = stripe.invoices._shouldRetry(
+        const res = stripe.invoices._shouldRetry(
           {
             statusCode: 200,
             req: {_requestEvent: {method: 'POST'}},
@@ -456,13 +452,13 @@ describe('StripeResource', function() {
       });
     });
 
-    describe('_getSleepTimeInMS', function() {
-      it('should not exceed the maximum or minimum values', function() {
-        var sleepSeconds;
-        var max = stripe.getMaxNetworkRetryDelay();
-        var min = stripe.getInitialNetworkRetryDelay();
+    describe('_getSleepTimeInMS', () => {
+      it('should not exceed the maximum or minimum values', () => {
+        let sleepSeconds;
+        const max = stripe.getMaxNetworkRetryDelay();
+        const min = stripe.getInitialNetworkRetryDelay();
 
-        for (var i = 0; i < 10; i++) {
+        for (let i = 0; i < 10; i++) {
           sleepSeconds = stripe.invoices._getSleepTimeInMS(i) / 1000;
 
           expect(sleepSeconds).to.be.at.most(max);
diff --git a/test/Webhook.spec.js b/test/Webhook.spec.js
index 2ba87061a9..23344013c0 100644
--- a/test/Webhook.spec.js
+++ b/test/Webhook.spec.js
@@ -1,25 +1,25 @@
 'use strict';
 
-var stripe = require('../testUtils').getSpyableStripe();
-var expect = require('chai').expect;
-var Buffer = require('safe-buffer').Buffer;
+const stripe = require('../testUtils').getSpyableStripe();
+const expect = require('chai').expect;
+const Buffer = require('safe-buffer').Buffer;
 
-var EVENT_PAYLOAD = {
+const EVENT_PAYLOAD = {
   id: 'evt_test_webhook',
   object: 'event',
 };
-var EVENT_PAYLOAD_STRING = JSON.stringify(EVENT_PAYLOAD, null, 2);
+const EVENT_PAYLOAD_STRING = JSON.stringify(EVENT_PAYLOAD, null, 2);
 
-var SECRET = 'whsec_test_secret';
+const SECRET = 'whsec_test_secret';
 
-describe('Webhooks', function() {
-  describe('.constructEvent', function() {
-    it('should return an Event instance from a valid JSON payload and valid signature header', function() {
-      var header = generateHeaderString({
+describe('Webhooks', () => {
+  describe('.constructEvent', () => {
+    it('should return an Event instance from a valid JSON payload and valid signature header', () => {
+      const header = generateHeaderString({
         payload: EVENT_PAYLOAD_STRING,
       });
 
-      var event = stripe.webhooks.constructEvent(
+      const event = stripe.webhooks.constructEvent(
         EVENT_PAYLOAD_STRING,
         header,
         SECRET
@@ -28,11 +28,11 @@ describe('Webhooks', function() {
       expect(event.id).to.equal(EVENT_PAYLOAD.id);
     });
 
-    it('should raise a JSON error from invalid JSON payload', function() {
-      var header = generateHeaderString({
+    it('should raise a JSON error from invalid JSON payload', () => {
+      const header = generateHeaderString({
         payload: '} I am not valid JSON; 123][',
       });
-      expect(function() {
+      expect(() => {
         stripe.webhooks.constructEvent(
           '} I am not valid JSON; 123][',
           header,
@@ -41,22 +41,22 @@ describe('Webhooks', function() {
       }).to.throw(/Unexpected token/);
     });
 
-    it('should raise a SignatureVerificationError from a valid JSON payload and an invalid signature header', function() {
-      var header = 'bad_header';
+    it('should raise a SignatureVerificationError from a valid JSON payload and an invalid signature header', () => {
+      const header = 'bad_header';
 
-      expect(function() {
+      expect(() => {
         stripe.webhooks.constructEvent(EVENT_PAYLOAD_STRING, header, SECRET);
       }).to.throw(/Unable to extract timestamp and signatures from header/);
     });
   });
 
-  describe('.verifySignatureHeader', function() {
-    it('should raise a SignatureVerificationError when the header does not have the expected format', function() {
-      var header = "I'm not even a real signature header";
+  describe('.verifySignatureHeader', () => {
+    it('should raise a SignatureVerificationError when the header does not have the expected format', () => {
+      const header = "I'm not even a real signature header";
 
-      var expectedMessage = /Unable to extract timestamp and signatures from header/;
+      const expectedMessage = /Unable to extract timestamp and signatures from header/;
 
-      expect(function() {
+      expect(() => {
         stripe.webhooks.signature.verifyHeader(
           EVENT_PAYLOAD_STRING,
           header,
@@ -64,7 +64,7 @@ describe('Webhooks', function() {
         );
       }).to.throw(expectedMessage);
 
-      expect(function() {
+      expect(() => {
         stripe.webhooks.signature.verifyHeader(
           EVENT_PAYLOAD_STRING,
           null,
@@ -72,7 +72,7 @@ describe('Webhooks', function() {
         );
       }).to.throw(expectedMessage);
 
-      expect(function() {
+      expect(() => {
         stripe.webhooks.signature.verifyHeader(
           EVENT_PAYLOAD_STRING,
           undefined,
@@ -80,7 +80,7 @@ describe('Webhooks', function() {
         );
       }).to.throw(expectedMessage);
 
-      expect(function() {
+      expect(() => {
         stripe.webhooks.signature.verifyHeader(
           EVENT_PAYLOAD_STRING,
           '',
@@ -89,12 +89,12 @@ describe('Webhooks', function() {
       }).to.throw(expectedMessage);
     });
 
-    it('should raise a SignatureVerificationError when there are no signatures with the expected scheme', function() {
-      var header = generateHeaderString({
+    it('should raise a SignatureVerificationError when there are no signatures with the expected scheme', () => {
+      const header = generateHeaderString({
         scheme: 'v0',
       });
 
-      expect(function() {
+      expect(() => {
         stripe.webhooks.signature.verifyHeader(
           EVENT_PAYLOAD_STRING,
           header,
@@ -103,12 +103,12 @@ describe('Webhooks', function() {
       }).to.throw(/No signatures found with expected scheme/);
     });
 
-    it('should raise a SignatureVerificationError when there are no valid signatures for the payload', function() {
-      var header = generateHeaderString({
+    it('should raise a SignatureVerificationError when there are no valid signatures for the payload', () => {
+      const header = generateHeaderString({
         signature: 'bad_signature',
       });
 
-      expect(function() {
+      expect(() => {
         stripe.webhooks.signature.verifyHeader(
           EVENT_PAYLOAD_STRING,
           header,
@@ -119,12 +119,12 @@ describe('Webhooks', function() {
       );
     });
 
-    it('should raise a SignatureVerificationError when the timestamp is not within the tolerance', function() {
-      var header = generateHeaderString({
+    it('should raise a SignatureVerificationError when the timestamp is not within the tolerance', () => {
+      const header = generateHeaderString({
         timestamp: Date.now() / 1000 - 15,
       });
 
-      expect(function() {
+      expect(() => {
         stripe.webhooks.signature.verifyHeader(
           EVENT_PAYLOAD_STRING,
           header,
@@ -137,8 +137,8 @@ describe('Webhooks', function() {
     it(
       'should return true when the header contains a valid signature and ' +
         'the timestamp is within the tolerance',
-      function() {
-        var header = generateHeaderString({
+      () => {
+        const header = generateHeaderString({
           timestamp: Date.now() / 1000,
         });
 
@@ -153,8 +153,8 @@ describe('Webhooks', function() {
       }
     );
 
-    it('should return true when the header contains at least one valid signature', function() {
-      var header = generateHeaderString({
+    it('should return true when the header contains at least one valid signature', () => {
+      let header = generateHeaderString({
         timestamp: Date.now() / 1000,
       });
 
@@ -173,8 +173,8 @@ describe('Webhooks', function() {
     it(
       'should return true when the header contains a valid signature ' +
         'and the timestamp is off but no tolerance is provided',
-      function() {
-        var header = generateHeaderString({
+      () => {
+        const header = generateHeaderString({
           timestamp: 12345,
         });
 
@@ -188,8 +188,8 @@ describe('Webhooks', function() {
       }
     );
 
-    it('should accept Buffer instances for the payload and header', function() {
-      var header = generateHeaderString({
+    it('should accept Buffer instances for the payload and header', () => {
+      const header = generateHeaderString({
         timestamp: Date.now() / 1000,
       });
 
@@ -216,13 +216,13 @@ function generateHeaderString(opts) {
   opts.signature =
     opts.signature ||
     stripe.webhooks.signature._computeSignature(
-      opts.timestamp + '.' + opts.payload,
+      `${opts.timestamp}.${opts.payload}`,
       opts.secret
     );
 
-  var generatedHeader = [
-    't=' + opts.timestamp,
-    opts.scheme + '=' + opts.signature,
+  const generatedHeader = [
+    `t=${opts.timestamp}`,
+    `${opts.scheme}=${opts.signature}`,
   ].join(',');
 
   return generatedHeader;
diff --git a/test/autoPagination.spec.js b/test/autoPagination.spec.js
index 8aa204b129..3cc3d9e35c 100644
--- a/test/autoPagination.spec.js
+++ b/test/autoPagination.spec.js
@@ -2,55 +2,51 @@
 
 /* eslint-disable callback-return */
 
-var testUtils = require('../testUtils');
-var stripe = require('../lib/stripe')(testUtils.getUserStripeKey(), 'latest');
+const testUtils = require('../testUtils');
+const stripe = require('../lib/stripe')(testUtils.getUserStripeKey(), 'latest');
 
-var expect = require('chai').expect;
+const expect = require('chai').expect;
 
-var LIMIT = 7;
-var TOTAL_OBJECTS = 8;
+const LIMIT = 7;
+const TOTAL_OBJECTS = 8;
 
 describe('auto pagination', function() {
   this.timeout(20000);
 
-  var email = 'test.' + Date.now() + '@example.com';
-  var realCustomerIds;
-  before(function() {
-    return new Promise(function(resolve) {
-      var createReqs = [];
-      for (var i = 0; i < TOTAL_OBJECTS; i++) {
-        createReqs.push(stripe.customers.create({email: email}));
-      }
-      return Promise.all(createReqs)
-        .then(function() {
-          // re-fetch to ensure correct order
-          return stripe.customers
-            .list({email: email})
-            .then(function(customers) {
-              realCustomerIds = customers.data.map(function(customer) {
-                return customer.id;
-              });
-            });
-        })
-        .then(resolve);
-    });
-  });
-
-  after(function() {
-    return new Promise(function(resolve) {
-      Promise.all(
-        realCustomerIds.map(function(customerId) {
-          return stripe.customers.del(customerId);
-        })
-      ).then(resolve);
-    });
-  });
-
-  describe('callbacks', function() {
-    it('lets you call `next()` to iterate and `next(false)` to break', function() {
-      return expect(
-        new Promise(function(resolve, reject) {
-          var customerIds = [];
+  const email = `test.${Date.now()}@example.com`;
+  let realCustomerIds;
+  before(
+    () =>
+      new Promise((resolve) => {
+        const createReqs = [];
+        for (let i = 0; i < TOTAL_OBJECTS; i++) {
+          createReqs.push(stripe.customers.create({email}));
+        }
+        return Promise.all(createReqs)
+          .then(() =>
+            // re-fetch to ensure correct order
+            stripe.customers.list({email}).then((customers) => {
+              realCustomerIds = customers.data.map((customer) => customer.id);
+            })
+          )
+          .then(resolve);
+      })
+  );
+
+  after(
+    () =>
+      new Promise((resolve) => {
+        Promise.all(
+          realCustomerIds.map((customerId) => stripe.customers.del(customerId))
+        ).then(resolve);
+      })
+  );
+
+  describe('callbacks', () => {
+    it('lets you call `next()` to iterate and `next(false)` to break', () =>
+      expect(
+        new Promise((resolve, reject) => {
+          const customerIds = [];
           function onCustomer(customer, next) {
             customerIds.push(customer.id);
             if (customerIds.length === LIMIT) {
@@ -69,16 +65,15 @@ describe('auto pagination', function() {
           }
 
           stripe.customers
-            .list({limit: 3, email: email})
+            .list({limit: 3, email})
             .autoPagingEach(onCustomer, onDone);
         })
-      ).to.eventually.deep.equal(realCustomerIds.slice(0, LIMIT));
-    });
+      ).to.eventually.deep.equal(realCustomerIds.slice(0, LIMIT)));
 
-    it('lets you ignore the second arg and `return false` to break', function() {
-      return expect(
-        new Promise(function(resolve, reject) {
-          var customerIds = [];
+    it('lets you ignore the second arg and `return false` to break', () =>
+      expect(
+        new Promise((resolve, reject) => {
+          const customerIds = [];
           function onCustomer(customer) {
             customerIds.push(customer.id);
             if (customerIds.length === LIMIT) {
@@ -96,16 +91,15 @@ describe('auto pagination', function() {
           }
 
           stripe.customers
-            .list({limit: 3, email: email})
+            .list({limit: 3, email})
             .autoPagingEach(onCustomer, onDone);
         })
-      ).to.eventually.deep.equal(realCustomerIds.slice(0, LIMIT));
-    });
+      ).to.eventually.deep.equal(realCustomerIds.slice(0, LIMIT)));
 
-    it('lets you ignore the second arg and return a Promise which returns `false` to break', function() {
-      return expect(
-        new Promise(function(resolve, reject) {
-          var customerIds = [];
+    it('lets you ignore the second arg and return a Promise which returns `false` to break', () =>
+      expect(
+        new Promise((resolve, reject) => {
+          const customerIds = [];
           function onCustomer(customer) {
             customerIds.push(customer.id);
             if (customerIds.length === LIMIT) {
@@ -124,16 +118,15 @@ describe('auto pagination', function() {
           }
 
           stripe.customers
-            .list({limit: 3, email: email})
+            .list({limit: 3, email})
             .autoPagingEach(onCustomer, onDone);
         })
-      ).to.eventually.deep.equal(realCustomerIds.slice(0, LIMIT));
-    });
+      ).to.eventually.deep.equal(realCustomerIds.slice(0, LIMIT)));
 
-    it('can use a promise instead of a callback for onDone', function() {
-      return expect(
-        new Promise(function(resolve, reject) {
-          var customerIds = [];
+    it('can use a promise instead of a callback for onDone', () =>
+      expect(
+        new Promise((resolve, reject) => {
+          const customerIds = [];
           function onCustomer(customer) {
             customerIds.push(customer.id);
           }
@@ -142,69 +135,65 @@ describe('auto pagination', function() {
           }
 
           stripe.customers
-            .list({limit: 3, email: email})
+            .list({limit: 3, email})
             .autoPagingEach(onCustomer)
             .then(onDone)
             .catch(reject);
         })
-      ).to.eventually.deep.equal(realCustomerIds);
-    });
+      ).to.eventually.deep.equal(realCustomerIds));
 
-    it('handles the end of a list properly when the last page is full', function() {
-      return expect(
-        new Promise(function(resolve, reject) {
-          var customerIds = [];
+    it('handles the end of a list properly when the last page is full', () =>
+      expect(
+        new Promise((resolve, reject) => {
+          const customerIds = [];
           return stripe.customers
-            .list({email: email, limit: TOTAL_OBJECTS / 2})
-            .autoPagingEach(function(customer) {
+            .list({email, limit: TOTAL_OBJECTS / 2})
+            .autoPagingEach((customer) => {
               customerIds.push(customer.id);
             })
             .catch(reject)
-            .then(function() {
+            .then(() => {
               resolve(customerIds);
             });
         })
-      ).to.eventually.deep.equal(realCustomerIds);
-    });
+      ).to.eventually.deep.equal(realCustomerIds));
 
-    it('handles the end of a list properly when the last page is not full', function() {
-      return expect(
-        new Promise(function(resolve, reject) {
-          var customerIds = [];
+    it('handles the end of a list properly when the last page is not full', () =>
+      expect(
+        new Promise((resolve, reject) => {
+          const customerIds = [];
           return stripe.customers
-            .list({email: email, limit: TOTAL_OBJECTS - 2})
-            .autoPagingEach(function(customer) {
+            .list({email, limit: TOTAL_OBJECTS - 2})
+            .autoPagingEach((customer) => {
               customerIds.push(customer.id);
             })
             .catch(reject)
-            .then(function() {
+            .then(() => {
               resolve(customerIds);
             });
         })
-      ).to.eventually.deep.equal(realCustomerIds);
-    });
+      ).to.eventually.deep.equal(realCustomerIds));
 
-    it('handles a list which is shorter than the page size properly', function() {
-      return expect(
-        new Promise(function(resolve, reject) {
-          var customerIds = [];
+    it('handles a list which is shorter than the page size properly', () =>
+      expect(
+        new Promise((resolve, reject) => {
+          const customerIds = [];
           return stripe.customers
-            .list({email: email, limit: TOTAL_OBJECTS + 2})
-            .autoPagingEach(function(customer) {
+            .list({email, limit: TOTAL_OBJECTS + 2})
+            .autoPagingEach((customer) => {
               customerIds.push(customer.id);
             })
             .catch(reject)
-            .then(function() {
+            .then(() => {
               resolve(customerIds);
             });
         })
-      ).to.eventually.deep.equal(realCustomerIds);
-    });
+      ).to.eventually.deep.equal(realCustomerIds));
 
-    it('handles errors after the first page correctly (callback)', function() {
-      return expect(
-        new Promise(function(resolve, reject) {
-          var i = 0;
+    it('handles errors after the first page correctly (callback)', () =>
+      expect(
+        new Promise((resolve, reject) => {
+          let i = 0;
           function onCustomer() {
             i += 1;
             if (i > 4) {
@@ -212,8 +201,8 @@ describe('auto pagination', function() {
             }
           }
           return stripe.customers
-            .list({email: email, limit: 3})
-            .autoPagingEach(onCustomer, function(err) {
+            .list({email, limit: 3})
+            .autoPagingEach(onCustomer, (err) => {
               if (err) {
                 resolve(err.message);
               } else {
@@ -221,13 +210,12 @@ describe('auto pagination', function() {
               }
             });
         })
-      ).to.eventually.deep.equal('Simulated error');
-    });
+      ).to.eventually.deep.equal('Simulated error'));
 
-    it('handles errors after the first page correctly (promise)', function() {
-      return expect(
-        new Promise(function(resolve, reject) {
-          var i = 0;
+    it('handles errors after the first page correctly (promise)', () =>
+      expect(
+        new Promise((resolve, reject) => {
+          let i = 0;
           function onCustomer() {
             i += 1;
             if (i > 4) {
@@ -235,91 +223,73 @@ describe('auto pagination', function() {
             }
           }
           return stripe.customers
-            .list({email: email, limit: 3})
+            .list({email, limit: 3})
             .autoPagingEach(onCustomer)
-            .then(function() {
+            .then(() => {
               reject(Error('Expected an error, did not get one.'));
             })
-            .catch(function(err) {
+            .catch((err) => {
               resolve(err.message);
             });
         })
-      ).to.eventually.deep.equal('Simulated error');
-    });
+      ).to.eventually.deep.equal('Simulated error'));
   });
 
-  describe('async iterators', function() {
+  describe('async iterators', () => {
     if (testUtils.envSupportsForAwait()) {
       // `for await` throws a syntax error everywhere but node 10,
       // so we must conditionally require it.
-      var forAwaitUntil = require('../testUtils/forAwait.node10').forAwaitUntil;
-
-      it('works with `for await` when that feature exists (user break)', function() {
-        return expect(
-          new Promise(function(resolve, reject) {
-            forAwaitUntil(
-              stripe.customers.list({limit: 3, email: email}),
-              LIMIT
-            )
-              .then(function(customers) {
-                resolve(
-                  customers.map(function(customer) {
-                    return customer.id;
-                  })
-                );
+      const forAwaitUntil = require('../testUtils/forAwait.node10')
+        .forAwaitUntil;
+
+      it('works with `for await` when that feature exists (user break)', () =>
+        expect(
+          new Promise((resolve, reject) => {
+            forAwaitUntil(stripe.customers.list({limit: 3, email}), LIMIT)
+              .then((customers) => {
+                resolve(customers.map((customer) => customer.id));
               })
               .catch(reject);
           })
-        ).to.eventually.deep.equal(realCustomerIds.slice(0, LIMIT));
-      });
+        ).to.eventually.deep.equal(realCustomerIds.slice(0, LIMIT)));
 
-      it('works with `for await` when that feature exists (end of list)', function() {
-        return expect(
-          new Promise(function(resolve, reject) {
+      it('works with `for await` when that feature exists (end of list)', () =>
+        expect(
+          new Promise((resolve, reject) => {
             forAwaitUntil(
-              stripe.customers.list({limit: 3, email: email}),
+              stripe.customers.list({limit: 3, email}),
               TOTAL_OBJECTS + 1
             )
-              .then(function(customers) {
-                resolve(
-                  customers.map(function(customer) {
-                    return customer.id;
-                  })
-                );
+              .then((customers) => {
+                resolve(customers.map((customer) => customer.id));
               })
               .catch(reject);
           })
-        ).to.eventually.deep.equal(realCustomerIds);
-      });
+        ).to.eventually.deep.equal(realCustomerIds));
     }
 
     if (testUtils.envSupportsAwait()) {
       // Similarly, `await` throws a syntax error below Node 7.6.
-      var awaitUntil = require('../testUtils/await.node7').awaitUntil;
-
-      it('works with `await` and a while loop when await exists', function() {
-        return expect(
-          new Promise(function(resolve, reject) {
-            awaitUntil(stripe.customers.list({limit: 3, email: email}), LIMIT)
-              .then(function(customers) {
-                resolve(
-                  customers.map(function(customer) {
-                    return customer.id;
-                  })
-                );
+      const awaitUntil = require('../testUtils/await.node7').awaitUntil;
+
+      it('works with `await` and a while loop when await exists', () =>
+        expect(
+          new Promise((resolve, reject) => {
+            awaitUntil(stripe.customers.list({limit: 3, email}), LIMIT)
+              .then((customers) => {
+                resolve(customers.map((customer) => customer.id));
               })
               .catch(reject);
           })
-        ).to.eventually.deep.equal(realCustomerIds.slice(0, LIMIT));
-      });
+        ).to.eventually.deep.equal(realCustomerIds.slice(0, LIMIT)));
     }
 
-    it('returns an empty object from .return() so that `break;` works in for-await', function() {
-      return expect(
-        new Promise(function(resolve, reject) {
-          var iter = stripe.customers.list({limit: 3, email: email});
+    it('returns an empty object from .return() so that `break;` works in for-await', () =>
+      expect(
+        new Promise((resolve, reject) => {
+          const iter = stripe.customers.list({limit: 3, email});
 
-          var customerIds = [];
+          const customerIds = [];
           function handleIter(result) {
             customerIds.push(result.value.id);
             expect(iter.return()).to.deep.equal({});
@@ -328,20 +298,19 @@ describe('auto pagination', function() {
           iter
             .next()
             .then(handleIter)
-            .then(function() {
+            .then(() => {
               resolve(customerIds);
             })
             .catch(reject);
         })
-      ).to.eventually.deep.equal(realCustomerIds.slice(0, 1));
-    });
+      ).to.eventually.deep.equal(realCustomerIds.slice(0, 1)));
 
-    it('works when you call it sequentially', function() {
-      return expect(
-        new Promise(function(resolve, reject) {
-          var iter = stripe.customers.list({limit: 3, email: email});
+    it('works when you call it sequentially', () =>
+      expect(
+        new Promise((resolve, reject) => {
+          const iter = stripe.customers.list({limit: 3, email});
 
-          var customerIds = [];
+          const customerIds = [];
           function handleIter(result) {
             customerIds.push(result.value.id);
             if (customerIds.length < 7) {
@@ -351,20 +320,19 @@ describe('auto pagination', function() {
           iter
             .next()
             .then(handleIter)
-            .then(function() {
+            .then(() => {
               resolve(customerIds);
             })
             .catch(reject);
         })
-      ).to.eventually.deep.equal(realCustomerIds.slice(0, LIMIT));
-    });
+      ).to.eventually.deep.equal(realCustomerIds.slice(0, LIMIT)));
 
-    it('gives you the same result each time when you call it multiple times in parallel', function() {
-      return expect(
-        new Promise(function(resolve, reject) {
-          var iter = stripe.customers.list({limit: 3, email: email});
+    it('gives you the same result each time when you call it multiple times in parallel', () =>
+      expect(
+        new Promise((resolve, reject) => {
+          const iter = stripe.customers.list({limit: 3, email});
 
-          var customerIds = [];
+          const customerIds = [];
           function handleIter(result) {
             customerIds.push(result.value.id);
           }
@@ -374,102 +342,86 @@ describe('auto pagination', function() {
             iter
               .next()
               .then(handleIter)
-              .then(function() {
-                return Promise.all([
+              .then(() =>
+                Promise.all([
                   iter.next().then(handleIter),
                   iter.next().then(handleIter),
-                ]);
-              })
-              .then(function() {
-                return Promise.all([
+                ])
+              )
+              .then(() =>
+                Promise.all([
                   iter.next().then(handleIter),
                   iter.next().then(handleIter),
-                ]);
-              })
-              .then(function() {
-                return Promise.all([
+                ])
+              )
+              .then(() =>
+                Promise.all([
                   iter.next().then(handleIter),
                   iter.next().then(handleIter),
-                ]);
-              }),
+                ])
+              ),
           ])
-            .then(function() {
+            .then(() => {
               resolve(customerIds);
             })
             .catch(reject);
         })
       ).to.eventually.deep.equal(
-        realCustomerIds.slice(0, 4).reduce(function(acc, x) {
+        realCustomerIds.slice(0, 4).reduce((acc, x) => {
           acc.push(x);
           acc.push(x);
           return acc;
         }, [])
-      );
-    });
+      ));
   });
 
-  describe('autoPagingToArray', function() {
-    it('can go to the end', function() {
-      return expect(
-        new Promise(function(resolve, reject) {
+  describe('autoPagingToArray', () => {
+    it('can go to the end', () =>
+      expect(
+        new Promise((resolve, reject) => {
           stripe.customers
-            .list({limit: 3, email: email})
+            .list({limit: 3, email})
             .autoPagingToArray({limit: TOTAL_OBJECTS + 1})
-            .then(function(customers) {
-              return customers.map(function(customer) {
-                return customer.id;
-              });
-            })
+            .then((customers) => customers.map((customer) => customer.id))
             .then(resolve)
             .catch(reject);
         })
-      ).to.eventually.deep.equal(realCustomerIds);
-    });
+      ).to.eventually.deep.equal(realCustomerIds));
 
-    it('returns a promise of an array', function() {
-      return expect(
-        new Promise(function(resolve, reject) {
+    it('returns a promise of an array', () =>
+      expect(
+        new Promise((resolve, reject) => {
           stripe.customers
-            .list({limit: 3, email: email})
+            .list({limit: 3, email})
             .autoPagingToArray({limit: LIMIT})
-            .then(function(customers) {
-              return customers.map(function(customer) {
-                return customer.id;
-              });
-            })
+            .then((customers) => customers.map((customer) => customer.id))
             .then(resolve)
             .catch(reject);
         })
-      ).to.eventually.deep.equal(realCustomerIds.slice(0, LIMIT));
-    });
+      ).to.eventually.deep.equal(realCustomerIds.slice(0, LIMIT)));
 
-    it('accepts an onDone callback, passing an array', function() {
-      return expect(
-        new Promise(function(resolve, reject) {
+    it('accepts an onDone callback, passing an array', () =>
+      expect(
+        new Promise((resolve, reject) => {
           function onDone(err, customers) {
             if (err) {
               reject(err);
             } else {
-              resolve(
-                customers.map(function(customer) {
-                  return customer.id;
-                })
-              );
+              resolve(customers.map((customer) => customer.id));
             }
           }
 
           stripe.customers
-            .list({limit: 3, email: email})
+            .list({limit: 3, email})
             .autoPagingToArray({limit: LIMIT}, onDone);
         })
-      ).to.eventually.deep.equal(realCustomerIds.slice(0, LIMIT));
-    });
+      ).to.eventually.deep.equal(realCustomerIds.slice(0, LIMIT)));
 
-    it('enforces a `limit` arg', function() {
-      return expect(
-        new Promise(function(resolve, reject) {
+    it('enforces a `limit` arg', () =>
+      expect(
+        new Promise((resolve, reject) => {
           try {
-            stripe.customers.list({limit: 3, email: email}).autoPagingToArray();
+            stripe.customers.list({limit: 3, email}).autoPagingToArray();
             reject(Error('Should have thrown.'));
           } catch (err) {
             resolve(err.message);
@@ -477,15 +429,14 @@ describe('auto pagination', function() {
         })
       ).to.eventually.equal(
         'You must pass a `limit` option to autoPagingToArray, eg; `autoPagingToArray({limit: 1000});`.'
-      );
-    });
+      ));
 
-    it('caps the `limit` arg to a reasonable ceiling', function() {
-      return expect(
-        new Promise(function(resolve, reject) {
+    it('caps the `limit` arg to a reasonable ceiling', () =>
+      expect(
+        new Promise((resolve, reject) => {
           try {
             stripe.customers
-              .list({limit: 3, email: email})
+              .list({limit: 3, email})
               .autoPagingToArray({limit: 1000000});
             reject(Error('Should have thrown.'));
           } catch (err) {
@@ -494,37 +445,34 @@ describe('auto pagination', function() {
         })
       ).to.eventually.equal(
         'You cannot specify a limit of more than 10,000 items to fetch in `autoPagingToArray`; use `autoPagingEach` to iterate through longer lists.'
-      );
-    });
+      ));
   });
 
-  describe('api compat edge cases', function() {
-    it('lets you listen to the first request as its own promise, and separately each item, but only sends one request for the first page.', function() {
-      return expect(
-        new Promise(function(resolve, reject) {
+  describe('api compat edge cases', () => {
+    it('lets you listen to the first request as its own promise, and separately each item, but only sends one request for the first page.', () =>
+      expect(
+        new Promise((resolve, reject) => {
           // Count requests: we want one for the first page (not two), and then one for the second page.
-          var reqCount = 0;
+          let reqCount = 0;
           function onRequest() {
             reqCount += 1;
           }
           stripe.on('request', onRequest);
 
-          var customerIds = [];
-          var p = stripe.customers.list({email: email, limit: 4});
+          const customerIds = [];
+          const p = stripe.customers.list({email, limit: 4});
           Promise.all([
             p,
-            p.autoPagingEach(function(customer) {
+            p.autoPagingEach((customer) => {
               customerIds.push(customer.id);
             }),
           ])
-            .then(function(results) {
+            .then((results) => {
               stripe.off('request', onRequest);
               expect(reqCount).to.equal(2); // not 3.
 
               resolve({
-                firstReq: results[0].data.map(function(customer) {
-                  return customer.id;
-                }),
+                firstReq: results[0].data.map((customer) => customer.id),
                 paginated: customerIds,
               });
             })
@@ -533,7 +481,6 @@ describe('auto pagination', function() {
       ).to.eventually.deep.equal({
         firstReq: realCustomerIds.slice(0, 4),
         paginated: realCustomerIds,
-      });
-    });
+      }));
   });
 });
diff --git a/test/flows.spec.js b/test/flows.spec.js
index e577bcc90c..5df1029065 100644
--- a/test/flows.spec.js
+++ b/test/flows.spec.js
@@ -1,54 +1,53 @@
 'use strict';
 
-var testUtils = require('../testUtils');
-var chai = require('chai');
-var stripe = require('../lib/stripe')(testUtils.getUserStripeKey(), 'latest');
-var fs = require('fs');
-var path = require('path');
-var stream = require('stream');
-var expect = chai.expect;
-
-var CUSTOMER_DETAILS = {
+const testUtils = require('../testUtils');
+const chai = require('chai');
+const stripe = require('../lib/stripe')(testUtils.getUserStripeKey(), 'latest');
+const fs = require('fs');
+const path = require('path');
+const stream = require('stream');
+const expect = chai.expect;
+
+const CUSTOMER_DETAILS = {
   description: 'Some customer',
   card: 'tok_visa',
 };
 
-var CURRENCY = '_DEFAULT_CURRENCY_NOT_YET_GOTTEN_';
+let CURRENCY = '_DEFAULT_CURRENCY_NOT_YET_GOTTEN_';
 
 describe('Flows', function() {
   // Note: These tests must be run as one so we can retrieve the
   // default_currency (required in subsequent tests);
 
-  var cleanup = new testUtils.CleanupUtility();
+  const cleanup = new testUtils.CleanupUtility();
   this.timeout(30000);
 
-  it('Allows me to retrieve default_currency', function() {
-    return expect(
-      stripe.account.retrieve().then(function(acct) {
+  it('Allows me to retrieve default_currency', () =>
+    expect(
+      stripe.account.retrieve().then((acct) => {
         CURRENCY = acct.default_currency;
         return acct;
       })
-    ).to.eventually.have.property('default_currency');
-  });
+    ).to.eventually.have.property('default_currency'));
 
-  describe('Plan+Subscription flow', function() {
-    it('Allows me to: Create a plan and subscribe a customer to it', function() {
-      return expect(
+  describe('Plan+Subscription flow', () => {
+    it('Allows me to: Create a plan and subscribe a customer to it', () =>
+      expect(
         Promise.all([
           stripe.plans.create({
-            id: 'plan' + testUtils.getRandomString(),
+            id: `plan${testUtils.getRandomString()}`,
             amount: 1700,
             currency: CURRENCY,
             interval: 'month',
             nickname: 'Gold Super Amazing Tier',
             product: {
-              name: 'product' + testUtils.getRandomString(),
+              name: `product${testUtils.getRandomString()}`,
             },
           }),
           stripe.customers.create(CUSTOMER_DETAILS),
-        ]).then(function(j) {
-          var plan = j[0];
-          var customer = j[1];
+        ]).then((j) => {
+          const plan = j[0];
+          const customer = j[1];
 
           cleanup.deleteCustomer(customer.id);
           cleanup.deletePlan(plan.id);
@@ -57,28 +56,27 @@ describe('Flows', function() {
             plan: plan.id,
           });
         })
-      ).to.eventually.have.property('status', 'active');
-    });
+      ).to.eventually.have.property('status', 'active'));
 
-    it('Allows me to: Create a plan and subscribe a customer to it, and update subscription (multi-subs API)', function() {
-      var plan;
+    it('Allows me to: Create a plan and subscribe a customer to it, and update subscription (multi-subs API)', () => {
+      let plan;
       return expect(
         Promise.all([
           stripe.plans.create({
-            id: 'plan' + testUtils.getRandomString(),
+            id: `plan${testUtils.getRandomString()}`,
             amount: 1700,
             currency: CURRENCY,
             interval: 'month',
             nickname: 'Gold Super Amazing Tier',
             product: {
-              name: 'product' + testUtils.getRandomString(),
+              name: `product${testUtils.getRandomString()}`,
             },
           }),
           stripe.customers.create(CUSTOMER_DETAILS),
         ])
-          .then(function(j) {
+          .then((j) => {
             plan = j[0];
-            var customer = j[1];
+            const customer = j[1];
 
             cleanup.deleteCustomer(customer.id);
             cleanup.deletePlan(plan.id);
@@ -87,61 +85,59 @@ describe('Flows', function() {
               plan: plan.id,
             });
           })
-          .then(function(subscription) {
-            return stripe.customers.updateSubscription(
+          .then((subscription) =>
+            stripe.customers.updateSubscription(
               subscription.customer,
               subscription.id,
               {
                 plan: plan.id,
                 quantity: '3',
               }
-            );
-          })
-          .then(function(subscription) {
-            return [subscription.status, subscription.quantity];
-          })
+            )
+          )
+          .then((subscription) => [subscription.status, subscription.quantity])
       ).to.eventually.deep.equal(['active', 3]);
     });
 
-    it('Errors when I attempt to subscribe a customer to a non-existent plan', function() {
-      return expect(
-        stripe.customers.create(CUSTOMER_DETAILS).then(function(customer) {
+    it('Errors when I attempt to subscribe a customer to a non-existent plan', () =>
+      expect(
+        stripe.customers.create(CUSTOMER_DETAILS).then((customer) => {
           cleanup.deleteCustomer(customer.id);
 
           return stripe.customers
             .updateSubscription(customer.id, {
-              plan: 'someNonExistentPlan' + testUtils.getRandomString(),
+              plan: `someNonExistentPlan${testUtils.getRandomString()}`,
             })
-            .then(null, function(err) {
-              // Resolve with the error so we can inspect it below
-              return err;
-            });
+            .then(
+              null,
+              (err) =>
+                // Resolve with the error so we can inspect it below
+                err
+            );
         })
-      ).to.eventually.satisfy(function(err) {
-        return (
+      ).to.eventually.satisfy(
+        (err) =>
           err.type === 'StripeInvalidRequestError' &&
           err.rawType === 'invalid_request_error'
-        );
-      });
-    });
+      ));
 
-    it('Allows me to: subscribe then update with `cancel_at_period_end` defined', function() {
-      return expect(
+    it('Allows me to: subscribe then update with `cancel_at_period_end` defined', () =>
+      expect(
         Promise.all([
           stripe.plans.create({
-            id: 'plan' + testUtils.getRandomString(),
+            id: `plan${testUtils.getRandomString()}`,
             amount: 1700,
             currency: CURRENCY,
             interval: 'month',
             nickname: 'Silver Super Amazing Tier',
             product: {
-              name: 'product' + testUtils.getRandomString(),
+              name: `product${testUtils.getRandomString()}`,
             },
           }),
           stripe.customers.create(CUSTOMER_DETAILS),
-        ]).then(function(j) {
-          var plan = j[0];
-          var customer = j[1];
+        ]).then((j) => {
+          const plan = j[0];
+          const customer = j[1];
 
           cleanup.deleteCustomer(customer.id);
           cleanup.deletePlan(plan.id);
@@ -151,235 +147,186 @@ describe('Flows', function() {
             cancel_at_period_end: true,
           });
         })
-      ).to.eventually.have.property('cancel_at_period_end', true);
-    });
+      ).to.eventually.have.property('cancel_at_period_end', true));
 
-    describe('Plan name variations', function() {
+    describe('Plan name variations', () => {
       [
-        '34535_355453' + testUtils.getRandomString(),
-        'TEST_239291' + testUtils.getRandomString(),
-        'TEST_a-i' + testUtils.getRandomString(),
-        'foobarbazteston___etwothree' + testUtils.getRandomString(),
-      ].forEach(function(planID) {
-        it(
-          'Allows me to create and retrieve plan with ID: ' + planID,
-          function() {
-            return expect(
-              stripe.plans
-                .create({
-                  id: planID,
-                  amount: 1700,
-                  currency: CURRENCY,
-                  interval: 'month',
-                  nickname: 'generic',
-                  product: {
-                    name: 'product' + testUtils.getRandomString(),
-                  },
-                })
-                .then(function() {
-                  cleanup.deletePlan(planID);
-                  return stripe.plans.retrieve(planID);
-                })
-            ).to.eventually.have.property('id', planID);
-          }
-        );
+        `34535_355453${testUtils.getRandomString()}`,
+        `TEST_239291${testUtils.getRandomString()}`,
+        `TEST_a-i${testUtils.getRandomString()}`,
+        `foobarbazteston___etwothree${testUtils.getRandomString()}`,
+      ].forEach((planID) => {
+        it(`Allows me to create and retrieve plan with ID: ${planID}`, () =>
+          expect(
+            stripe.plans
+              .create({
+                id: planID,
+                amount: 1700,
+                currency: CURRENCY,
+                interval: 'month',
+                nickname: 'generic',
+                product: {
+                  name: `product${testUtils.getRandomString()}`,
+                },
+              })
+              .then(() => {
+                cleanup.deletePlan(planID);
+                return stripe.plans.retrieve(planID);
+              })
+          ).to.eventually.have.property('id', planID));
       });
     });
   });
 
-  describe('Coupon flow', function() {
-    var customer;
-    var coupon;
+  describe('Coupon flow', () => {
+    let customer;
+    let coupon;
 
-    describe('When I create a coupon & customer', function() {
-      it('Does so', function() {
-        return expect(
+    describe('When I create a coupon & customer', () => {
+      it('Does so', () =>
+        expect(
           Promise.all([
             stripe.coupons.create({
               percent_off: 20,
               duration: 'once',
             }),
             stripe.customers.create(CUSTOMER_DETAILS),
-          ]).then(function(joined) {
+          ]).then((joined) => {
             coupon = joined[0];
             customer = joined[1];
           })
-        ).to.not.be.eventually.rejected;
-      });
-      describe('And I apply the coupon to the customer', function() {
-        it('Does so', function() {
-          return expect(
+        ).to.not.be.eventually.rejected);
+      describe('And I apply the coupon to the customer', () => {
+        it('Does so', () =>
+          expect(
             stripe.customers.update(customer.id, {
               coupon: coupon.id,
             })
-          ).to.not.be.eventually.rejected;
-        });
-        it('Can be retrieved from that customer', function() {
-          return expect(
+          ).to.not.be.eventually.rejected);
+        it('Can be retrieved from that customer', () =>
+          expect(
             stripe.customers.retrieve(customer.id)
-          ).to.eventually.have.nested.property('discount.coupon.id', coupon.id);
-        });
-        describe('The resulting discount', function() {
-          it('Can be removed', function() {
-            return expect(
+          ).to.eventually.have.nested.property(
+            'discount.coupon.id',
+            coupon.id
+          ));
+        describe('The resulting discount', () => {
+          it('Can be removed', () =>
+            expect(
               stripe.customers.deleteDiscount(customer.id)
-            ).to.eventually.have.property('deleted', true);
-          });
-          describe('Re-querying it', function() {
-            it('Does indeed indicate that it is deleted', function() {
-              return expect(
+            ).to.eventually.have.property('deleted', true));
+          describe('Re-querying it', () => {
+            it('Does indeed indicate that it is deleted', () =>
+              expect(
                 stripe.customers.retrieve(customer.id)
-              ).to.eventually.have.property('discount', null);
-            });
+              ).to.eventually.have.property('discount', null));
           });
         });
       });
     });
   });
 
-  describe('Metadata flow', function() {
-    it('Can save and retrieve metadata', function() {
-      var customer;
+  describe('Metadata flow', () => {
+    it('Can save and retrieve metadata', () => {
+      let customer;
       return expect(
         stripe.customers
           .create(CUSTOMER_DETAILS)
-          .then(function(cust) {
+          .then((cust) => {
             customer = cust;
             cleanup.deleteCustomer(cust.id);
             return stripe.customers.setMetadata(cust.id, {foo: '123'});
           })
-          .then(function() {
-            return stripe.customers.getMetadata(customer.id);
-          })
+          .then(() => stripe.customers.getMetadata(customer.id))
       ).to.eventually.deep.equal({foo: '123'});
     });
-    it('Can reset metadata', function() {
-      var customer;
+    it('Can reset metadata', () => {
+      let customer;
       return expect(
         stripe.customers
           .create(CUSTOMER_DETAILS)
-          .then(function(cust) {
+          .then((cust) => {
             customer = cust;
             cleanup.deleteCustomer(cust.id);
             return stripe.customers.setMetadata(cust.id, {baz: '123'});
           })
-          .then(function() {
-            return stripe.customers.setMetadata(customer.id, null);
-          })
-          .then(function() {
-            return stripe.customers.getMetadata(customer.id);
-          })
+          .then(() => stripe.customers.setMetadata(customer.id, null))
+          .then(() => stripe.customers.getMetadata(customer.id))
       ).to.eventually.deep.equal({});
     });
-    it('Resets metadata when setting new metadata', function() {
-      var customer;
+    it('Resets metadata when setting new metadata', () => {
+      let customer;
       return expect(
         stripe.customers
           .create(CUSTOMER_DETAILS)
-          .then(function(cust) {
+          .then((cust) => {
             customer = cust;
             cleanup.deleteCustomer(cust.id);
             return stripe.customers.setMetadata(cust.id, {foo: '123'});
           })
-          .then(function() {
-            return stripe.customers.setMetadata(customer.id, {baz: '456'});
-          })
+          .then(() => stripe.customers.setMetadata(customer.id, {baz: '456'}))
       ).to.eventually.deep.equal({baz: '456'});
     });
-    it('Can set individual key/value pairs', function() {
-      var customer;
+    it('Can set individual key/value pairs', () => {
+      let customer;
       return expect(
         stripe.customers
           .create(CUSTOMER_DETAILS)
-          .then(function(cust) {
+          .then((cust) => {
             customer = cust;
             cleanup.deleteCustomer(cust.id);
           })
-          .then(function() {
-            return stripe.customers.setMetadata(customer.id, 'baz', 456);
-          })
-          .then(function() {
-            return stripe.customers.setMetadata(customer.id, '_other_', 999);
-          })
-          .then(function() {
-            return stripe.customers.setMetadata(customer.id, 'foo', 123);
-          })
-          .then(function() {
+          .then(() => stripe.customers.setMetadata(customer.id, 'baz', 456))
+          .then(() => stripe.customers.setMetadata(customer.id, '_other_', 999))
+          .then(() => stripe.customers.setMetadata(customer.id, 'foo', 123))
+          .then(() =>
             // Change foo
-            return stripe.customers.setMetadata(customer.id, 'foo', 222);
-          })
-          .then(function() {
+            stripe.customers.setMetadata(customer.id, 'foo', 222)
+          )
+          .then(() =>
             // Delete baz
-            return stripe.customers.setMetadata(customer.id, 'baz', null);
-          })
-          .then(function() {
-            return stripe.customers.getMetadata(customer.id);
-          })
+            stripe.customers.setMetadata(customer.id, 'baz', null)
+          )
+          .then(() => stripe.customers.getMetadata(customer.id))
       ).to.eventually.deep.equal({_other_: '999', foo: '222'});
     });
-    it('Can set individual key/value pairs [with per request token]', function() {
-      var customer;
-      var authToken = testUtils.getUserStripeKey();
+    it('Can set individual key/value pairs [with per request token]', () => {
+      let customer;
+      const authToken = testUtils.getUserStripeKey();
       return expect(
         stripe.customers
           .create(CUSTOMER_DETAILS)
-          .then(function(cust) {
+          .then((cust) => {
             customer = cust;
             cleanup.deleteCustomer(cust.id);
           })
-          .then(function() {
-            return stripe.customers.setMetadata(
-              customer.id,
-              {baz: 456},
-              authToken
-            );
-          })
-          .then(function() {
-            return stripe.customers.setMetadata(
-              customer.id,
-              '_other_',
-              999,
-              authToken
-            );
-          })
-          .then(function() {
-            return stripe.customers.setMetadata(
-              customer.id,
-              'foo',
-              123,
-              authToken
-            );
-          })
-          .then(function() {
+          .then(() =>
+            stripe.customers.setMetadata(customer.id, {baz: 456}, authToken)
+          )
+          .then(() =>
+            stripe.customers.setMetadata(customer.id, '_other_', 999, authToken)
+          )
+          .then(() =>
+            stripe.customers.setMetadata(customer.id, 'foo', 123, authToken)
+          )
+          .then(() =>
             // Change foo
-            return stripe.customers.setMetadata(
-              customer.id,
-              'foo',
-              222,
-              authToken
-            );
-          })
-          .then(function() {
+            stripe.customers.setMetadata(customer.id, 'foo', 222, authToken)
+          )
+          .then(() =>
             // Delete baz
-            return stripe.customers.setMetadata(
-              customer.id,
-              'baz',
-              null,
-              authToken
-            );
-          })
-          .then(function() {
-            return stripe.customers.getMetadata(customer.id, authToken);
-          })
+            stripe.customers.setMetadata(customer.id, 'baz', null, authToken)
+          )
+          .then(() => stripe.customers.getMetadata(customer.id, authToken))
       ).to.eventually.deep.equal({_other_: '999', foo: '222'});
     });
   });
 
-  describe('Expanding', function() {
-    describe('A customer within a charge', function() {
-      it('Allows you to expand a customer object', function() {
-        return expect(
-          stripe.customers.create(CUSTOMER_DETAILS).then(function(cust) {
+  describe('Expanding', () => {
+    describe('A customer within a charge', () => {
+      it('Allows you to expand a customer object', () =>
+        expect(
+          stripe.customers.create(CUSTOMER_DETAILS).then((cust) => {
             cleanup.deleteCustomer(cust.id);
             return stripe.charges.create({
               customer: cust.id,
@@ -388,31 +335,29 @@ describe('Flows', function() {
               expand: ['customer'],
             });
           })
-        ).to.eventually.have.nested.property('customer.created');
-      });
+        ).to.eventually.have.nested.property('customer.created'));
     });
-    describe("A customer's default source", function() {
-      it('Allows you to expand a default_source', function() {
-        return expect(
+    describe("A customer's default source", () => {
+      it('Allows you to expand a default_source', () =>
+        expect(
           stripe.customers
             .create({
               description: 'Some customer',
               source: 'tok_visa',
               expand: ['default_source'],
             })
-            .then(function(cust) {
+            .then((cust) => {
               cleanup.deleteCustomer(cust.id);
               return cust;
             })
           // Confirm it's expanded by checking that some prop (e.g. exp_year) exists:
-        ).to.eventually.have.nested.property('default_source.exp_year');
-      });
+        ).to.eventually.have.nested.property('default_source.exp_year'));
     });
   });
 
-  describe('Charge', function() {
-    it('Allows you to create a charge', function() {
-      return expect(
+  describe('Charge', () => {
+    it('Allows you to create a charge', () =>
+      expect(
         stripe.charges
           .create({
             amount: 1234,
@@ -425,37 +370,32 @@ describe('Flows', function() {
               },
             },
           })
-          .then(null, function(error) {
-            return error;
-          })
-      ).to.eventually.have.nested.property('raw.charge');
-    });
+          .then(null, (error) => error)
+      ).to.eventually.have.nested.property('raw.charge'));
   });
 
-  describe('Getting balance', function() {
-    it('Allows me to do so', function() {
-      return expect(stripe.balance.retrieve()).to.eventually.have.property(
+  describe('Getting balance', () => {
+    it('Allows me to do so', () =>
+      expect(stripe.balance.retrieve()).to.eventually.have.property(
         'object',
         'balance'
-      );
-    });
-    it('Allows me to do so with specified auth key', function() {
-      return expect(
+      ));
+    it('Allows me to do so with specified auth key', () =>
+      expect(
         stripe.balance.retrieve(testUtils.getUserStripeKey())
-      ).to.eventually.have.property('object', 'balance');
-    });
+      ).to.eventually.have.property('object', 'balance'));
   });
 
-  describe('Request/Response Events', function() {
-    var connectedAccountId;
+  describe('Request/Response Events', () => {
+    let connectedAccountId;
 
-    before(function(done) {
+    before((done) => {
       // Pick a random connected account to use in the `Stripe-Account` header
       stripe.accounts
         .list({
           limit: 1,
         })
-        .then(function(accounts) {
+        .then((accounts) => {
           if (accounts.data.length < 1) {
             return done(
               new Error(
@@ -470,9 +410,9 @@ describe('Flows', function() {
         });
     });
 
-    it('should emit a `request` event to listeners on request', function(done) {
-      var apiVersion = '2017-06-05';
-      var idempotencyKey = Math.random()
+    it('should emit a `request` event to listeners on request', (done) => {
+      const apiVersion = '2017-06-05';
+      const idempotencyKey = Math.random()
         .toString(36)
         .slice(2);
 
@@ -505,14 +445,14 @@ describe('Flows', function() {
             stripe_account: connectedAccountId,
           }
         )
-        .then(null, function() {
+        .then(null, () => {
           // I expect there to be an error here.
         });
     });
 
-    it('should emit a `response` event to listeners on response', function(done) {
-      var apiVersion = '2017-06-05';
-      var idempotencyKey = Math.random()
+    it('should emit a `response` event to listeners on response', (done) => {
+      const apiVersion = '2017-06-05';
+      const idempotencyKey = Math.random()
         .toString(36)
         .slice(2);
 
@@ -552,12 +492,12 @@ describe('Flows', function() {
             stripe_account: connectedAccountId,
           }
         )
-        .then(null, function() {
+        .then(null, () => {
           // I expect there to be an error here.
         });
     });
 
-    it('should not emit a `response` event to removed listeners on response', function(done) {
+    it('should not emit a `response` event to removed listeners on response', (done) => {
       function onResponse(response) {
         done(new Error('How did you get here?'));
       }
@@ -571,16 +511,16 @@ describe('Flows', function() {
           currency: 'usd',
           card: 'tok_visa',
         })
-        .then(function() {
+        .then(() => {
           done();
         });
     });
   });
 
-  describe('FileUpload', function() {
-    it('Allows you to upload a file as a stream', function() {
-      var testFilename = path.join(__dirname, 'resources/data/minimal.pdf');
-      var f = fs.createReadStream(testFilename);
+  describe('FileUpload', () => {
+    it('Allows you to upload a file as a stream', () => {
+      const testFilename = path.join(__dirname, 'resources/data/minimal.pdf');
+      const f = fs.createReadStream(testFilename);
 
       return expect(
         stripe.fileUploads
@@ -592,15 +532,13 @@ describe('Flows', function() {
               type: 'application/octet-stream',
             },
           })
-          .then(null, function(error) {
-            return error;
-          })
+          .then(null, (error) => error)
       ).to.eventually.have.nested.property('size', 739);
     });
 
-    it('Allows you to upload a file synchronously', function() {
-      var testFilename = path.join(__dirname, 'resources/data/minimal.pdf');
-      var f = fs.readFileSync(testFilename);
+    it('Allows you to upload a file synchronously', () => {
+      const testFilename = path.join(__dirname, 'resources/data/minimal.pdf');
+      const f = fs.readFileSync(testFilename);
 
       return expect(
         stripe.fileUploads
@@ -612,19 +550,17 @@ describe('Flows', function() {
               type: 'application/octet-stream',
             },
           })
-          .then(null, function(error) {
-            return error;
-          })
+          .then(null, (error) => error)
       ).to.eventually.have.nested.property('size', 739);
     });
 
-    it('Surfaces stream errors correctly', function(done) {
-      var mockedStream = new stream.Readable();
-      mockedStream._read = function() {};
+    it('Surfaces stream errors correctly', (done) => {
+      const mockedStream = new stream.Readable();
+      mockedStream._read = () => {};
 
-      var fakeError = new Error('I am a fake error');
+      const fakeError = new Error('I am a fake error');
 
-      process.nextTick(function() {
+      process.nextTick(() => {
         mockedStream.emit('error', fakeError);
       });
 
@@ -637,7 +573,7 @@ describe('Flows', function() {
             type: 'application/octet-stream',
           },
         })
-        .catch(function(error) {
+        .catch((error) => {
           expect(error.message).to.equal(
             'An error occurred while attempting to process the file for upload.'
           );
diff --git a/test/resources/Account.spec.js b/test/resources/Account.spec.js
index 5eaef72394..e09d6f2c06 100644
--- a/test/resources/Account.spec.js
+++ b/test/resources/Account.spec.js
@@ -1,17 +1,17 @@
 'use strict';
 
-var stripe = require('../../testUtils').getSpyableStripe();
-var expect = require('chai').expect;
+const stripe = require('../../testUtils').getSpyableStripe();
+const expect = require('chai').expect;
 
-var TEST_AUTH_KEY = 'aGN0bIwXnHdw5645VABjPdSn8nWY7G11';
+const TEST_AUTH_KEY = 'aGN0bIwXnHdw5645VABjPdSn8nWY7G11';
 
-describe('Account Resource', function() {
+describe('Account Resource', () => {
   function uniqueEmail() {
-    return Math.random() + 'bob@example.com';
+    return `${Math.random()}bob@example.com`;
   }
-  describe('create', function() {
-    it('Sends the correct request', function() {
-      var data = {
+  describe('create', () => {
+    it('Sends the correct request', () => {
+      const data = {
         managed: false,
         country: 'US',
         email: uniqueEmail(),
@@ -20,14 +20,14 @@ describe('Account Resource', function() {
       expect(stripe.LAST_REQUEST).to.deep.equal({
         method: 'POST',
         url: '/v1/accounts',
-        data: data,
+        data,
         headers: {},
       });
     });
   });
 
-  describe('delete', function() {
-    it('deletes an account successfully', function() {
+  describe('delete', () => {
+    it('deletes an account successfully', () => {
       stripe.account.del('acct_16Tzq6DBahdM4C8s');
       expect(stripe.LAST_REQUEST).to.deep.equal({
         method: 'DELETE',
@@ -38,8 +38,8 @@ describe('Account Resource', function() {
     });
   });
 
-  describe('reject', function() {
-    it('rejects an account successfully', function() {
+  describe('reject', () => {
+    it('rejects an account successfully', () => {
       stripe.account.reject('acct_16Tzq6DBahdM4C8s', {reason: 'fraud'});
       expect(stripe.LAST_REQUEST).to.deep.equal({
         method: 'POST',
@@ -50,8 +50,8 @@ describe('Account Resource', function() {
     });
   });
 
-  describe('retrieve', function() {
-    it('Sends the correct request with no params', function() {
+  describe('retrieve', () => {
+    it('Sends the correct request with no params', () => {
       stripe.account.retrieve();
       expect(stripe.LAST_REQUEST).to.deep.equal({
         method: 'GET',
@@ -61,7 +61,7 @@ describe('Account Resource', function() {
       });
     });
 
-    it('Sends the correct request with ID param', function() {
+    it('Sends the correct request with ID param', () => {
       stripe.account.retrieve('foo');
       expect(stripe.LAST_REQUEST).to.deep.equal({
         method: 'GET',
@@ -71,8 +71,8 @@ describe('Account Resource', function() {
       });
     });
 
-    it('Sends the correct request with secret key', function() {
-      var key = 'sk_12345678901234567890123456789012';
+    it('Sends the correct request with secret key', () => {
+      const key = 'sk_12345678901234567890123456789012';
       stripe.account.retrieve(null, key);
       expect(stripe.LAST_REQUEST).to.deep.equal({
         auth: key,
@@ -83,8 +83,8 @@ describe('Account Resource', function() {
       });
     });
 
-    it('Sends the correct request with secret key as first object', function() {
-      var params = {api_key: 'sk_12345678901234567890123456789012'};
+    it('Sends the correct request with secret key as first object', () => {
+      const params = {api_key: 'sk_12345678901234567890123456789012'};
       stripe.account.retrieve(params);
       expect(stripe.LAST_REQUEST).to.deep.equal({
         auth: params.api_key,
@@ -95,8 +95,8 @@ describe('Account Resource', function() {
       });
     });
 
-    it('Sends the correct request with a callback', function() {
-      stripe.account.retrieve(function(err, account) {});
+    it('Sends the correct request with a callback', () => {
+      stripe.account.retrieve((err, account) => {});
       expect(stripe.LAST_REQUEST).to.deep.equal({
         method: 'GET',
         url: '/v1/account',
@@ -106,9 +106,9 @@ describe('Account Resource', function() {
     });
   });
 
-  describe('External account methods', function() {
-    describe('retrieveExternalAccount', function() {
-      it('Sends the correct request', function() {
+  describe('External account methods', () => {
+    describe('retrieveExternalAccount', () => {
+      it('Sends the correct request', () => {
         stripe.account.retrieveExternalAccount(
           'accountIdFoo321',
           'externalAccountIdFoo456'
@@ -122,7 +122,7 @@ describe('Account Resource', function() {
         });
       });
 
-      it('Sends the correct request [with specified auth]', function() {
+      it('Sends the correct request [with specified auth]', () => {
         stripe.account.retrieveExternalAccount(
           'accountIdFoo321',
           'externalAccountIdFoo456',
@@ -139,8 +139,8 @@ describe('Account Resource', function() {
       });
     });
 
-    describe('createExternalAccount', function() {
-      it('Sends the correct request', function() {
+    describe('createExternalAccount', () => {
+      it('Sends the correct request', () => {
         stripe.account.createExternalAccount('accountIdFoo321', {
           number: '123456',
           currency: 'usd',
@@ -154,7 +154,7 @@ describe('Account Resource', function() {
         });
       });
 
-      it('Sends the correct request [with specified auth]', function() {
+      it('Sends the correct request [with specified auth]', () => {
         stripe.account.createExternalAccount(
           'accountIdFoo321',
           {
@@ -174,8 +174,8 @@ describe('Account Resource', function() {
       });
     });
 
-    describe('updateExternalAccount', function() {
-      it('Sends the correct request', function() {
+    describe('updateExternalAccount', () => {
+      it('Sends the correct request', () => {
         stripe.account.updateExternalAccount(
           'accountIdFoo321',
           'externalAccountIdFoo456',
@@ -193,8 +193,8 @@ describe('Account Resource', function() {
       });
     });
 
-    describe('deleteExternalAccount', function() {
-      it('Sends the correct request', function() {
+    describe('deleteExternalAccount', () => {
+      it('Sends the correct request', () => {
         stripe.account.deleteExternalAccount(
           'accountIdFoo321',
           'externalAccountIdFoo456'
@@ -208,7 +208,7 @@ describe('Account Resource', function() {
         });
       });
 
-      it('Sends the correct request [with specified auth]', function() {
+      it('Sends the correct request [with specified auth]', () => {
         stripe.account.deleteExternalAccount(
           'accountIdFoo321',
           'externalAccountIdFoo456',
@@ -225,8 +225,8 @@ describe('Account Resource', function() {
       });
     });
 
-    describe('listExternalAccounts', function() {
-      it('Sends the correct request', function() {
+    describe('listExternalAccounts', () => {
+      it('Sends the correct request', () => {
         stripe.account.listExternalAccounts('accountIdFoo321');
         expect(stripe.LAST_REQUEST).to.deep.equal({
           method: 'GET',
@@ -236,7 +236,7 @@ describe('Account Resource', function() {
         });
       });
 
-      it('Sends the correct request [with specified auth]', function() {
+      it('Sends the correct request [with specified auth]', () => {
         stripe.account.listExternalAccounts('accountIdFoo321', TEST_AUTH_KEY);
         expect(stripe.LAST_REQUEST).to.deep.equal({
           method: 'GET',
@@ -249,9 +249,9 @@ describe('Account Resource', function() {
     });
   });
 
-  describe('LoginLink methods', function() {
-    describe('createLoginLink', function() {
-      it('Sends the correct request', function() {
+  describe('LoginLink methods', () => {
+    describe('createLoginLink', () => {
+      it('Sends the correct request', () => {
         stripe.account.createLoginLink('acct_EXPRESS');
         expect(stripe.LAST_REQUEST).to.deep.equal({
           method: 'POST',
@@ -263,9 +263,9 @@ describe('Account Resource', function() {
     });
   });
 
-  describe('Person methods', function() {
-    describe('retrievePerson', function() {
-      it('Sends the correct request', function() {
+  describe('Person methods', () => {
+    describe('retrievePerson', () => {
+      it('Sends the correct request', () => {
         stripe.account.retrievePerson('acct_123', 'person_123');
         expect(stripe.LAST_REQUEST).to.deep.equal({
           method: 'GET',
@@ -275,7 +275,7 @@ describe('Account Resource', function() {
         });
       });
 
-      it('Sends the correct request [with specified auth]', function() {
+      it('Sends the correct request [with specified auth]', () => {
         stripe.account.retrievePerson('acct_123', 'person_123', TEST_AUTH_KEY);
         expect(stripe.LAST_REQUEST).to.deep.equal({
           method: 'GET',
@@ -287,8 +287,8 @@ describe('Account Resource', function() {
       });
     });
 
-    describe('createPerson', function() {
-      it('Sends the correct request', function() {
+    describe('createPerson', () => {
+      it('Sends the correct request', () => {
         stripe.account.createPerson('acct_123', {
           first_name: 'John',
         });
@@ -300,7 +300,7 @@ describe('Account Resource', function() {
         });
       });
 
-      it('Sends the correct request [with specified auth]', function() {
+      it('Sends the correct request [with specified auth]', () => {
         stripe.account.createPerson(
           'acct_123',
           {
@@ -318,8 +318,8 @@ describe('Account Resource', function() {
       });
     });
 
-    describe('updatePerson', function() {
-      it('Sends the correct request', function() {
+    describe('updatePerson', () => {
+      it('Sends the correct request', () => {
         stripe.account.updatePerson('acct_123', 'person_123', {
           first_name: 'John',
         });
@@ -331,7 +331,7 @@ describe('Account Resource', function() {
         });
       });
 
-      it('Sends the correct request [with specified auth]', function() {
+      it('Sends the correct request [with specified auth]', () => {
         stripe.account.updatePerson(
           'acct_123',
           'person_123',
@@ -350,8 +350,8 @@ describe('Account Resource', function() {
       });
     });
 
-    describe('deletePerson', function() {
-      it('Sends the correct request', function() {
+    describe('deletePerson', () => {
+      it('Sends the correct request', () => {
         stripe.account.deletePerson('acct_123', 'person_123');
         expect(stripe.LAST_REQUEST).to.deep.equal({
           method: 'DELETE',
@@ -361,7 +361,7 @@ describe('Account Resource', function() {
         });
       });
 
-      it('Sends the correct request [with specified auth]', function() {
+      it('Sends the correct request [with specified auth]', () => {
         stripe.account.deletePerson('acct_123', 'person_123', TEST_AUTH_KEY);
         expect(stripe.LAST_REQUEST).to.deep.equal({
           method: 'DELETE',
@@ -373,8 +373,8 @@ describe('Account Resource', function() {
       });
     });
 
-    describe('listPersons', function() {
-      it('Sends the correct request', function() {
+    describe('listPersons', () => {
+      it('Sends the correct request', () => {
         stripe.account.listPersons('acct_123');
         expect(stripe.LAST_REQUEST).to.deep.equal({
           method: 'GET',
@@ -384,7 +384,7 @@ describe('Account Resource', function() {
         });
       });
 
-      it('Sends the correct request [with specified auth]', function() {
+      it('Sends the correct request [with specified auth]', () => {
         stripe.account.listPersons('acct_123', TEST_AUTH_KEY);
         expect(stripe.LAST_REQUEST).to.deep.equal({
           method: 'GET',
diff --git a/test/resources/AccountLinks.spec.js b/test/resources/AccountLinks.spec.js
index 74dfe1caef..bd42414fcd 100644
--- a/test/resources/AccountLinks.spec.js
+++ b/test/resources/AccountLinks.spec.js
@@ -1,11 +1,11 @@
 'use strict';
 
-var stripe = require('../../testUtils').getSpyableStripe();
-var expect = require('chai').expect;
+const stripe = require('../../testUtils').getSpyableStripe();
+const expect = require('chai').expect;
 
-describe('AccountLinks Resource', function() {
-  describe('create', function() {
-    it('Sends the correct request', function() {
+describe('AccountLinks Resource', () => {
+  describe('create', () => {
+    it('Sends the correct request', () => {
       stripe.accountLinks.create({
         account: 'acct_123',
         failure_url: 'https://stripe.com/failure',
diff --git a/test/resources/ApplePayDomains.spec.js b/test/resources/ApplePayDomains.spec.js
index 106e3fd9fd..38fd741849 100644
--- a/test/resources/ApplePayDomains.spec.js
+++ b/test/resources/ApplePayDomains.spec.js
@@ -1,11 +1,11 @@
 'use strict';
 
-var stripe = require('../../testUtils').getSpyableStripe();
-var expect = require('chai').expect;
+const stripe = require('../../testUtils').getSpyableStripe();
+const expect = require('chai').expect;
 
-describe('ApplePayDomains Resource', function() {
-  describe('retrieve', function() {
-    it('Sends the correct request', function() {
+describe('ApplePayDomains Resource', () => {
+  describe('retrieve', () => {
+    it('Sends the correct request', () => {
       stripe.applePayDomains.retrieve('apwc_retrieve');
       expect(stripe.LAST_REQUEST).to.deep.equal({
         method: 'GET',
@@ -16,8 +16,8 @@ describe('ApplePayDomains Resource', function() {
     });
   });
 
-  describe('del', function() {
-    it('Sends the correct request', function() {
+  describe('del', () => {
+    it('Sends the correct request', () => {
       stripe.applePayDomains.del('apwc_delete');
       expect(stripe.LAST_REQUEST).to.deep.equal({
         method: 'DELETE',
@@ -28,8 +28,8 @@ describe('ApplePayDomains Resource', function() {
     });
   });
 
-  describe('create', function() {
-    it('Sends the correct request', function() {
+  describe('create', () => {
+    it('Sends the correct request', () => {
       stripe.applePayDomains.create({
         domain_name: 'example.com',
       });
@@ -45,8 +45,8 @@ describe('ApplePayDomains Resource', function() {
     });
   });
 
-  describe('list', function() {
-    it('Sends the correct request', function() {
+  describe('list', () => {
+    it('Sends the correct request', () => {
       stripe.applePayDomains.list();
       expect(stripe.LAST_REQUEST).to.deep.equal({
         method: 'GET',
diff --git a/test/resources/ApplicationFeeRefunds.spec.js b/test/resources/ApplicationFeeRefunds.spec.js
index 8f97d8db0e..75be78aa2e 100644
--- a/test/resources/ApplicationFeeRefunds.spec.js
+++ b/test/resources/ApplicationFeeRefunds.spec.js
@@ -1,61 +1,53 @@
 'use strict';
 
-var resources = require('../../lib/stripe').resources;
-var stripe = require('../../testUtils').getSpyableStripe();
-var expect = require('chai').expect;
+const resources = require('../../lib/stripe').resources;
+const stripe = require('../../testUtils').getSpyableStripe();
+const expect = require('chai').expect;
 
-var APPFEE_TEST_ID = 'appFeeIdTest999';
-var REFUND_TEST_ID = 'refundIdTest999';
+const APPFEE_TEST_ID = 'appFeeIdTest999';
+const REFUND_TEST_ID = 'refundIdTest999';
 
 // Create new CustomerCard instance with pre-filled customerId:
-var appFeeRefund = new resources.ApplicationFeeRefunds(stripe, {
+const appFeeRefund = new resources.ApplicationFeeRefunds(stripe, {
   feeId: APPFEE_TEST_ID,
 });
 
 // Use spy from existing resource:
 appFeeRefund._request = stripe.customers._request;
 
-describe('ApplicationFeeRefund Resource', function() {
-  describe('retrieve', function() {
-    it('Sends the correct request', function() {
+describe('ApplicationFeeRefund Resource', () => {
+  describe('retrieve', () => {
+    it('Sends the correct request', () => {
       appFeeRefund.retrieve(REFUND_TEST_ID);
       expect(stripe.LAST_REQUEST).to.deep.equal({
         method: 'GET',
-        url:
-          '/v1/application_fees/' +
-          APPFEE_TEST_ID +
-          '/refunds/' +
-          REFUND_TEST_ID,
+        url: `/v1/application_fees/${APPFEE_TEST_ID}/refunds/${REFUND_TEST_ID}`,
         data: {},
         headers: {},
       });
     });
   });
 
-  describe('update', function() {
-    it('Sends the correct request', function() {
+  describe('update', () => {
+    it('Sends the correct request', () => {
       appFeeRefund.update(REFUND_TEST_ID, {
         metadata: {key: 'value'},
       });
       expect(stripe.LAST_REQUEST).to.deep.equal({
         method: 'POST',
-        url:
-          '/v1/application_fees/' +
-          APPFEE_TEST_ID +
-          '/refunds/' +
-          REFUND_TEST_ID,
+        url: `/v1/application_fees/${APPFEE_TEST_ID}/refunds/${REFUND_TEST_ID}`,
         data: {metadata: {key: 'value'}},
         headers: {},
       });
     });
   });
 
-  describe('list', function() {
-    it('Sends the correct request', function() {
+  describe('list', () => {
+    it('Sends the correct request', () => {
       appFeeRefund.list();
       expect(stripe.LAST_REQUEST).to.deep.equal({
         method: 'GET',
-        url: '/v1/application_fees/' + APPFEE_TEST_ID + '/refunds',
+        url: `/v1/application_fees/${APPFEE_TEST_ID}/refunds`,
         data: {},
         headers: {},
       });
diff --git a/test/resources/ApplicationFees.spec.js b/test/resources/ApplicationFees.spec.js
index 34dbac595e..74487d76df 100644
--- a/test/resources/ApplicationFees.spec.js
+++ b/test/resources/ApplicationFees.spec.js
@@ -1,11 +1,11 @@
 'use strict';
 
-var stripe = require('../../testUtils').getSpyableStripe();
-var expect = require('chai').expect;
+const stripe = require('../../testUtils').getSpyableStripe();
+const expect = require('chai').expect;
 
-describe('ApplicationFee Resource', function() {
-  describe('list', function() {
-    it('Sends the correct request', function() {
+describe('ApplicationFee Resource', () => {
+  describe('list', () => {
+    it('Sends the correct request', () => {
       stripe.applicationFees.list();
       expect(stripe.LAST_REQUEST).to.deep.equal({
         method: 'GET',
@@ -16,8 +16,8 @@ describe('ApplicationFee Resource', function() {
     });
   });
 
-  describe('refund', function() {
-    it('Sends the correct request', function() {
+  describe('refund', () => {
+    it('Sends the correct request', () => {
       stripe.applicationFees.refund('applicationFeeIdExample3242', {
         amount: 23,
       });
@@ -30,8 +30,8 @@ describe('ApplicationFee Resource', function() {
     });
   });
 
-  describe('refunds', function() {
-    it('Sends the correct update request', function() {
+  describe('refunds', () => {
+    it('Sends the correct update request', () => {
       stripe.applicationFees.updateRefund(
         'appFeeIdExample3242',
         'refundIdExample2312',
@@ -46,7 +46,7 @@ describe('ApplicationFee Resource', function() {
       });
     });
 
-    it('Sends the correct create request', function() {
+    it('Sends the correct create request', () => {
       stripe.applicationFees.createRefund('appFeeIdExample3242', {amount: 100});
       expect(stripe.LAST_REQUEST).to.deep.equal({
         method: 'POST',
@@ -56,7 +56,7 @@ describe('ApplicationFee Resource', function() {
       });
     });
 
-    it('Sends the correct list request', function() {
+    it('Sends the correct list request', () => {
       stripe.applicationFees.listRefunds('appFeeIdExample3242');
       expect(stripe.LAST_REQUEST).to.deep.equal({
         method: 'GET',
@@ -66,7 +66,7 @@ describe('ApplicationFee Resource', function() {
       });
     });
 
-    it('Sends the correct retrieve request', function() {
+    it('Sends the correct retrieve request', () => {
       stripe.applicationFees.retrieveRefund(
         'appFeeIdExample3242',
         'refundIdExample2312'
diff --git a/test/resources/Balance.spec.js b/test/resources/Balance.spec.js
index 4951f9e151..c1e8a3f686 100644
--- a/test/resources/Balance.spec.js
+++ b/test/resources/Balance.spec.js
@@ -1,11 +1,11 @@
 'use strict';
 
-var stripe = require('../../testUtils').getSpyableStripe();
-var expect = require('chai').expect;
+const stripe = require('../../testUtils').getSpyableStripe();
+const expect = require('chai').expect;
 
-describe('Balance Resource', function() {
-  describe('retrieve', function() {
-    it('Sends the correct request', function() {
+describe('Balance Resource', () => {
+  describe('retrieve', () => {
+    it('Sends the correct request', () => {
       stripe.balance.retrieve();
       expect(stripe.LAST_REQUEST).to.deep.equal({
         method: 'GET',
@@ -15,7 +15,7 @@ describe('Balance Resource', function() {
       });
     });
 
-    it('Sends the correct request [with specified auth]', function() {
+    it('Sends the correct request [with specified auth]', () => {
       stripe.balance.retrieve('aGN0bIwXnHdw5645VABjPdSn8nWY7G11');
       expect(stripe.LAST_REQUEST).to.deep.equal({
         method: 'GET',
@@ -27,8 +27,8 @@ describe('Balance Resource', function() {
     });
   });
 
-  describe('listTransactions', function() {
-    it('Sends the correct request', function() {
+  describe('listTransactions', () => {
+    it('Sends the correct request', () => {
       stripe.balance.listTransactions();
       expect(stripe.LAST_REQUEST).to.deep.equal({
         method: 'GET',
@@ -38,7 +38,7 @@ describe('Balance Resource', function() {
       });
     });
 
-    it('Sends the correct request [with specified auth]', function() {
+    it('Sends the correct request [with specified auth]', () => {
       stripe.balance.listTransactions('aGN0bIwXnHdw5645VABjPdSn8nWY7G11');
       expect(stripe.LAST_REQUEST).to.deep.equal({
         method: 'GET',
@@ -50,8 +50,8 @@ describe('Balance Resource', function() {
     });
   });
 
-  describe('retrieveTransaction', function() {
-    it('Sends the correct request', function() {
+  describe('retrieveTransaction', () => {
+    it('Sends the correct request', () => {
       stripe.balance.retrieveTransaction('transactionIdFoo');
       expect(stripe.LAST_REQUEST).to.deep.equal({
         method: 'GET',
@@ -61,7 +61,7 @@ describe('Balance Resource', function() {
       });
     });
 
-    it('Sends the correct request [with specified auth]', function() {
+    it('Sends the correct request [with specified auth]', () => {
       stripe.balance.retrieveTransaction(
         'transactionIdFoo',
         'aGN0bIwXnHdw5645VABjPdSn8nWY7G11'
diff --git a/test/resources/BitcoinReceivers.spec.js b/test/resources/BitcoinReceivers.spec.js
index ada4e055db..494840c17a 100644
--- a/test/resources/BitcoinReceivers.spec.js
+++ b/test/resources/BitcoinReceivers.spec.js
@@ -1,11 +1,11 @@
 'use strict';
 
-var stripe = require('../../testUtils').getSpyableStripe();
-var expect = require('chai').expect;
+const stripe = require('../../testUtils').getSpyableStripe();
+const expect = require('chai').expect;
 
-describe('BitcoinReceivers Resource', function() {
-  describe('retrieve', function() {
-    it('Sends the correct request', function() {
+describe('BitcoinReceivers Resource', () => {
+  describe('retrieve', () => {
+    it('Sends the correct request', () => {
       stripe.bitcoinReceivers.retrieve('receiverId1');
       expect(stripe.LAST_REQUEST).to.deep.equal({
         method: 'GET',
@@ -16,8 +16,8 @@ describe('BitcoinReceivers Resource', function() {
     });
   });
 
-  describe('list', function() {
-    it('Sends the correct request', function() {
+  describe('list', () => {
+    it('Sends the correct request', () => {
       stripe.bitcoinReceivers.list();
       expect(stripe.LAST_REQUEST).to.deep.equal({
         method: 'GET',
@@ -28,8 +28,8 @@ describe('BitcoinReceivers Resource', function() {
     });
   });
 
-  describe('listTransactions', function() {
-    it('Sends the correct request', function() {
+  describe('listTransactions', () => {
+    it('Sends the correct request', () => {
       stripe.bitcoinReceivers.listTransactions('receiverId', {
         limit: 1,
       });
diff --git a/test/resources/ChargeRefunds.spec.js b/test/resources/ChargeRefunds.spec.js
index 843930073f..4bba3c1cae 100644
--- a/test/resources/ChargeRefunds.spec.js
+++ b/test/resources/ChargeRefunds.spec.js
@@ -1,67 +1,67 @@
 'use strict';
 
-var resources = require('../../lib/stripe').resources;
-var stripe = require('../../testUtils').getSpyableStripe();
-var expect = require('chai').expect;
+const resources = require('../../lib/stripe').resources;
+const stripe = require('../../testUtils').getSpyableStripe();
+const expect = require('chai').expect;
 
-var CHARGE_TEST_ID = 'chargeIdTest999';
-var REFUND_TEST_ID = 'refundIdTest999';
+const CHARGE_TEST_ID = 'chargeIdTest999';
+const REFUND_TEST_ID = 'refundIdTest999';
 
 // Create new CustomerCard instance with pre-filled customerId:
-var chargeRefund = new resources.ChargeRefunds(stripe, {
+const chargeRefund = new resources.ChargeRefunds(stripe, {
   chargeId: CHARGE_TEST_ID,
 });
 
 // Use spy from existing resource:
 chargeRefund._request = stripe.customers._request;
 
-describe('ChargeRefund Resource', function() {
-  describe('retrieve', function() {
-    it('Sends the correct request', function() {
+describe('ChargeRefund Resource', () => {
+  describe('retrieve', () => {
+    it('Sends the correct request', () => {
       chargeRefund.retrieve(REFUND_TEST_ID);
       expect(stripe.LAST_REQUEST).to.deep.equal({
         method: 'GET',
-        url: '/v1/charges/' + CHARGE_TEST_ID + '/refunds/' + REFUND_TEST_ID,
+        url: `/v1/charges/${CHARGE_TEST_ID}/refunds/${REFUND_TEST_ID}`,
         data: {},
         headers: {},
       });
     });
   });
 
-  describe('create', function() {
-    it('Sends the correct request', function() {
+  describe('create', () => {
+    it('Sends the correct request', () => {
       chargeRefund.create({
         amount: 100,
       });
       expect(stripe.LAST_REQUEST).to.deep.equal({
         method: 'POST',
-        url: '/v1/charges/' + CHARGE_TEST_ID + '/refunds',
+        url: `/v1/charges/${CHARGE_TEST_ID}/refunds`,
         data: {amount: 100},
         headers: {},
       });
     });
   });
 
-  describe('update', function() {
-    it('Sends the correct request', function() {
+  describe('update', () => {
+    it('Sends the correct request', () => {
       chargeRefund.update(REFUND_TEST_ID, {
         metadata: {key: 'value'},
       });
       expect(stripe.LAST_REQUEST).to.deep.equal({
         method: 'POST',
-        url: '/v1/charges/' + CHARGE_TEST_ID + '/refunds/' + REFUND_TEST_ID,
+        url: `/v1/charges/${CHARGE_TEST_ID}/refunds/${REFUND_TEST_ID}`,
         data: {metadata: {key: 'value'}},
         headers: {},
       });
     });
   });
 
-  describe('list', function() {
-    it('Sends the correct request', function() {
+  describe('list', () => {
+    it('Sends the correct request', () => {
       chargeRefund.list();
       expect(stripe.LAST_REQUEST).to.deep.equal({
         method: 'GET',
-        url: '/v1/charges/' + CHARGE_TEST_ID + '/refunds',
+        url: `/v1/charges/${CHARGE_TEST_ID}/refunds`,
         data: {},
         headers: {},
       });
diff --git a/test/resources/Charges.spec.js b/test/resources/Charges.spec.js
index dc68e835bf..f0c82a4362 100644
--- a/test/resources/Charges.spec.js
+++ b/test/resources/Charges.spec.js
@@ -1,11 +1,11 @@
 'use strict';
 
-var stripe = require('../../testUtils').getSpyableStripe();
-var expect = require('chai').expect;
+const stripe = require('../../testUtils').getSpyableStripe();
+const expect = require('chai').expect;
 
-describe('Charge Resource', function() {
-  describe('retrieve', function() {
-    it('Sends the correct request', function() {
+describe('Charge Resource', () => {
+  describe('retrieve', () => {
+    it('Sends the correct request', () => {
       stripe.charges.retrieve('chargeIdFoo123');
       expect(stripe.LAST_REQUEST).to.deep.equal({
         method: 'GET',
@@ -16,8 +16,8 @@ describe('Charge Resource', function() {
     });
   });
 
-  describe('create', function() {
-    it('Sends the correct request', function() {
+  describe('create', () => {
+    it('Sends the correct request', () => {
       stripe.charges.create({
         amount: '1500',
         currency: 'usd',
@@ -44,8 +44,8 @@ describe('Charge Resource', function() {
     });
   });
 
-  describe('list', function() {
-    it('Sends the correct request', function() {
+  describe('list', () => {
+    it('Sends the correct request', () => {
       stripe.charges.list();
       expect(stripe.LAST_REQUEST).to.deep.equal({
         method: 'GET',
@@ -56,8 +56,8 @@ describe('Charge Resource', function() {
     });
   });
 
-  describe('capture', function() {
-    it('Sends the correct request', function() {
+  describe('capture', () => {
+    it('Sends the correct request', () => {
       stripe.charges.capture('chargeIdExample3242', {amount: 23});
       expect(stripe.LAST_REQUEST).to.deep.equal({
         method: 'POST',
@@ -68,8 +68,8 @@ describe('Charge Resource', function() {
     });
   });
 
-  describe('update', function() {
-    it('Sends the correct request', function() {
+  describe('update', () => {
+    it('Sends the correct request', () => {
       stripe.charges.update('chargeIdExample3242', {description: 'foo321'});
       expect(stripe.LAST_REQUEST).to.deep.equal({
         method: 'POST',
@@ -80,8 +80,8 @@ describe('Charge Resource', function() {
     });
   });
 
-  describe('refund', function() {
-    it('Sends the correct request', function() {
+  describe('refund', () => {
+    it('Sends the correct request', () => {
       stripe.charges.refund('chargeIdExample3242', {amount: 23});
       expect(stripe.LAST_REQUEST).to.deep.equal({
         method: 'POST',
@@ -91,7 +91,7 @@ describe('Charge Resource', function() {
       });
     });
 
-    it('Incorrect arguments result in an error', function() {
+    it('Incorrect arguments result in an error', () => {
       expect(
         stripe.charges.refund('chargeIdExample123', 39392)
       ).to.be.eventually.rejectedWith(/unknown arguments/i);
@@ -106,8 +106,8 @@ describe('Charge Resource', function() {
     });
   });
 
-  describe('refunds', function() {
-    it('Sends the correct update request', function() {
+  describe('refunds', () => {
+    it('Sends the correct update request', () => {
       stripe.charges.updateRefund(
         'chargeIdExample3242',
         'refundIdExample2312',
@@ -121,7 +121,7 @@ describe('Charge Resource', function() {
       });
     });
 
-    it('Sends the correct create request', function() {
+    it('Sends the correct create request', () => {
       stripe.charges.createRefund('chargeIdExample3242', {amount: 100});
       expect(stripe.LAST_REQUEST).to.deep.equal({
         method: 'POST',
@@ -131,7 +131,7 @@ describe('Charge Resource', function() {
       });
     });
 
-    it('Sends the correct list request', function() {
+    it('Sends the correct list request', () => {
       stripe.charges.listRefunds('chargeIdExample3242');
       expect(stripe.LAST_REQUEST).to.deep.equal({
         method: 'GET',
@@ -141,7 +141,7 @@ describe('Charge Resource', function() {
       });
     });
 
-    it('Sends the correct retrieve request', function() {
+    it('Sends the correct retrieve request', () => {
       stripe.charges.retrieveRefund(
         'chargeIdExample3242',
         'refundIdExample2312'
@@ -155,8 +155,8 @@ describe('Charge Resource', function() {
     });
   });
 
-  describe('updateDispute', function() {
-    it('Sends the correct request', function() {
+  describe('updateDispute', () => {
+    it('Sends the correct request', () => {
       stripe.charges.updateDispute('chargeIdExample3242', {evidence: 'foo'});
       expect(stripe.LAST_REQUEST).to.deep.equal({
         method: 'POST',
@@ -167,8 +167,8 @@ describe('Charge Resource', function() {
     });
   });
 
-  describe('closeDispute', function() {
-    it('Sends the correct request', function() {
+  describe('closeDispute', () => {
+    it('Sends the correct request', () => {
       stripe.charges.closeDispute('chargeIdExample3242', {});
       expect(stripe.LAST_REQUEST).to.deep.equal({
         method: 'POST',
@@ -179,8 +179,8 @@ describe('Charge Resource', function() {
     });
   });
 
-  describe('markAsFraudulent', function() {
-    it('Sends the correct request', function() {
+  describe('markAsFraudulent', () => {
+    it('Sends the correct request', () => {
       stripe.charges.markAsFraudulent('chargeIdExample3242');
       expect(stripe.LAST_REQUEST).to.deep.equal({
         method: 'POST',
@@ -191,8 +191,8 @@ describe('Charge Resource', function() {
     });
   });
 
-  describe('markAsSafe', function() {
-    it('Sends the correct request', function() {
+  describe('markAsSafe', () => {
+    it('Sends the correct request', () => {
       stripe.charges.markAsSafe('chargeIdExample3242');
       expect(stripe.LAST_REQUEST).to.deep.equal({
         method: 'POST',
diff --git a/test/resources/Checkout/Sessions.spec.js b/test/resources/Checkout/Sessions.spec.js
index 1b071806f0..2ab499b94f 100644
--- a/test/resources/Checkout/Sessions.spec.js
+++ b/test/resources/Checkout/Sessions.spec.js
@@ -1,14 +1,14 @@
 'use strict';
 
-var stripe = require('../../../testUtils').getSpyableStripe();
+const stripe = require('../../../testUtils').getSpyableStripe();
 
-var expect = require('chai').expect;
+const expect = require('chai').expect;
 
-describe('Checkout', function() {
-  describe('Sessions Resource', function() {
-    describe('create', function() {
-      it('Sends the correct request', function() {
-        var params = {
+describe('Checkout', () => {
+  describe('Sessions Resource', () => {
+    describe('create', () => {
+      it('Sends the correct request', () => {
+        const params = {
           cancel_url: 'https://stripe.com/cancel',
           client_reference_id: '1234',
           line_items: [
@@ -38,8 +38,8 @@ describe('Checkout', function() {
       });
     });
 
-    describe('retrieve', function() {
-      it('Sends the correct request', function() {
+    describe('retrieve', () => {
+      it('Sends the correct request', () => {
         stripe.checkout.sessions.retrieve('cs_123');
         expect(stripe.LAST_REQUEST).to.deep.equal({
           method: 'GET',
diff --git a/test/resources/CountrySpecs.spec.js b/test/resources/CountrySpecs.spec.js
index b896cdb91d..d5f6252aa4 100644
--- a/test/resources/CountrySpecs.spec.js
+++ b/test/resources/CountrySpecs.spec.js
@@ -1,11 +1,11 @@
 'use strict';
 
-var stripe = require('../../testUtils').getSpyableStripe();
-var expect = require('chai').expect;
+const stripe = require('../../testUtils').getSpyableStripe();
+const expect = require('chai').expect;
 
-describe('CountrySpecs Resource', function() {
-  describe('list', function() {
-    it('Sends the correct request', function() {
+describe('CountrySpecs Resource', () => {
+  describe('list', () => {
+    it('Sends the correct request', () => {
       stripe.countrySpecs.list();
       expect(stripe.LAST_REQUEST).to.deep.equal({
         method: 'GET',
@@ -16,13 +16,13 @@ describe('CountrySpecs Resource', function() {
     });
   });
 
-  describe('retrieve', function() {
-    it('Sends the correct request', function() {
-      var country = 'US';
+  describe('retrieve', () => {
+    it('Sends the correct request', () => {
+      const country = 'US';
       stripe.countrySpecs.retrieve(country);
       expect(stripe.LAST_REQUEST).to.deep.equal({
         method: 'GET',
-        url: '/v1/country_specs/' + country,
+        url: `/v1/country_specs/${country}`,
         data: {},
         headers: {},
       });
diff --git a/test/resources/Coupons.spec.js b/test/resources/Coupons.spec.js
index 199845993e..f906a02d79 100644
--- a/test/resources/Coupons.spec.js
+++ b/test/resources/Coupons.spec.js
@@ -1,11 +1,11 @@
 'use strict';
 
-var stripe = require('../../testUtils').getSpyableStripe();
-var expect = require('chai').expect;
+const stripe = require('../../testUtils').getSpyableStripe();
+const expect = require('chai').expect;
 
-describe('Coupons Resource', function() {
-  describe('retrieve', function() {
-    it('Sends the correct request', function() {
+describe('Coupons Resource', () => {
+  describe('retrieve', () => {
+    it('Sends the correct request', () => {
       stripe.coupons.retrieve('couponId123');
       expect(stripe.LAST_REQUEST).to.deep.equal({
         method: 'GET',
@@ -16,8 +16,8 @@ describe('Coupons Resource', function() {
     });
   });
 
-  describe('del', function() {
-    it('Sends the correct request', function() {
+  describe('del', () => {
+    it('Sends the correct request', () => {
       stripe.coupons.del('couponId123');
       expect(stripe.LAST_REQUEST).to.deep.equal({
         method: 'DELETE',
@@ -28,8 +28,8 @@ describe('Coupons Resource', function() {
     });
   });
 
-  describe('update', function() {
-    it('Sends the correct request', function() {
+  describe('update', () => {
+    it('Sends the correct request', () => {
       stripe.coupons.update('couponId123', {
         metadata: {a: '1234'},
       });
@@ -44,8 +44,8 @@ describe('Coupons Resource', function() {
     });
   });
 
-  describe('create', function() {
-    it('Sends the correct request', function() {
+  describe('create', () => {
+    it('Sends the correct request', () => {
       stripe.coupons.create({
         percent_off: 25,
         duration: 'repeating',
@@ -65,8 +65,8 @@ describe('Coupons Resource', function() {
     });
   });
 
-  describe('list', function() {
-    it('Sends the correct request', function() {
+  describe('list', () => {
+    it('Sends the correct request', () => {
       stripe.coupons.list();
       expect(stripe.LAST_REQUEST).to.deep.equal({
         method: 'GET',
diff --git a/test/resources/CreditNotes.spec.js b/test/resources/CreditNotes.spec.js
index b0343e1eca..025e01a176 100644
--- a/test/resources/CreditNotes.spec.js
+++ b/test/resources/CreditNotes.spec.js
@@ -1,11 +1,11 @@
 'use strict';
 
-var stripe = require('../../testUtils').getSpyableStripe();
-var expect = require('chai').expect;
+const stripe = require('../../testUtils').getSpyableStripe();
+const expect = require('chai').expect;
 
-describe('CreditNotes Resource', function() {
-  describe('retrieve', function() {
-    it('Sends the correct request', function() {
+describe('CreditNotes Resource', () => {
+  describe('retrieve', () => {
+    it('Sends the correct request', () => {
       stripe.creditNotes.retrieve('cn_123');
       expect(stripe.LAST_REQUEST).to.deep.equal({
         method: 'GET',
@@ -16,9 +16,9 @@ describe('CreditNotes Resource', function() {
     });
   });
 
-  describe('create', function() {
-    it('Sends the correct request', function() {
-      var data = {
+  describe('create', () => {
+    it('Sends the correct request', () => {
+      const data = {
         amount: 100,
         invoice: 'in_123',
         reason: 'duplicate',
@@ -28,13 +28,13 @@ describe('CreditNotes Resource', function() {
         method: 'POST',
         url: '/v1/credit_notes',
         headers: {},
-        data: data,
+        data,
       });
     });
   });
 
-  describe('list', function() {
-    it('Sends the correct request', function() {
+  describe('list', () => {
+    it('Sends the correct request', () => {
       stripe.creditNotes.list({count: 25});
       expect(stripe.LAST_REQUEST).to.deep.equal({
         method: 'GET',
@@ -45,8 +45,8 @@ describe('CreditNotes Resource', function() {
     });
   });
 
-  describe('update', function() {
-    it('Sends the correct request', function() {
+  describe('update', () => {
+    it('Sends the correct request', () => {
       stripe.creditNotes.update('cn_123', {application_fee: 200});
       expect(stripe.LAST_REQUEST).to.deep.equal({
         method: 'POST',
@@ -57,8 +57,8 @@ describe('CreditNotes Resource', function() {
     });
   });
 
-  describe('voidCreditNote', function() {
-    it('Sends the correct request', function() {
+  describe('voidCreditNote', () => {
+    it('Sends the correct request', () => {
       stripe.creditNotes.voidCreditNote('cn_123');
       expect(stripe.LAST_REQUEST).to.deep.equal({
         method: 'POST',
diff --git a/test/resources/CustomerCards.spec.js b/test/resources/CustomerCards.spec.js
index 0354d9fb7e..4a9812f792 100644
--- a/test/resources/CustomerCards.spec.js
+++ b/test/resources/CustomerCards.spec.js
@@ -1,79 +1,79 @@
 'use strict';
 
-var resources = require('../../lib/stripe').resources;
-var stripe = require('../../testUtils').getSpyableStripe();
-var expect = require('chai').expect;
+const resources = require('../../lib/stripe').resources;
+const stripe = require('../../testUtils').getSpyableStripe();
+const expect = require('chai').expect;
 
-var CUSTOMER_TEST_ID = 'customerIdTest999';
+const CUSTOMER_TEST_ID = 'customerIdTest999';
 
 // Create new CustomerCard instance with pre-filled customerId:
-var customerCard = new resources.CustomerCards(stripe, {
+const customerCard = new resources.CustomerCards(stripe, {
   customerId: CUSTOMER_TEST_ID,
 });
 
 // Use spy from existing resource:
 customerCard._request = stripe.customers._request;
 
-describe('CustomerCard Resource', function() {
-  describe('retrieve', function() {
-    it('Sends the correct request', function() {
+describe('CustomerCard Resource', () => {
+  describe('retrieve', () => {
+    it('Sends the correct request', () => {
       customerCard.retrieve('cardIdFoo456');
       expect(stripe.LAST_REQUEST).to.deep.equal({
         method: 'GET',
-        url: '/v1/customers/' + CUSTOMER_TEST_ID + '/cards/cardIdFoo456',
+        url: `/v1/customers/${CUSTOMER_TEST_ID}/cards/cardIdFoo456`,
         headers: {},
         data: {},
       });
     });
   });
 
-  describe('create', function() {
-    it('Sends the correct request', function() {
+  describe('create', () => {
+    it('Sends the correct request', () => {
       customerCard.create({
         number: '123456',
         exp_month: '12',
       });
       expect(stripe.LAST_REQUEST).to.deep.equal({
         method: 'POST',
-        url: '/v1/customers/' + CUSTOMER_TEST_ID + '/cards',
+        url: `/v1/customers/${CUSTOMER_TEST_ID}/cards`,
         headers: {},
         data: {number: '123456', exp_month: '12'},
       });
     });
   });
 
-  describe('update', function() {
-    it('Sends the correct request', function() {
+  describe('update', () => {
+    it('Sends the correct request', () => {
       customerCard.update('cardIdFoo456', {
         name: 'Bob M. Baz',
       });
       expect(stripe.LAST_REQUEST).to.deep.equal({
         method: 'POST',
-        url: '/v1/customers/' + CUSTOMER_TEST_ID + '/cards/cardIdFoo456',
+        url: `/v1/customers/${CUSTOMER_TEST_ID}/cards/cardIdFoo456`,
         headers: {},
         data: {name: 'Bob M. Baz'},
       });
     });
   });
 
-  describe('del', function() {
-    it('Sends the correct request', function() {
+  describe('del', () => {
+    it('Sends the correct request', () => {
       customerCard.del('cardIdFoo456');
       expect(stripe.LAST_REQUEST).to.deep.equal({
         method: 'DELETE',
-        url: '/v1/customers/' + CUSTOMER_TEST_ID + '/cards/cardIdFoo456',
+        url: `/v1/customers/${CUSTOMER_TEST_ID}/cards/cardIdFoo456`,
         headers: {},
         data: {},
       });
     });
   });
 
-  describe('list', function() {
-    it('Sends the correct request', function() {
+  describe('list', () => {
+    it('Sends the correct request', () => {
       customerCard.list();
       expect(stripe.LAST_REQUEST).to.deep.equal({
         method: 'GET',
-        url: '/v1/customers/' + CUSTOMER_TEST_ID + '/cards',
+        url: `/v1/customers/${CUSTOMER_TEST_ID}/cards`,
         headers: {},
         data: {},
       });
diff --git a/test/resources/CustomerSubscriptions.spec.js b/test/resources/CustomerSubscriptions.spec.js
index 6d9b71933e..842902464c 100644
--- a/test/resources/CustomerSubscriptions.spec.js
+++ b/test/resources/CustomerSubscriptions.spec.js
@@ -1,97 +1,88 @@
 'use strict';
 
-var resources = require('../../lib/stripe').resources;
-var stripe = require('../../testUtils').getSpyableStripe();
-var expect = require('chai').expect;
+const resources = require('../../lib/stripe').resources;
+const stripe = require('../../testUtils').getSpyableStripe();
+const expect = require('chai').expect;
 
-var CUSTOMER_TEST_ID = 'customerIdTest999';
+const CUSTOMER_TEST_ID = 'customerIdTest999';
 
 // Create new CustomerSubscription instance with pre-filled customerId:
-var customerSubscription = new resources.CustomerSubscriptions(stripe, {
+const customerSubscription = new resources.CustomerSubscriptions(stripe, {
   customerId: CUSTOMER_TEST_ID,
 });
 
 // Use spy from existing resource:
 customerSubscription._request = stripe.customers._request;
 
-describe('CustomerSubscription Resource', function() {
-  describe('retrieve', function() {
-    it('Sends the correct request', function() {
+describe('CustomerSubscription Resource', () => {
+  describe('retrieve', () => {
+    it('Sends the correct request', () => {
       customerSubscription.retrieve('subscriptionIdFoo456');
       expect(stripe.LAST_REQUEST).to.deep.equal({
         method: 'GET',
-        url:
-          '/v1/customers/' +
-          CUSTOMER_TEST_ID +
-          '/subscriptions/subscriptionIdFoo456',
+        url: `/v1/customers/${CUSTOMER_TEST_ID}/subscriptions/subscriptionIdFoo456`,
         headers: {},
         data: {},
       });
     });
   });
 
-  describe('create', function() {
-    it('Sends the correct request', function() {
+  describe('create', () => {
+    it('Sends the correct request', () => {
       customerSubscription.create({
         plan: 'gold',
         quantity: '12',
       });
       expect(stripe.LAST_REQUEST).to.deep.equal({
         method: 'POST',
-        url: '/v1/customers/' + CUSTOMER_TEST_ID + '/subscriptions',
+        url: `/v1/customers/${CUSTOMER_TEST_ID}/subscriptions`,
         headers: {},
         data: {plan: 'gold', quantity: '12'},
       });
     });
   });
 
-  describe('update', function() {
-    it('Sends the correct request', function() {
+  describe('update', () => {
+    it('Sends the correct request', () => {
       customerSubscription.update('subscriptionIdFoo456', {
         name: 'Bob M. Baz',
       });
       expect(stripe.LAST_REQUEST).to.deep.equal({
         method: 'POST',
-        url:
-          '/v1/customers/' +
-          CUSTOMER_TEST_ID +
-          '/subscriptions/subscriptionIdFoo456',
+        url: `/v1/customers/${CUSTOMER_TEST_ID}/subscriptions/subscriptionIdFoo456`,
         headers: {},
         data: {name: 'Bob M. Baz'},
       });
     });
   });
 
-  describe('del', function() {
-    it('Sends the correct request', function() {
+  describe('del', () => {
+    it('Sends the correct request', () => {
       customerSubscription.del('subscriptionIdFoo456');
       expect(stripe.LAST_REQUEST).to.deep.equal({
         method: 'DELETE',
-        url:
-          '/v1/customers/' +
-          CUSTOMER_TEST_ID +
-          '/subscriptions/subscriptionIdFoo456',
+        url: `/v1/customers/${CUSTOMER_TEST_ID}/subscriptions/subscriptionIdFoo456`,
         headers: {},
         data: {},
       });
     });
   });
 
-  describe('list', function() {
-    it('Sends the correct request', function() {
+  describe('list', () => {
+    it('Sends the correct request', () => {
       customerSubscription.list();
       expect(stripe.LAST_REQUEST).to.deep.equal({
         method: 'GET',
-        url: '/v1/customers/' + CUSTOMER_TEST_ID + '/subscriptions',
+        url: `/v1/customers/${CUSTOMER_TEST_ID}/subscriptions`,
         headers: {},
         data: {},
       });
     });
   });
 
-  describe('Discount methods', function() {
-    describe('deleteDiscount', function() {
-      it('Sends the correct request', function() {
+  describe('Discount methods', () => {
+    describe('deleteDiscount', () => {
+      it('Sends the correct request', () => {
         customerSubscription.deleteDiscount(
           'customerIdFoo321',
           'subscriptionIdBar654'
diff --git a/test/resources/Customers.spec.js b/test/resources/Customers.spec.js
index 3193585e12..ee3d0adf3b 100644
--- a/test/resources/Customers.spec.js
+++ b/test/resources/Customers.spec.js
@@ -1,13 +1,13 @@
 'use strict';
 
-var stripe = require('../../testUtils').getSpyableStripe();
-var expect = require('chai').expect;
+const stripe = require('../../testUtils').getSpyableStripe();
+const expect = require('chai').expect;
 
-var TEST_AUTH_KEY = 'aGN0bIwXnHdw5645VABjPdSn8nWY7G11';
+const TEST_AUTH_KEY = 'aGN0bIwXnHdw5645VABjPdSn8nWY7G11';
 
-describe('Customers Resource', function() {
-  describe('retrieve', function() {
-    it('Sends the correct request', function() {
+describe('Customers Resource', () => {
+  describe('retrieve', () => {
+    it('Sends the correct request', () => {
       stripe.customers.retrieve('cus_123');
       expect(stripe.LAST_REQUEST).to.deep.equal({
         method: 'GET',
@@ -17,7 +17,7 @@ describe('Customers Resource', function() {
       });
     });
 
-    it('Sends the correct request [with specified auth]', function() {
+    it('Sends the correct request [with specified auth]', () => {
       stripe.customers.retrieve('cus_123', TEST_AUTH_KEY);
       expect(stripe.LAST_REQUEST).to.deep.equal({
         method: 'GET',
@@ -29,8 +29,8 @@ describe('Customers Resource', function() {
     });
   });
 
-  describe('create', function() {
-    it('Sends the correct request', function() {
+  describe('create', () => {
+    it('Sends the correct request', () => {
       stripe.customers.create({description: 'Some customer'});
       expect(stripe.LAST_REQUEST).to.deep.equal({
         method: 'POST',
@@ -40,7 +40,7 @@ describe('Customers Resource', function() {
       });
     });
 
-    it('Sends the correct request [with specified auth]', function() {
+    it('Sends the correct request [with specified auth]', () => {
       stripe.customers.create({description: 'Some customer'}, TEST_AUTH_KEY);
       expect(stripe.LAST_REQUEST).to.deep.equal({
         method: 'POST',
@@ -51,7 +51,7 @@ describe('Customers Resource', function() {
       });
     });
 
-    it('Sends the correct request [with specified auth and no body]', function() {
+    it('Sends the correct request [with specified auth and no body]', () => {
       stripe.customers.create(TEST_AUTH_KEY);
       expect(stripe.LAST_REQUEST).to.deep.equal({
         method: 'POST',
@@ -62,7 +62,7 @@ describe('Customers Resource', function() {
       });
     });
 
-    it('Sends the correct request [with specified idempotency_key in options]', function() {
+    it('Sends the correct request [with specified idempotency_key in options]', () => {
       stripe.customers.create(
         {description: 'Some customer'},
         {idempotency_key: 'foo'}
@@ -75,7 +75,7 @@ describe('Customers Resource', function() {
       });
     });
 
-    it('Sends the correct request [with specified auth in options]', function() {
+    it('Sends the correct request [with specified auth in options]', () => {
       stripe.customers.create(
         {description: 'Some customer'},
         {api_key: TEST_AUTH_KEY}
@@ -89,7 +89,7 @@ describe('Customers Resource', function() {
       });
     });
 
-    it('Sends the correct request [with specified auth and idempotent key in options]', function() {
+    it('Sends the correct request [with specified auth and idempotent key in options]', () => {
       stripe.customers.create(
         {description: 'Some customer'},
         {api_key: TEST_AUTH_KEY, idempotency_key: 'foo'}
@@ -103,7 +103,7 @@ describe('Customers Resource', function() {
       });
     });
 
-    it('Sends the correct request [with specified auth in options and no body]', function() {
+    it('Sends the correct request [with specified auth in options and no body]', () => {
       stripe.customers.create({api_key: TEST_AUTH_KEY});
       expect(stripe.LAST_REQUEST).to.deep.equal({
         method: 'POST',
@@ -115,8 +115,8 @@ describe('Customers Resource', function() {
     });
   });
 
-  describe('update', function() {
-    it('Sends the correct request', function() {
+  describe('update', () => {
+    it('Sends the correct request', () => {
       stripe.customers.update('cus_123', {
         description: 'Foo "baz"',
       });
@@ -129,8 +129,8 @@ describe('Customers Resource', function() {
     });
   });
 
-  describe('del', function() {
-    it('Sends the correct request', function() {
+  describe('del', () => {
+    it('Sends the correct request', () => {
       stripe.customers.del('cus_123');
       expect(stripe.LAST_REQUEST).to.deep.equal({
         method: 'DELETE',
@@ -141,8 +141,8 @@ describe('Customers Resource', function() {
     });
   });
 
-  describe('list', function() {
-    it('Sends the correct request', function() {
+  describe('list', () => {
+    it('Sends the correct request', () => {
       stripe.customers.list();
       expect(stripe.LAST_REQUEST).to.deep.equal({
         method: 'GET',
@@ -152,7 +152,7 @@ describe('Customers Resource', function() {
       });
     });
 
-    it('Sends the correct request [with specified auth]', function() {
+    it('Sends the correct request [with specified auth]', () => {
       stripe.customers.list(TEST_AUTH_KEY);
       expect(stripe.LAST_REQUEST).to.deep.equal({
         method: 'GET',
@@ -164,9 +164,9 @@ describe('Customers Resource', function() {
     });
   });
 
-  describe('Subscription methods', function() {
-    describe('updateSubscription', function() {
-      it('Sends the correct request', function() {
+  describe('Subscription methods', () => {
+    describe('updateSubscription', () => {
+      it('Sends the correct request', () => {
         stripe.customers.updateSubscription('cus_123', {
           plan: 'fooPlan',
         });
@@ -178,7 +178,7 @@ describe('Customers Resource', function() {
         });
       });
 
-      it('Sends the correct request [with specified auth]', function() {
+      it('Sends the correct request [with specified auth]', () => {
         stripe.customers.updateSubscription(
           'cus_123',
           {
@@ -196,8 +196,8 @@ describe('Customers Resource', function() {
       });
     });
 
-    describe('cancelSubscription', function() {
-      it('Sends the correct request', function() {
+    describe('cancelSubscription', () => {
+      it('Sends the correct request', () => {
         stripe.customers.cancelSubscription('cus_123');
         expect(stripe.LAST_REQUEST).to.deep.equal({
           method: 'DELETE',
@@ -207,7 +207,7 @@ describe('Customers Resource', function() {
         });
       });
 
-      it('Sends the correct request [with specified auth]', function() {
+      it('Sends the correct request [with specified auth]', () => {
         stripe.customers.cancelSubscription('cus_123', TEST_AUTH_KEY);
         expect(stripe.LAST_REQUEST).to.deep.equal({
           method: 'DELETE',
@@ -218,8 +218,8 @@ describe('Customers Resource', function() {
         });
       });
 
-      describe('With at_period_end defined', function() {
-        it('Sends the correct request', function() {
+      describe('With at_period_end defined', () => {
+        it('Sends the correct request', () => {
           stripe.customers.cancelSubscription('cus_123', {at_period_end: true});
           expect(stripe.LAST_REQUEST).to.deep.equal({
             method: 'DELETE',
@@ -230,8 +230,8 @@ describe('Customers Resource', function() {
         });
       });
 
-      describe('With at_period_end defined [with specified auth]', function() {
-        it('Sends the correct request', function() {
+      describe('With at_period_end defined [with specified auth]', () => {
+        it('Sends the correct request', () => {
           stripe.customers.cancelSubscription(
             'cus_123',
             {at_period_end: true},
@@ -249,9 +249,9 @@ describe('Customers Resource', function() {
     });
   });
 
-  describe('Discount methods', function() {
-    describe('deleteDiscount', function() {
-      it('Sends the correct request', function() {
+  describe('Discount methods', () => {
+    describe('deleteDiscount', () => {
+      it('Sends the correct request', () => {
         stripe.customers.deleteDiscount('cus_123');
         expect(stripe.LAST_REQUEST).to.deep.equal({
           method: 'DELETE',
@@ -262,8 +262,8 @@ describe('Customers Resource', function() {
       });
     });
 
-    describe('deleteSubscriptionDiscount', function() {
-      it('Sends the correct request', function() {
+    describe('deleteSubscriptionDiscount', () => {
+      it('Sends the correct request', () => {
         stripe.customers.deleteSubscriptionDiscount('cus_123', 'sub_123');
         expect(stripe.LAST_REQUEST).to.deep.equal({
           method: 'DELETE',
@@ -275,10 +275,10 @@ describe('Customers Resource', function() {
     });
   });
 
-  describe('Metadata methods', function() {
-    describe('setMetadata', function() {
-      describe('When deleting metadata', function() {
-        it('Sends the correct request', function() {
+  describe('Metadata methods', () => {
+    describe('setMetadata', () => {
+      describe('When deleting metadata', () => {
+        it('Sends the correct request', () => {
           stripe.customers.setMetadata('cus_123', null);
           expect(stripe.LAST_REQUEST).to.deep.equal({
             method: 'POST',
@@ -291,17 +291,17 @@ describe('Customers Resource', function() {
         });
       });
 
-      describe('When setting new metadata', function() {
-        it('Sends one request to get current, and another to set new data', function() {
-          return expect(
-            new Promise(function(resolve, reject) {
+      describe('When setting new metadata', () => {
+        it('Sends one request to get current, and another to set new data', () =>
+          expect(
+            new Promise((resolve, reject) => {
               stripe.customers
                 .setMetadata('cus_123', {
                   foo: 123,
                   baz: 456,
                 })
-                .then(function() {
-                  var reqs = stripe.REQUESTS;
+                .then(() => {
+                  const reqs = stripe.REQUESTS;
                   resolve([
                     // Last two requests
                     reqs[reqs.length - 2],
@@ -326,12 +326,11 @@ describe('Customers Resource', function() {
                 metadata: {foo: 123, baz: 456},
               },
             },
-          ]);
-        });
+          ]));
       });
 
-      describe('When setting with an auth key', function() {
-        it('Sends the correct request, including the specified auth key', function() {
+      describe('When setting with an auth key', () => {
+        it('Sends the correct request, including the specified auth key', () => {
           stripe.customers.setMetadata('cus_123', null, TEST_AUTH_KEY);
           expect(stripe.LAST_REQUEST).to.deep.equal({
             method: 'POST',
@@ -367,9 +366,9 @@ describe('Customers Resource', function() {
     });
   });
 
-  describe('Card methods', function() {
-    describe('retrieveCard', function() {
-      it('Sends the correct request', function() {
+  describe('Card methods', () => {
+    describe('retrieveCard', () => {
+      it('Sends the correct request', () => {
         stripe.customers.retrieveCard('cus_123', 'card_123');
         expect(stripe.LAST_REQUEST).to.deep.equal({
           method: 'GET',
@@ -379,7 +378,7 @@ describe('Customers Resource', function() {
         });
       });
 
-      it('Sends the correct request [with specified auth]', function() {
+      it('Sends the correct request [with specified auth]', () => {
         stripe.customers.retrieveCard('cus_123', 'card_123', TEST_AUTH_KEY);
         expect(stripe.LAST_REQUEST).to.deep.equal({
           method: 'GET',
@@ -391,8 +390,8 @@ describe('Customers Resource', function() {
       });
     });
 
-    describe('createCard', function() {
-      it('Sends the correct request', function() {
+    describe('createCard', () => {
+      it('Sends the correct request', () => {
         stripe.customers.createCard('cus_123', {
           number: '123456',
           exp_month: '12',
@@ -405,7 +404,7 @@ describe('Customers Resource', function() {
         });
       });
 
-      it('Sends the correct request [with specified auth]', function() {
+      it('Sends the correct request [with specified auth]', () => {
         stripe.customers.createCard(
           'cus_123',
           {
@@ -424,8 +423,8 @@ describe('Customers Resource', function() {
       });
     });
 
-    describe('updateCard', function() {
-      it('Sends the correct request', function() {
+    describe('updateCard', () => {
+      it('Sends the correct request', () => {
         stripe.customers.updateCard('cus_123', 'card_123', {
           name: 'Bob M. Baz',
         });
@@ -438,8 +437,8 @@ describe('Customers Resource', function() {
       });
     });
 
-    describe('deleteCard', function() {
-      it('Sends the correct request', function() {
+    describe('deleteCard', () => {
+      it('Sends the correct request', () => {
         stripe.customers.deleteCard('cus_123', 'card_123');
         expect(stripe.LAST_REQUEST).to.deep.equal({
           method: 'DELETE',
@@ -449,7 +448,7 @@ describe('Customers Resource', function() {
         });
       });
 
-      it('Sends the correct request [with specified auth]', function() {
+      it('Sends the correct request [with specified auth]', () => {
         stripe.customers.deleteCard('cus_123', 'card_123', TEST_AUTH_KEY);
         expect(stripe.LAST_REQUEST).to.deep.equal({
           method: 'DELETE',
@@ -461,8 +460,8 @@ describe('Customers Resource', function() {
       });
     });
 
-    describe('listCards', function() {
-      it('Sends the correct request', function() {
+    describe('listCards', () => {
+      it('Sends the correct request', () => {
         stripe.customers.listCards('cus_123');
         expect(stripe.LAST_REQUEST).to.deep.equal({
           method: 'GET',
@@ -472,7 +471,7 @@ describe('Customers Resource', function() {
         });
       });
 
-      it('Sends the correct request [with specified auth]', function() {
+      it('Sends the correct request [with specified auth]', () => {
         stripe.customers.listCards('cus_123', TEST_AUTH_KEY);
         expect(stripe.LAST_REQUEST).to.deep.equal({
           method: 'GET',
@@ -485,9 +484,9 @@ describe('Customers Resource', function() {
     });
   });
 
-  describe('Source methods', function() {
-    describe('retrieveSource', function() {
-      it('Sends the correct request', function() {
+  describe('Source methods', () => {
+    describe('retrieveSource', () => {
+      it('Sends the correct request', () => {
         stripe.customers.retrieveSource('cus_123', 'card_123');
         expect(stripe.LAST_REQUEST).to.deep.equal({
           method: 'GET',
@@ -497,7 +496,7 @@ describe('Customers Resource', function() {
         });
       });
 
-      it('Sends the correct request [with specified auth]', function() {
+      it('Sends the correct request [with specified auth]', () => {
         stripe.customers.retrieveSource('cus_123', 'card_123', TEST_AUTH_KEY);
         expect(stripe.LAST_REQUEST).to.deep.equal({
           method: 'GET',
@@ -509,8 +508,8 @@ describe('Customers Resource', function() {
       });
     });
 
-    describe('createSource', function() {
-      it('Sends the correct request', function() {
+    describe('createSource', () => {
+      it('Sends the correct request', () => {
         stripe.customers.createSource('cus_123', {
           object: 'card',
           number: '123456',
@@ -524,7 +523,7 @@ describe('Customers Resource', function() {
         });
       });
 
-      it('Sends the correct request [with specified auth]', function() {
+      it('Sends the correct request [with specified auth]', () => {
         stripe.customers.createSource(
           'cus_123',
           {
@@ -544,8 +543,8 @@ describe('Customers Resource', function() {
       });
     });
 
-    describe('updateSource', function() {
-      it('Sends the correct request', function() {
+    describe('updateSource', () => {
+      it('Sends the correct request', () => {
         stripe.customers.updateSource('cus_123', 'card_123', {
           name: 'Bob M. Baz',
         });
@@ -558,8 +557,8 @@ describe('Customers Resource', function() {
       });
     });
 
-    describe('deleteSource', function() {
-      it('Sends the correct request', function() {
+    describe('deleteSource', () => {
+      it('Sends the correct request', () => {
         stripe.customers.deleteSource('cus_123', 'card_123');
         expect(stripe.LAST_REQUEST).to.deep.equal({
           method: 'DELETE',
@@ -569,7 +568,7 @@ describe('Customers Resource', function() {
         });
       });
 
-      it('Sends the correct request [with specified auth]', function() {
+      it('Sends the correct request [with specified auth]', () => {
         stripe.customers.deleteSource('cus_123', 'card_123', TEST_AUTH_KEY);
         expect(stripe.LAST_REQUEST).to.deep.equal({
           method: 'DELETE',
@@ -581,8 +580,8 @@ describe('Customers Resource', function() {
       });
     });
 
-    describe('listSources', function() {
-      it('Sends the correct request', function() {
+    describe('listSources', () => {
+      it('Sends the correct request', () => {
         stripe.customers.listSources('cus_123');
         expect(stripe.LAST_REQUEST).to.deep.equal({
           method: 'GET',
@@ -592,7 +591,7 @@ describe('Customers Resource', function() {
         });
       });
 
-      it('Sends the correct request [with specified auth]', function() {
+      it('Sends the correct request [with specified auth]', () => {
         stripe.customers.listSources('cus_123', TEST_AUTH_KEY);
         expect(stripe.LAST_REQUEST).to.deep.equal({
           method: 'GET',
@@ -604,9 +603,9 @@ describe('Customers Resource', function() {
       });
     });
 
-    describe('verifySource', function() {
-      it('Sends the correct request', function() {
-        var data = {amounts: [32, 45]};
+    describe('verifySource', () => {
+      it('Sends the correct request', () => {
+        const data = {amounts: [32, 45]};
 
         stripe.customers.verifySource(
           'cus_123',
@@ -618,16 +617,16 @@ describe('Customers Resource', function() {
           method: 'POST',
           url: '/v1/customers/cus_123/sources/card_123/verify',
           headers: {},
-          data: data,
+          data,
           auth: TEST_AUTH_KEY,
         });
       });
     });
   });
 
-  describe('Subscription methods', function() {
-    describe('retrieveSubscription', function() {
-      it('Sends the correct request', function() {
+  describe('Subscription methods', () => {
+    describe('retrieveSubscription', () => {
+      it('Sends the correct request', () => {
         stripe.customers.retrieveSubscription('cus_123', 'sub_123');
         expect(stripe.LAST_REQUEST).to.deep.equal({
           method: 'GET',
@@ -637,7 +636,7 @@ describe('Customers Resource', function() {
         });
       });
 
-      it('Sends the correct request [with specified auth]', function() {
+      it('Sends the correct request [with specified auth]', () => {
         stripe.customers.retrieveSubscription(
           'cus_123',
           'sub_123',
@@ -653,8 +652,8 @@ describe('Customers Resource', function() {
       });
     });
 
-    describe('createSubscription', function() {
-      it('Sends the correct request', function() {
+    describe('createSubscription', () => {
+      it('Sends the correct request', () => {
         stripe.customers.createSubscription('cus_123', {
           plan: 'gold',
           quantity: '12',
@@ -667,7 +666,7 @@ describe('Customers Resource', function() {
         });
       });
 
-      it('Sends the correct request [with specified auth]', function() {
+      it('Sends the correct request [with specified auth]', () => {
         stripe.customers.createSubscription(
           'cus_123',
           {
@@ -686,8 +685,8 @@ describe('Customers Resource', function() {
       });
     });
 
-    describe('updateSubscription (new-style api)', function() {
-      it('Sends the correct request', function() {
+    describe('updateSubscription (new-style api)', () => {
+      it('Sends the correct request', () => {
         stripe.customers.updateSubscription('cus_123', 'sub_123', {
           quantity: '2',
         });
@@ -699,7 +698,7 @@ describe('Customers Resource', function() {
         });
       });
 
-      it('Sends the correct request [with specified auth]', function() {
+      it('Sends the correct request [with specified auth]', () => {
         stripe.customers.updateSubscription(
           'cus_123',
           'sub_123',
@@ -718,8 +717,8 @@ describe('Customers Resource', function() {
       });
     });
 
-    describe('cancelSubscription (new-style api)', function() {
-      it('Sends the correct request', function() {
+    describe('cancelSubscription (new-style api)', () => {
+      it('Sends the correct request', () => {
         stripe.customers.cancelSubscription('cus_123', 'sub_123');
         expect(stripe.LAST_REQUEST).to.deep.equal({
           method: 'DELETE',
@@ -729,7 +728,7 @@ describe('Customers Resource', function() {
         });
       });
 
-      it('Sends the correct request [with specified auth]', function() {
+      it('Sends the correct request [with specified auth]', () => {
         stripe.customers.cancelSubscription(
           'cus_123',
           'sub_123',
@@ -744,8 +743,8 @@ describe('Customers Resource', function() {
         });
       });
 
-      describe('With at_period_end defined', function() {
-        it('Sends the correct request', function() {
+      describe('With at_period_end defined', () => {
+        it('Sends the correct request', () => {
           stripe.customers.cancelSubscription('cus_123', 'sub_123', {
             at_period_end: true,
           });
@@ -758,8 +757,8 @@ describe('Customers Resource', function() {
         });
       });
 
-      describe('With at_period_end defined [with specified auth]', function() {
-        it('Sends the correct request', function() {
+      describe('With at_period_end defined [with specified auth]', () => {
+        it('Sends the correct request', () => {
           stripe.customers.cancelSubscription(
             'cus_123',
             'sub_123',
@@ -777,8 +776,8 @@ describe('Customers Resource', function() {
       });
     });
 
-    describe('listSubscriptions', function() {
-      it('Sends the correct request', function() {
+    describe('listSubscriptions', () => {
+      it('Sends the correct request', () => {
         stripe.customers.listSubscriptions('cus_123');
         expect(stripe.LAST_REQUEST).to.deep.equal({
           method: 'GET',
@@ -788,7 +787,7 @@ describe('Customers Resource', function() {
         });
       });
 
-      it('Sends the correct request [with specified auth]', function() {
+      it('Sends the correct request [with specified auth]', () => {
         stripe.customers.listSubscriptions('cus_123', TEST_AUTH_KEY);
         expect(stripe.LAST_REQUEST).to.deep.equal({
           method: 'GET',
@@ -801,9 +800,9 @@ describe('Customers Resource', function() {
     });
   });
 
-  describe('TaxId methods', function() {
-    describe('retrieveTaxId', function() {
-      it('Sends the correct request', function() {
+  describe('TaxId methods', () => {
+    describe('retrieveTaxId', () => {
+      it('Sends the correct request', () => {
         stripe.customers.retrieveTaxId('cus_123', 'txi_123');
         expect(stripe.LAST_REQUEST).to.deep.equal({
           method: 'GET',
@@ -814,9 +813,9 @@ describe('Customers Resource', function() {
       });
     });
 
-    describe('createTaxId', function() {
-      it('Sends the correct request', function() {
-        var data = {
+    describe('createTaxId', () => {
+      it('Sends the correct request', () => {
+        const data = {
           type: 'eu_vat',
           value: '11111',
         };
@@ -825,13 +824,13 @@ describe('Customers Resource', function() {
           method: 'POST',
           url: '/v1/customers/cus_123/tax_ids',
           headers: {},
-          data: data,
+          data,
         });
       });
     });
 
-    describe('deleteTaxId', function() {
-      it('Sends the correct request', function() {
+    describe('deleteTaxId', () => {
+      it('Sends the correct request', () => {
         stripe.customers.deleteTaxId('cus_123', 'txi_123');
         expect(stripe.LAST_REQUEST).to.deep.equal({
           method: 'DELETE',
@@ -842,8 +841,8 @@ describe('Customers Resource', function() {
       });
     });
 
-    describe('listTaxIds', function() {
-      it('Sends the correct request', function() {
+    describe('listTaxIds', () => {
+      it('Sends the correct request', () => {
         stripe.customers.listTaxIds('cus_123');
         expect(stripe.LAST_REQUEST).to.deep.equal({
           method: 'GET',
diff --git a/test/resources/Disputes.spec.js b/test/resources/Disputes.spec.js
index d227adfe3d..6bee3ed7f8 100644
--- a/test/resources/Disputes.spec.js
+++ b/test/resources/Disputes.spec.js
@@ -1,11 +1,11 @@
 'use strict';
 
-var stripe = require('../../testUtils').getSpyableStripe();
-var expect = require('chai').expect;
+const stripe = require('../../testUtils').getSpyableStripe();
+const expect = require('chai').expect;
 
-describe('Dispute Resource', function() {
-  describe('retrieve', function() {
-    it('Sends the correct request', function() {
+describe('Dispute Resource', () => {
+  describe('retrieve', () => {
+    it('Sends the correct request', () => {
       stripe.disputes.retrieve('dp_123');
       expect(stripe.LAST_REQUEST).to.deep.equal({
         method: 'GET',
@@ -16,8 +16,8 @@ describe('Dispute Resource', function() {
     });
   });
 
-  describe('list', function() {
-    it('Sends the correct request', function() {
+  describe('list', () => {
+    it('Sends the correct request', () => {
       stripe.disputes.list();
       expect(stripe.LAST_REQUEST).to.deep.equal({
         method: 'GET',
@@ -28,8 +28,8 @@ describe('Dispute Resource', function() {
     });
   });
 
-  describe('close', function() {
-    it('Sends the correct request', function() {
+  describe('close', () => {
+    it('Sends the correct request', () => {
       stripe.disputes.close('dp_123');
       expect(stripe.LAST_REQUEST).to.deep.equal({
         method: 'POST',
@@ -40,8 +40,8 @@ describe('Dispute Resource', function() {
     });
   });
 
-  describe('update', function() {
-    it('Sends the correct request', function() {
+  describe('update', () => {
+    it('Sends the correct request', () => {
       stripe.disputes.update('dp_123', {evidence: {customer_name: 'Bob'}});
       expect(stripe.LAST_REQUEST).to.deep.equal({
         method: 'POST',
diff --git a/test/resources/EphemeralKeys.spec.js b/test/resources/EphemeralKeys.spec.js
index 565a066186..1a5a2ca171 100644
--- a/test/resources/EphemeralKeys.spec.js
+++ b/test/resources/EphemeralKeys.spec.js
@@ -1,7 +1,7 @@
 'use strict';
 
-var stripe = require('../../testUtils').getSpyableStripe();
-var expect = require('chai').expect;
+const stripe = require('../../testUtils').getSpyableStripe();
+const expect = require('chai').expect;
 
 function errorsOnNoStripeVersion() {
   return expect(
@@ -27,9 +27,9 @@ function sendsCorrectStripeVersion() {
   });
 }
 
-describe('EphemeralKey Resource', function() {
-  describe('create', function() {
-    it('Sends the correct request', function() {
+describe('EphemeralKey Resource', () => {
+  describe('create', () => {
+    it('Sends the correct request', () => {
       stripe.ephemeralKeys.create(
         {customer: 'cus_123'},
         {stripe_version: '2017-05-25'}
@@ -46,7 +46,7 @@ describe('EphemeralKey Resource', function() {
       });
     });
 
-    describe('when an api version is set', function() {
+    describe('when an api version is set', () => {
       beforeEach(function() {
         this.oldVersion = stripe.getApiField('version');
         stripe.setApiVersion('2017-05-25');
@@ -56,16 +56,15 @@ describe('EphemeralKey Resource', function() {
         stripe.setApiVersion(this.oldVersion);
       });
 
-      it('Errors if no stripe-version is specified', function() {
-        return errorsOnNoStripeVersion();
-      });
+      it('Errors if no stripe-version is specified', () =>
+        errorsOnNoStripeVersion());
 
-      it('Sends the correct stripe-version', function() {
+      it('Sends the correct stripe-version', () => {
         sendsCorrectStripeVersion();
       });
     });
 
-    describe('when no api version is set', function() {
+    describe('when no api version is set', () => {
       beforeEach(function() {
         this.oldVersion = stripe.getApiField('version');
         stripe.setApiVersion(null);
@@ -75,18 +74,17 @@ describe('EphemeralKey Resource', function() {
         stripe.setApiVersion(this.oldVersion);
       });
 
-      it('Errors if no stripe-version is specified', function() {
-        return errorsOnNoStripeVersion();
-      });
+      it('Errors if no stripe-version is specified', () =>
+        errorsOnNoStripeVersion());
 
-      it('Sends the correct stripe-version', function() {
+      it('Sends the correct stripe-version', () => {
         sendsCorrectStripeVersion();
       });
     });
   });
 
-  describe('delete', function() {
-    it('Sends the correct request', function() {
+  describe('delete', () => {
+    it('Sends the correct request', () => {
       stripe.ephemeralKeys.del('ephkey_123');
       expect(stripe.LAST_REQUEST).to.deep.equal({
         method: 'DELETE',
diff --git a/test/resources/Events.spec.js b/test/resources/Events.spec.js
index c5e77fdfdb..28fbe98ed4 100644
--- a/test/resources/Events.spec.js
+++ b/test/resources/Events.spec.js
@@ -1,11 +1,11 @@
 'use strict';
 
-var stripe = require('../../testUtils').getSpyableStripe();
-var expect = require('chai').expect;
+const stripe = require('../../testUtils').getSpyableStripe();
+const expect = require('chai').expect;
 
-describe('Events Resource', function() {
-  describe('retrieve', function() {
-    it('Sends the correct request', function() {
+describe('Events Resource', () => {
+  describe('retrieve', () => {
+    it('Sends the correct request', () => {
       stripe.events.retrieve('eventIdBaz');
       expect(stripe.LAST_REQUEST).to.deep.equal({
         method: 'GET',
@@ -16,8 +16,8 @@ describe('Events Resource', function() {
     });
   });
 
-  describe('list', function() {
-    it('Sends the correct request', function() {
+  describe('list', () => {
+    it('Sends the correct request', () => {
       stripe.events.list({count: 25});
       expect(stripe.LAST_REQUEST).to.deep.equal({
         method: 'GET',
diff --git a/test/resources/ExchangeRates.spec.js b/test/resources/ExchangeRates.spec.js
index 9f7108b77f..233119cd57 100644
--- a/test/resources/ExchangeRates.spec.js
+++ b/test/resources/ExchangeRates.spec.js
@@ -1,11 +1,11 @@
 'use strict';
 
-var stripe = require('../../testUtils').getSpyableStripe();
-var expect = require('chai').expect;
+const stripe = require('../../testUtils').getSpyableStripe();
+const expect = require('chai').expect;
 
-describe('ExchangeRates Resource', function() {
-  describe('list', function() {
-    it('Sends the correct request', function() {
+describe('ExchangeRates Resource', () => {
+  describe('list', () => {
+    it('Sends the correct request', () => {
       stripe.exchangeRates.list();
       expect(stripe.LAST_REQUEST).to.deep.equal({
         method: 'GET',
@@ -16,13 +16,13 @@ describe('ExchangeRates Resource', function() {
     });
   });
 
-  describe('retrieve', function() {
-    it('Sends the correct request', function() {
-      var currency = 'USD';
+  describe('retrieve', () => {
+    it('Sends the correct request', () => {
+      const currency = 'USD';
       stripe.exchangeRates.retrieve(currency);
       expect(stripe.LAST_REQUEST).to.deep.equal({
         method: 'GET',
-        url: '/v1/exchange_rates/' + currency,
+        url: `/v1/exchange_rates/${currency}`,
         data: {},
         headers: {},
       });
diff --git a/test/resources/FileLinks.spec.js b/test/resources/FileLinks.spec.js
index 8bf6677b21..1071d22ec0 100644
--- a/test/resources/FileLinks.spec.js
+++ b/test/resources/FileLinks.spec.js
@@ -1,11 +1,11 @@
 'use strict';
 
-var stripe = require('../../testUtils').getSpyableStripe();
-var expect = require('chai').expect;
+const stripe = require('../../testUtils').getSpyableStripe();
+const expect = require('chai').expect;
 
-describe('FileLinks Resource', function() {
-  describe('retrieve', function() {
-    it('Sends the correct request', function() {
+describe('FileLinks Resource', () => {
+  describe('retrieve', () => {
+    it('Sends the correct request', () => {
       stripe.fileLinks.retrieve('link_123');
       expect(stripe.LAST_REQUEST).to.deep.equal({
         method: 'GET',
@@ -16,8 +16,8 @@ describe('FileLinks Resource', function() {
     });
   });
 
-  describe('create', function() {
-    it('Sends the correct request', function() {
+  describe('create', () => {
+    it('Sends the correct request', () => {
       stripe.fileLinks.create({file: 'file_123'});
       expect(stripe.LAST_REQUEST).to.deep.equal({
         method: 'POST',
@@ -28,8 +28,8 @@ describe('FileLinks Resource', function() {
     });
   });
 
-  describe('update', function() {
-    it('Sends the correct request', function() {
+  describe('update', () => {
+    it('Sends the correct request', () => {
       stripe.fileLinks.update('link_123', {metadata: {key: 'value'}});
       expect(stripe.LAST_REQUEST).to.deep.equal({
         method: 'POST',
@@ -40,8 +40,8 @@ describe('FileLinks Resource', function() {
     });
   });
 
-  describe('list', function() {
-    it('Sends the correct request', function() {
+  describe('list', () => {
+    it('Sends the correct request', () => {
       stripe.fileLinks.list();
       expect(stripe.LAST_REQUEST).to.deep.equal({
         method: 'GET',
diff --git a/test/resources/FileUploads.spec.js b/test/resources/FileUploads.spec.js
index 1a846356be..06669b84ed 100644
--- a/test/resources/FileUploads.spec.js
+++ b/test/resources/FileUploads.spec.js
@@ -1,15 +1,15 @@
 'use strict';
 
-var stripe = require('../../testUtils').getSpyableStripe();
-var expect = require('chai').expect;
-var fs = require('fs');
-var path = require('path');
+const stripe = require('../../testUtils').getSpyableStripe();
+const expect = require('chai').expect;
+const fs = require('fs');
+const path = require('path');
 
-var TEST_AUTH_KEY = 'aGN0bIwXnHdw5645VABjPdSn8nWY7G11';
+const TEST_AUTH_KEY = 'aGN0bIwXnHdw5645VABjPdSn8nWY7G11';
 
-describe('File Uploads Resource', function() {
-  describe('retrieve', function() {
-    it('Sends the correct request', function() {
+describe('File Uploads Resource', () => {
+  describe('retrieve', () => {
+    it('Sends the correct request', () => {
       stripe.fileUploads.retrieve('fil_12345');
       expect(stripe.LAST_REQUEST).to.deep.equal({
         method: 'GET',
@@ -19,7 +19,7 @@ describe('File Uploads Resource', function() {
       });
     });
 
-    it('Sends the correct request [with specified auth]', function() {
+    it('Sends the correct request [with specified auth]', () => {
       stripe.fileUploads.retrieve('fil_12345', TEST_AUTH_KEY);
       expect(stripe.LAST_REQUEST).to.deep.equal({
         method: 'GET',
@@ -31,8 +31,8 @@ describe('File Uploads Resource', function() {
     });
   });
 
-  describe('list', function() {
-    it('Sends the correct request', function() {
+  describe('list', () => {
+    it('Sends the correct request', () => {
       stripe.fileUploads.list();
       expect(stripe.LAST_REQUEST).to.deep.equal({
         method: 'GET',
@@ -43,10 +43,10 @@ describe('File Uploads Resource', function() {
     });
   });
 
-  describe('create', function() {
-    it('Sends the correct file upload request', function() {
-      var testFilename = path.join(__dirname, 'data/minimal.pdf');
-      var f = fs.readFileSync(testFilename);
+  describe('create', () => {
+    it('Sends the correct file upload request', () => {
+      const testFilename = path.join(__dirname, 'data/minimal.pdf');
+      const f = fs.readFileSync(testFilename);
 
       stripe.fileUploads.create({
         purpose: 'dispute_evidence',
@@ -63,9 +63,9 @@ describe('File Uploads Resource', function() {
       expect(stripe.LAST_REQUEST).to.deep.property('url', '/v1/files');
     });
 
-    it('Sends the correct file upload request [with specified auth]', function() {
-      var testFilename = path.join(__dirname, 'data/minimal.pdf');
-      var f = fs.readFileSync(testFilename);
+    it('Sends the correct file upload request [with specified auth]', () => {
+      const testFilename = path.join(__dirname, 'data/minimal.pdf');
+      const f = fs.readFileSync(testFilename);
 
       stripe.fileUploads.create(
         {
@@ -86,9 +86,9 @@ describe('File Uploads Resource', function() {
       expect(stripe.LAST_REQUEST).to.deep.property('auth', TEST_AUTH_KEY);
     });
 
-    it('Streams a file and sends the correct file upload request', function() {
-      var testFilename = path.join(__dirname, 'data/minimal.pdf');
-      var f = fs.createReadStream(testFilename);
+    it('Streams a file and sends the correct file upload request', () => {
+      const testFilename = path.join(__dirname, 'data/minimal.pdf');
+      const f = fs.createReadStream(testFilename);
 
       return stripe.fileUploads
         .create({
@@ -100,7 +100,7 @@ describe('File Uploads Resource', function() {
           },
           file_link_data: {create: true},
         })
-        .then(function() {
+        .then(() => {
           expect(stripe.LAST_REQUEST).to.deep.property(
             'host',
             'files.stripe.com'
@@ -110,9 +110,9 @@ describe('File Uploads Resource', function() {
         });
     });
 
-    it('Streams a file and sends the correct file upload request [with specified auth]', function() {
-      var testFilename = path.join(__dirname, 'data/minimal.pdf');
-      var f = fs.createReadStream(testFilename);
+    it('Streams a file and sends the correct file upload request [with specified auth]', () => {
+      const testFilename = path.join(__dirname, 'data/minimal.pdf');
+      const f = fs.createReadStream(testFilename);
 
       return stripe.fileUploads
         .create(
@@ -127,7 +127,7 @@ describe('File Uploads Resource', function() {
           },
           TEST_AUTH_KEY
         )
-        .then(function() {
+        .then(() => {
           expect(stripe.LAST_REQUEST).to.deep.property(
             'host',
             'files.stripe.com'
diff --git a/test/resources/Files.spec.js b/test/resources/Files.spec.js
index 280eec92c1..52487ffe07 100644
--- a/test/resources/Files.spec.js
+++ b/test/resources/Files.spec.js
@@ -1,15 +1,15 @@
 'use strict';
 
-var stripe = require('../../testUtils').getSpyableStripe();
-var expect = require('chai').expect;
-var fs = require('fs');
-var path = require('path');
+const stripe = require('../../testUtils').getSpyableStripe();
+const expect = require('chai').expect;
+const fs = require('fs');
+const path = require('path');
 
-var TEST_AUTH_KEY = 'aGN0bIwXnHdw5645VABjPdSn8nWY7G11';
+const TEST_AUTH_KEY = 'aGN0bIwXnHdw5645VABjPdSn8nWY7G11';
 
-describe('Files Resource', function() {
-  describe('retrieve', function() {
-    it('Sends the correct request', function() {
+describe('Files Resource', () => {
+  describe('retrieve', () => {
+    it('Sends the correct request', () => {
       stripe.files.retrieve('fil_12345');
       expect(stripe.LAST_REQUEST).to.deep.equal({
         method: 'GET',
@@ -19,7 +19,7 @@ describe('Files Resource', function() {
       });
     });
 
-    it('Sends the correct request [with specified auth]', function() {
+    it('Sends the correct request [with specified auth]', () => {
       stripe.files.retrieve('fil_12345', TEST_AUTH_KEY);
       expect(stripe.LAST_REQUEST).to.deep.equal({
         method: 'GET',
@@ -31,8 +31,8 @@ describe('Files Resource', function() {
     });
   });
 
-  describe('list', function() {
-    it('Sends the correct request', function() {
+  describe('list', () => {
+    it('Sends the correct request', () => {
       stripe.files.list();
       expect(stripe.LAST_REQUEST).to.deep.equal({
         method: 'GET',
@@ -43,10 +43,10 @@ describe('Files Resource', function() {
     });
   });
 
-  describe('create', function() {
-    it('Sends the correct file upload request', function() {
-      var testFilename = path.join(__dirname, 'data/minimal.pdf');
-      var f = fs.readFileSync(testFilename);
+  describe('create', () => {
+    it('Sends the correct file upload request', () => {
+      const testFilename = path.join(__dirname, 'data/minimal.pdf');
+      const f = fs.readFileSync(testFilename);
 
       stripe.files.create({
         purpose: 'dispute_evidence',
@@ -63,9 +63,9 @@ describe('Files Resource', function() {
       expect(stripe.LAST_REQUEST).to.deep.property('url', '/v1/files');
     });
 
-    it('Sends the correct file upload request [with specified auth]', function() {
-      var testFilename = path.join(__dirname, 'data/minimal.pdf');
-      var f = fs.readFileSync(testFilename);
+    it('Sends the correct file upload request [with specified auth]', () => {
+      const testFilename = path.join(__dirname, 'data/minimal.pdf');
+      const f = fs.readFileSync(testFilename);
 
       stripe.files.create(
         {
@@ -86,9 +86,9 @@ describe('Files Resource', function() {
       expect(stripe.LAST_REQUEST).to.deep.property('auth', TEST_AUTH_KEY);
     });
 
-    it('Streams a file and sends the correct file upload request', function() {
-      var testFilename = path.join(__dirname, 'data/minimal.pdf');
-      var f = fs.createReadStream(testFilename);
+    it('Streams a file and sends the correct file upload request', () => {
+      const testFilename = path.join(__dirname, 'data/minimal.pdf');
+      const f = fs.createReadStream(testFilename);
 
       return stripe.files
         .create({
@@ -100,7 +100,7 @@ describe('Files Resource', function() {
           },
           file_link_data: {create: true},
         })
-        .then(function() {
+        .then(() => {
           expect(stripe.LAST_REQUEST).to.deep.property(
             'host',
             'files.stripe.com'
@@ -110,9 +110,9 @@ describe('Files Resource', function() {
         });
     });
 
-    it('Streams a file and sends the correct file upload request [with specified auth]', function() {
-      var testFilename = path.join(__dirname, 'data/minimal.pdf');
-      var f = fs.createReadStream(testFilename);
+    it('Streams a file and sends the correct file upload request [with specified auth]', () => {
+      const testFilename = path.join(__dirname, 'data/minimal.pdf');
+      const f = fs.createReadStream(testFilename);
 
       return stripe.files
         .create(
@@ -127,7 +127,7 @@ describe('Files Resource', function() {
           },
           TEST_AUTH_KEY
         )
-        .then(function() {
+        .then(() => {
           expect(stripe.LAST_REQUEST).to.deep.property(
             'host',
             'files.stripe.com'
diff --git a/test/resources/InvoiceItems.spec.js b/test/resources/InvoiceItems.spec.js
index 6a6352968a..61de4fd338 100644
--- a/test/resources/InvoiceItems.spec.js
+++ b/test/resources/InvoiceItems.spec.js
@@ -1,11 +1,11 @@
 'use strict';
 
-var stripe = require('../../testUtils').getSpyableStripe();
-var expect = require('chai').expect;
+const stripe = require('../../testUtils').getSpyableStripe();
+const expect = require('chai').expect;
 
-describe('InvoiceItems Resource', function() {
-  describe('retrieve', function() {
-    it('Sends the correct request', function() {
+describe('InvoiceItems Resource', () => {
+  describe('retrieve', () => {
+    it('Sends the correct request', () => {
       stripe.invoiceItems.retrieve('invoiceItemIdTesting123');
       expect(stripe.LAST_REQUEST).to.deep.equal({
         method: 'GET',
@@ -16,8 +16,8 @@ describe('InvoiceItems Resource', function() {
     });
   });
 
-  describe('create', function() {
-    it('Sends the correct request', function() {
+  describe('create', () => {
+    it('Sends the correct request', () => {
       stripe.invoiceItems.create({
         customer: 'cust_id_888',
       });
@@ -30,8 +30,8 @@ describe('InvoiceItems Resource', function() {
     });
   });
 
-  describe('update', function() {
-    it('Sends the correct request', function() {
+  describe('update', () => {
+    it('Sends the correct request', () => {
       stripe.invoiceItems.update('invoiceItemId1', {
         amount: 1900,
       });
@@ -44,8 +44,8 @@ describe('InvoiceItems Resource', function() {
     });
   });
 
-  describe('del', function() {
-    it('Sends the correct request', function() {
+  describe('del', () => {
+    it('Sends the correct request', () => {
       stripe.invoiceItems.del('invoiceItemId2');
       expect(stripe.LAST_REQUEST).to.deep.equal({
         method: 'DELETE',
@@ -56,8 +56,8 @@ describe('InvoiceItems Resource', function() {
     });
   });
 
-  describe('list', function() {
-    it('Sends the correct request', function() {
+  describe('list', () => {
+    it('Sends the correct request', () => {
       stripe.invoiceItems.list();
       expect(stripe.LAST_REQUEST).to.deep.equal({
         method: 'GET',
diff --git a/test/resources/Invoices.spec.js b/test/resources/Invoices.spec.js
index cfd8657b2d..b6598d8c38 100644
--- a/test/resources/Invoices.spec.js
+++ b/test/resources/Invoices.spec.js
@@ -1,11 +1,11 @@
 'use strict';
 
-var stripe = require('../../testUtils').getSpyableStripe();
-var expect = require('chai').expect;
+const stripe = require('../../testUtils').getSpyableStripe();
+const expect = require('chai').expect;
 
-describe('Invoices Resource', function() {
-  describe('retrieve', function() {
-    it('Sends the correct request', function() {
+describe('Invoices Resource', () => {
+  describe('retrieve', () => {
+    it('Sends the correct request', () => {
       stripe.invoices.retrieve('in_123');
       expect(stripe.LAST_REQUEST).to.deep.equal({
         method: 'GET',
@@ -16,8 +16,8 @@ describe('Invoices Resource', function() {
     });
   });
 
-  describe('create', function() {
-    it('Sends the correct request', function() {
+  describe('create', () => {
+    it('Sends the correct request', () => {
       stripe.invoices.create({application_fee: 111});
       expect(stripe.LAST_REQUEST).to.deep.equal({
         method: 'POST',
@@ -28,8 +28,8 @@ describe('Invoices Resource', function() {
     });
   });
 
-  describe('list', function() {
-    it('Sends the correct request', function() {
+  describe('list', () => {
+    it('Sends the correct request', () => {
       stripe.invoices.list({count: 25});
       expect(stripe.LAST_REQUEST).to.deep.equal({
         method: 'GET',
@@ -40,8 +40,8 @@ describe('Invoices Resource', function() {
     });
   });
 
-  describe('update', function() {
-    it('Sends the correct request', function() {
+  describe('update', () => {
+    it('Sends the correct request', () => {
       stripe.invoices.update('in_123', {application_fee: 200});
       expect(stripe.LAST_REQUEST).to.deep.equal({
         method: 'POST',
@@ -52,8 +52,8 @@ describe('Invoices Resource', function() {
     });
   });
 
-  describe('del', function() {
-    it('Sends the correct request', function() {
+  describe('del', () => {
+    it('Sends the correct request', () => {
       stripe.invoices.del('in_123');
       expect(stripe.LAST_REQUEST).to.deep.equal({
         method: 'DELETE',
@@ -64,8 +64,8 @@ describe('Invoices Resource', function() {
     });
   });
 
-  describe('retrieveLines', function() {
-    it('Sends the correct request', function() {
+  describe('retrieveLines', () => {
+    it('Sends the correct request', () => {
       stripe.invoices.retrieveLines('in_123');
       expect(stripe.LAST_REQUEST).to.deep.equal({
         method: 'GET',
@@ -76,9 +76,9 @@ describe('Invoices Resource', function() {
     });
   });
 
-  describe('retrieveUpcoming', function() {
-    describe('With just a customer ID', function() {
-      it('Sends the correct request', function() {
+  describe('retrieveUpcoming', () => {
+    describe('With just a customer ID', () => {
+      it('Sends the correct request', () => {
         stripe.invoices.retrieveUpcoming('cus_123');
         expect(stripe.LAST_REQUEST).to.deep.equal({
           method: 'GET',
@@ -89,8 +89,8 @@ describe('Invoices Resource', function() {
       });
     });
 
-    describe('With a subscription ID in addition to a customer ID', function() {
-      it('Sends the correct request', function() {
+    describe('With a subscription ID in addition to a customer ID', () => {
+      it('Sends the correct request', () => {
         stripe.invoices.retrieveUpcoming('cus_123', 'sub_123');
         expect(stripe.LAST_REQUEST).to.deep.equal({
           method: 'GET',
@@ -101,8 +101,8 @@ describe('Invoices Resource', function() {
       });
     });
 
-    describe('With an options object that includes `subscription_items`', function() {
-      it('Sends the correct request', function() {
+    describe('With an options object that includes `subscription_items`', () => {
+      it('Sends the correct request', () => {
         stripe.invoices.retrieveUpcoming('cus_123', {
           subscription_items: [{plan: 'potato'}, {plan: 'rutabaga'}],
         });
@@ -118,8 +118,8 @@ describe('Invoices Resource', function() {
       });
     });
 
-    describe('Without a customer id but options', function() {
-      it('Sends the correct request', function() {
+    describe('Without a customer id but options', () => {
+      it('Sends the correct request', () => {
         stripe.invoices.retrieveUpcoming({
           customer: 'cus_abc',
           subscription_items: [{plan: 'potato'}, {plan: 'rutabaga'}],
@@ -136,8 +136,8 @@ describe('Invoices Resource', function() {
       });
     });
 
-    describe('With an options object that includes `subscription_items` in addition to a subscription ID', function() {
-      it('Sends the correct request', function() {
+    describe('With an options object that includes `subscription_items` in addition to a subscription ID', () => {
+      it('Sends the correct request', () => {
         stripe.invoices.retrieveUpcoming('cus_123', 'sub_123', {
           subscription_items: [
             {plan: 'potato'},
@@ -163,8 +163,8 @@ describe('Invoices Resource', function() {
       });
     });
 
-    describe('With a options object in addition to a customer ID', function() {
-      it('Sends the correct request', function() {
+    describe('With a options object in addition to a customer ID', () => {
+      it('Sends the correct request', () => {
         stripe.invoices.retrieveUpcoming('cus_123', {plan: 'planId123'});
         expect(stripe.LAST_REQUEST).to.deep.equal({
           method: 'GET',
@@ -176,8 +176,8 @@ describe('Invoices Resource', function() {
     });
   });
 
-  describe('finalizeInvoice', function() {
-    it('Sends the correct request', function() {
+  describe('finalizeInvoice', () => {
+    it('Sends the correct request', () => {
       stripe.invoices.finalizeInvoice('in_123');
       expect(stripe.LAST_REQUEST).to.deep.equal({
         method: 'POST',
@@ -188,8 +188,8 @@ describe('Invoices Resource', function() {
     });
   });
 
-  describe('mark uncollectible', function() {
-    it('Sends the correct request', function() {
+  describe('mark uncollectible', () => {
+    it('Sends the correct request', () => {
       stripe.invoices.markUncollectible('in_123');
       expect(stripe.LAST_REQUEST).to.deep.equal({
         method: 'POST',
@@ -200,8 +200,8 @@ describe('Invoices Resource', function() {
     });
   });
 
-  describe('pay', function() {
-    it('Sends the correct request', function() {
+  describe('pay', () => {
+    it('Sends the correct request', () => {
       stripe.invoices.pay('in_123', {
         source: 'tok_FooBar',
       });
@@ -214,8 +214,8 @@ describe('Invoices Resource', function() {
     });
   });
 
-  describe('sendInvoice', function() {
-    it('Sends the correct request', function() {
+  describe('sendInvoice', () => {
+    it('Sends the correct request', () => {
       stripe.invoices.sendInvoice('in_123');
       expect(stripe.LAST_REQUEST).to.deep.equal({
         method: 'POST',
@@ -226,8 +226,8 @@ describe('Invoices Resource', function() {
     });
   });
 
-  describe('voidInvoice', function() {
-    it('Sends the correct request', function() {
+  describe('voidInvoice', () => {
+    it('Sends the correct request', () => {
       stripe.invoices.voidInvoice('in_123');
       expect(stripe.LAST_REQUEST).to.deep.equal({
         method: 'POST',
diff --git a/test/resources/IssuerFraudRecords.spec.js b/test/resources/IssuerFraudRecords.spec.js
index 46842348c7..6a4bd87265 100644
--- a/test/resources/IssuerFraudRecords.spec.js
+++ b/test/resources/IssuerFraudRecords.spec.js
@@ -1,11 +1,11 @@
 'use strict';
 
-var stripe = require('../../testUtils').getSpyableStripe();
-var expect = require('chai').expect;
+const stripe = require('../../testUtils').getSpyableStripe();
+const expect = require('chai').expect;
 
-describe('IssuerFraudRecord Resource', function() {
-  describe('retrieve', function() {
-    it('Sends the correct request', function() {
+describe('IssuerFraudRecord Resource', () => {
+  describe('retrieve', () => {
+    it('Sends the correct request', () => {
       stripe.issuerFraudRecords.retrieve('issfr_123');
       expect(stripe.LAST_REQUEST).to.deep.equal({
         method: 'GET',
@@ -16,8 +16,8 @@ describe('IssuerFraudRecord Resource', function() {
     });
   });
 
-  describe('list', function() {
-    it('Sends the correct request', function() {
+  describe('list', () => {
+    it('Sends the correct request', () => {
       stripe.issuerFraudRecords.list();
       expect(stripe.LAST_REQUEST).to.deep.equal({
         method: 'GET',
diff --git a/test/resources/Issuing/Authorization.spec.js b/test/resources/Issuing/Authorization.spec.js
index bb3e3d5d01..e58774d59f 100644
--- a/test/resources/Issuing/Authorization.spec.js
+++ b/test/resources/Issuing/Authorization.spec.js
@@ -1,12 +1,12 @@
 'use strict';
 
-var stripe = require('../../../testUtils').getSpyableStripe();
-var expect = require('chai').expect;
+const stripe = require('../../../testUtils').getSpyableStripe();
+const expect = require('chai').expect;
 
-describe('Issuing', function() {
-  describe('Authorization Resource', function() {
-    describe('retrieve', function() {
-      it('Sends the correct request', function() {
+describe('Issuing', () => {
+  describe('Authorization Resource', () => {
+    describe('retrieve', () => {
+      it('Sends the correct request', () => {
         stripe.issuing.authorizations.retrieve('iauth_123');
         expect(stripe.LAST_REQUEST).to.deep.equal({
           method: 'GET',
@@ -17,8 +17,8 @@ describe('Issuing', function() {
       });
     });
 
-    describe('list', function() {
-      it('Sends the correct request', function() {
+    describe('list', () => {
+      it('Sends the correct request', () => {
         stripe.issuing.authorizations.list();
         expect(stripe.LAST_REQUEST).to.deep.equal({
           method: 'GET',
@@ -29,8 +29,8 @@ describe('Issuing', function() {
       });
     });
 
-    describe('update', function() {
-      it('Sends the correct request', function() {
+    describe('update', () => {
+      it('Sends the correct request', () => {
         stripe.issuing.authorizations.update('iauth_123', {
           metadata: {
             thing1: true,
@@ -51,8 +51,8 @@ describe('Issuing', function() {
       });
     });
 
-    describe('approve', function() {
-      it('Sends the correct request', function() {
+    describe('approve', () => {
+      it('Sends the correct request', () => {
         stripe.issuing.authorizations.approve('iauth_123');
         expect(stripe.LAST_REQUEST).to.deep.equal({
           method: 'POST',
@@ -63,8 +63,8 @@ describe('Issuing', function() {
       });
     });
 
-    describe('decline', function() {
-      it('Sends the correct request', function() {
+    describe('decline', () => {
+      it('Sends the correct request', () => {
         stripe.issuing.authorizations.decline('iauth_123');
         expect(stripe.LAST_REQUEST).to.deep.equal({
           method: 'POST',
diff --git a/test/resources/Issuing/Cardholders.spec.js b/test/resources/Issuing/Cardholders.spec.js
index 646e735c40..16bf3b531b 100644
--- a/test/resources/Issuing/Cardholders.spec.js
+++ b/test/resources/Issuing/Cardholders.spec.js
@@ -1,13 +1,13 @@
 'use strict';
 
-var stripe = require('../../../testUtils').getSpyableStripe();
+const stripe = require('../../../testUtils').getSpyableStripe();
 
-var expect = require('chai').expect;
+const expect = require('chai').expect;
 
-describe('Issuing', function() {
-  describe('Cardholders Resource', function() {
-    describe('retrieve', function() {
-      it('Sends the correct request', function() {
+describe('Issuing', () => {
+  describe('Cardholders Resource', () => {
+    describe('retrieve', () => {
+      it('Sends the correct request', () => {
         stripe.issuing.cardholders.retrieve('ich_123');
 
         expect(stripe.LAST_REQUEST).to.deep.equal({
@@ -19,8 +19,8 @@ describe('Issuing', function() {
       });
     });
 
-    describe('create', function() {
-      it('Sends the correct request', function() {
+    describe('create', () => {
+      it('Sends the correct request', () => {
         stripe.issuing.cardholders.create({
           billing: {},
           name: 'Tim Testperson',
@@ -39,8 +39,8 @@ describe('Issuing', function() {
       });
     });
 
-    describe('update', function() {
-      it('Sends the correct request', function() {
+    describe('update', () => {
+      it('Sends the correct request', () => {
         stripe.issuing.cardholders.update('ich_123', {
           metadata: {
             thing1: true,
@@ -61,8 +61,8 @@ describe('Issuing', function() {
       });
     });
 
-    describe('list', function() {
-      it('Sends the correct request', function() {
+    describe('list', () => {
+      it('Sends the correct request', () => {
         stripe.issuing.cardholders.list();
         expect(stripe.LAST_REQUEST).to.deep.equal({
           method: 'GET',
diff --git a/test/resources/Issuing/Cards.spec.js b/test/resources/Issuing/Cards.spec.js
index 90e8111ff3..551667064b 100644
--- a/test/resources/Issuing/Cards.spec.js
+++ b/test/resources/Issuing/Cards.spec.js
@@ -1,13 +1,13 @@
 'use strict';
 
-var stripe = require('../../../testUtils').getSpyableStripe();
+const stripe = require('../../../testUtils').getSpyableStripe();
 
-var expect = require('chai').expect;
+const expect = require('chai').expect;
 
-describe('Issuing', function() {
-  describe('Cards Resource', function() {
-    describe('retrieve', function() {
-      it('Sends the correct request', function() {
+describe('Issuing', () => {
+  describe('Cards Resource', () => {
+    describe('retrieve', () => {
+      it('Sends the correct request', () => {
         stripe.issuing.cards.retrieve('ic_123');
 
         expect(stripe.LAST_REQUEST).to.deep.equal({
@@ -19,8 +19,8 @@ describe('Issuing', function() {
       });
     });
 
-    describe('create', function() {
-      it('Sends the correct request', function() {
+    describe('create', () => {
+      it('Sends the correct request', () => {
         stripe.issuing.cards.create({
           currency: 'usd',
           type: 'physical',
@@ -37,8 +37,8 @@ describe('Issuing', function() {
       });
     });
 
-    describe('update', function() {
-      it('Sends the correct request', function() {
+    describe('update', () => {
+      it('Sends the correct request', () => {
         stripe.issuing.cards.update('ic_123', {
           metadata: {
             thing1: true,
@@ -59,8 +59,8 @@ describe('Issuing', function() {
       });
     });
 
-    describe('list', function() {
-      it('Sends the correct request', function() {
+    describe('list', () => {
+      it('Sends the correct request', () => {
         stripe.issuing.cards.list();
         expect(stripe.LAST_REQUEST).to.deep.equal({
           method: 'GET',
@@ -72,9 +72,9 @@ describe('Issuing', function() {
     });
   });
 
-  describe('Virtual Cards Resource', function() {
-    describe('retrieve', function() {
-      it('Sends the correct request', function() {
+  describe('Virtual Cards Resource', () => {
+    describe('retrieve', () => {
+      it('Sends the correct request', () => {
         stripe.issuing.cards.retrieveDetails('ic_123');
 
         expect(stripe.LAST_REQUEST).to.deep.equal({
diff --git a/test/resources/Issuing/Disputes.spec.js b/test/resources/Issuing/Disputes.spec.js
index 2af1f0cd43..4ae2d3ac5a 100644
--- a/test/resources/Issuing/Disputes.spec.js
+++ b/test/resources/Issuing/Disputes.spec.js
@@ -1,13 +1,13 @@
 'use strict';
 
-var stripe = require('../../../testUtils').getSpyableStripe();
+const stripe = require('../../../testUtils').getSpyableStripe();
 
-var expect = require('chai').expect;
+const expect = require('chai').expect;
 
-describe('Issuing', function() {
-  describe('Disputes Resource', function() {
-    describe('retrieve', function() {
-      it('Sends the correct request', function() {
+describe('Issuing', () => {
+  describe('Disputes Resource', () => {
+    describe('retrieve', () => {
+      it('Sends the correct request', () => {
         stripe.issuing.disputes.retrieve('idp_123');
 
         expect(stripe.LAST_REQUEST).to.deep.equal({
@@ -19,8 +19,8 @@ describe('Issuing', function() {
       });
     });
 
-    describe('create', function() {
-      it('Sends the correct request', function() {
+    describe('create', () => {
+      it('Sends the correct request', () => {
         stripe.issuing.disputes.create({
           transaction: 'ipi_123',
         });
@@ -35,8 +35,8 @@ describe('Issuing', function() {
       });
     });
 
-    describe('update', function() {
-      it('Sends the correct request', function() {
+    describe('update', () => {
+      it('Sends the correct request', () => {
         stripe.issuing.disputes.update('idp_123', {
           metadata: {
             thing1: true,
@@ -57,8 +57,8 @@ describe('Issuing', function() {
       });
     });
 
-    describe('list', function() {
-      it('Sends the correct request', function() {
+    describe('list', () => {
+      it('Sends the correct request', () => {
         stripe.issuing.disputes.list();
         expect(stripe.LAST_REQUEST).to.deep.equal({
           method: 'GET',
diff --git a/test/resources/Issuing/Transactions.spec.js b/test/resources/Issuing/Transactions.spec.js
index 89e6ffac5f..2fa9854d56 100644
--- a/test/resources/Issuing/Transactions.spec.js
+++ b/test/resources/Issuing/Transactions.spec.js
@@ -1,13 +1,13 @@
 'use strict';
 
-var stripe = require('../../../testUtils').getSpyableStripe();
+const stripe = require('../../../testUtils').getSpyableStripe();
 
-var expect = require('chai').expect;
+const expect = require('chai').expect;
 
-describe('Issuing', function() {
-  describe('Transactions Resource', function() {
-    describe('retrieve', function() {
-      it('Sends the correct request', function() {
+describe('Issuing', () => {
+  describe('Transactions Resource', () => {
+    describe('retrieve', () => {
+      it('Sends the correct request', () => {
         stripe.issuing.transactions.retrieve('ipi_123');
 
         expect(stripe.LAST_REQUEST).to.deep.equal({
@@ -19,8 +19,8 @@ describe('Issuing', function() {
       });
     });
 
-    describe('update', function() {
-      it('Sends the correct request', function() {
+    describe('update', () => {
+      it('Sends the correct request', () => {
         stripe.issuing.transactions.update('ipi_123', {
           metadata: {
             thing1: true,
@@ -41,8 +41,8 @@ describe('Issuing', function() {
       });
     });
 
-    describe('list', function() {
-      it('Sends the correct request', function() {
+    describe('list', () => {
+      it('Sends the correct request', () => {
         stripe.issuing.transactions.list();
         expect(stripe.LAST_REQUEST).to.deep.equal({
           method: 'GET',
diff --git a/test/resources/LoginLinks.spec.js b/test/resources/LoginLinks.spec.js
index f46361259a..f8c6c36183 100644
--- a/test/resources/LoginLinks.spec.js
+++ b/test/resources/LoginLinks.spec.js
@@ -1,24 +1,24 @@
 'use strict';
 
-var resources = require('../../lib/stripe').resources;
-var stripe = require('../../testUtils').getSpyableStripe();
-var expect = require('chai').expect;
+const resources = require('../../lib/stripe').resources;
+const stripe = require('../../testUtils').getSpyableStripe();
+const expect = require('chai').expect;
 
-var ACCOUNT_ID = 'acct_EXPRESS';
+const ACCOUNT_ID = 'acct_EXPRESS';
 
 // Create new LoginLink instance with pre-filled accountId:
-var loginLink = new resources.LoginLinks(stripe, {accountId: ACCOUNT_ID});
+const loginLink = new resources.LoginLinks(stripe, {accountId: ACCOUNT_ID});
 
 // Use spy from existing resource:
 loginLink._request = stripe.customers._request;
 
-describe('LoginLink Resource', function() {
-  describe('create', function() {
-    it('Sends the correct request', function() {
+describe('LoginLink Resource', () => {
+  describe('create', () => {
+    it('Sends the correct request', () => {
       loginLink.create();
       expect(stripe.LAST_REQUEST).to.deep.equal({
         method: 'POST',
-        url: '/v1/accounts/' + ACCOUNT_ID + '/login_links',
+        url: `/v1/accounts/${ACCOUNT_ID}/login_links`,
         headers: {},
         data: {},
       });
diff --git a/test/resources/OAuth.spec.js b/test/resources/OAuth.spec.js
index 7db3345309..c04af14f3c 100644
--- a/test/resources/OAuth.spec.js
+++ b/test/resources/OAuth.spec.js
@@ -1,38 +1,38 @@
 'use strict';
 
-var stripe = require('../../testUtils').getSpyableStripe();
+const stripe = require('../../testUtils').getSpyableStripe();
 
-var expect = require('chai').expect;
-var URL = require('url');
-var qs = require('qs');
+const expect = require('chai').expect;
+const URL = require('url');
+const qs = require('qs');
 
-describe('OAuth', function() {
-  describe('authorize', function() {
-    describe('when a default client_id is set', function() {
-      beforeEach(function() {
+describe('OAuth', () => {
+  describe('authorize', () => {
+    describe('when a default client_id is set', () => {
+      beforeEach(() => {
         stripe.setClientId('default_client_id');
       });
 
-      it('Uses the correct host', function() {
-        var url = stripe.oauth.authorizeUrl();
+      it('Uses the correct host', () => {
+        const url = stripe.oauth.authorizeUrl();
 
-        var host = URL.parse(url).hostname;
+        const host = URL.parse(url).hostname;
 
         expect(host).to.equal('connect.stripe.com');
       });
 
-      it('Uses the correct path', function() {
-        var url = stripe.oauth.authorizeUrl({state: 'some_state'});
+      it('Uses the correct path', () => {
+        const url = stripe.oauth.authorizeUrl({state: 'some_state'});
 
-        var pathname = URL.parse(url).pathname;
+        const pathname = URL.parse(url).pathname;
 
         expect(pathname).to.equal('/oauth/authorize');
       });
 
-      it('Uses the correct query', function() {
-        var url = stripe.oauth.authorizeUrl({state: 'some_state'});
+      it('Uses the correct query', () => {
+        const url = stripe.oauth.authorizeUrl({state: 'some_state'});
 
-        var query = qs.parse(URL.parse(url).query);
+        const query = qs.parse(URL.parse(url).query);
 
         expect(query.client_id).to.equal('default_client_id');
         expect(query.response_type).to.equal('code');
@@ -40,19 +40,19 @@ describe('OAuth', function() {
         expect(query.state).to.equal('some_state');
       });
 
-      it('Uses a provided client_id instead of the default', function() {
-        var url = stripe.oauth.authorizeUrl({client_id: '123abc'});
+      it('Uses a provided client_id instead of the default', () => {
+        const url = stripe.oauth.authorizeUrl({client_id: '123abc'});
 
-        var query = qs.parse(URL.parse(url).query);
+        const query = qs.parse(URL.parse(url).query);
 
         expect(query.client_id).to.equal('123abc');
       });
 
-      describe('for Express account', function() {
-        it('Uses the correct path', function() {
-          var url = stripe.oauth.authorizeUrl({}, {express: true});
+      describe('for Express account', () => {
+        it('Uses the correct path', () => {
+          const url = stripe.oauth.authorizeUrl({}, {express: true});
 
-          var pathname = URL.parse(url).pathname;
+          const pathname = URL.parse(url).pathname;
 
           expect(pathname).to.equal('/express/oauth/authorize');
         });
@@ -60,8 +60,8 @@ describe('OAuth', function() {
     });
   });
 
-  describe('token', function() {
-    it('Sends the correct request', function() {
+  describe('token', () => {
+    it('Sends the correct request', () => {
       stripe.oauth.token({
         code: '123abc',
         grant_type: 'authorization_code',
@@ -80,12 +80,12 @@ describe('OAuth', function() {
     });
   });
 
-  describe('deauthorize', function() {
-    beforeEach(function() {
+  describe('deauthorize', () => {
+    beforeEach(() => {
       stripe.setClientId('default_client_id');
     });
 
-    it('Sends the correct request without explicit client_id', function() {
+    it('Sends the correct request without explicit client_id', () => {
       stripe.oauth.deauthorize({
         stripe_user_id: 'some_user_id',
       });
@@ -102,7 +102,7 @@ describe('OAuth', function() {
       });
     });
 
-    it('Sends the correct request with explicit client_id', function() {
+    it('Sends the correct request with explicit client_id', () => {
       stripe.oauth.deauthorize({
         stripe_user_id: 'some_user_id',
         client_id: '123abc',
diff --git a/test/resources/OrderReturns.spec.js b/test/resources/OrderReturns.spec.js
index 2dff709fe9..f39052edb2 100644
--- a/test/resources/OrderReturns.spec.js
+++ b/test/resources/OrderReturns.spec.js
@@ -1,11 +1,11 @@
 'use strict';
 
-var stripe = require('../../testUtils').getSpyableStripe();
-var expect = require('chai').expect;
+const stripe = require('../../testUtils').getSpyableStripe();
+const expect = require('chai').expect;
 
-describe('OrderReturn Resource', function() {
-  describe('retrieve', function() {
-    it('Sends the correct request', function() {
+describe('OrderReturn Resource', () => {
+  describe('retrieve', () => {
+    it('Sends the correct request', () => {
       stripe.orderReturns.retrieve('orderReturnIdFoo123');
       expect(stripe.LAST_REQUEST).to.deep.equal({
         method: 'GET',
@@ -16,8 +16,8 @@ describe('OrderReturn Resource', function() {
     });
   });
 
-  describe('list', function() {
-    it('Sends the correct request', function() {
+  describe('list', () => {
+    it('Sends the correct request', () => {
       stripe.orderReturns.list({
         limit: 3,
       });
@@ -31,7 +31,7 @@ describe('OrderReturn Resource', function() {
       });
     });
 
-    it('Supports filtering by order', function() {
+    it('Supports filtering by order', () => {
       stripe.orderReturns.list({
         order: 'orderIdFoo123',
       });
diff --git a/test/resources/Orders.spec.js b/test/resources/Orders.spec.js
index d0d05de51c..c367271577 100644
--- a/test/resources/Orders.spec.js
+++ b/test/resources/Orders.spec.js
@@ -1,11 +1,11 @@
 'use strict';
 
-var stripe = require('../../testUtils').getSpyableStripe();
-var expect = require('chai').expect;
+const stripe = require('../../testUtils').getSpyableStripe();
+const expect = require('chai').expect;
 
-describe('Order Resource', function() {
-  describe('retrieve', function() {
-    it('Sends the correct request', function() {
+describe('Order Resource', () => {
+  describe('retrieve', () => {
+    it('Sends the correct request', () => {
       stripe.orders.retrieve('orderIdFoo123');
       expect(stripe.LAST_REQUEST).to.deep.equal({
         method: 'GET',
@@ -16,8 +16,8 @@ describe('Order Resource', function() {
     });
   });
 
-  describe('create', function() {
-    it('Sends the correct request', function() {
+  describe('create', () => {
+    it('Sends the correct request', () => {
       stripe.orders.create({
         currency: 'usd',
         items: [
@@ -58,8 +58,8 @@ describe('Order Resource', function() {
     });
   });
 
-  describe('list', function() {
-    it('Sends the correct request', function() {
+  describe('list', () => {
+    it('Sends the correct request', () => {
       stripe.orders.list({
         limit: 3,
       });
@@ -73,7 +73,7 @@ describe('Order Resource', function() {
       });
     });
 
-    it('Supports filtering by status', function() {
+    it('Supports filtering by status', () => {
       stripe.orders.list({
         status: 'active',
       });
@@ -88,8 +88,8 @@ describe('Order Resource', function() {
     });
   });
 
-  describe('pay', function() {
-    it('Sends the correct request', function() {
+  describe('pay', () => {
+    it('Sends the correct request', () => {
       stripe.orders.pay('orderIdFoo3242', {
         source: 'tok_FooBar',
       });
@@ -102,8 +102,8 @@ describe('Order Resource', function() {
     });
   });
 
-  describe('returnOrder', function() {
-    it('Sends the correct request', function() {
+  describe('returnOrder', () => {
+    it('Sends the correct request', () => {
       stripe.orders.returnOrder('orderIdFoo3242', {
         items: [{parent: 'sku_123'}],
       });
@@ -116,8 +116,8 @@ describe('Order Resource', function() {
     });
   });
 
-  describe('update', function() {
-    it('Sends the correct request', function() {
+  describe('update', () => {
+    it('Sends the correct request', () => {
       stripe.orders.update('orderIdFoo3242', {status: 'fulfilled'});
       expect(stripe.LAST_REQUEST).to.deep.equal({
         method: 'POST',
diff --git a/test/resources/PaymentIntents.spec.js b/test/resources/PaymentIntents.spec.js
index 81f8f2c639..757b900ff5 100644
--- a/test/resources/PaymentIntents.spec.js
+++ b/test/resources/PaymentIntents.spec.js
@@ -1,14 +1,14 @@
 'use strict';
 
-var stripe = require('../../testUtils').getSpyableStripe();
-var expect = require('chai').expect;
+const stripe = require('../../testUtils').getSpyableStripe();
+const expect = require('chai').expect;
 
-var PAYMENT_INTENT_TEST_ID = 'pi_123';
+const PAYMENT_INTENT_TEST_ID = 'pi_123';
 
-describe('Payment Intents Resource', function() {
-  describe('create', function() {
-    it('Sends the correct request', function() {
-      var params = {
+describe('Payment Intents Resource', () => {
+  describe('create', () => {
+    it('Sends the correct request', () => {
+      const params = {
         amount: 200,
         currency: 'usd',
         payment_method_types: ['card'],
@@ -23,8 +23,8 @@ describe('Payment Intents Resource', function() {
     });
   });
 
-  describe('list', function() {
-    it('Sends the correct request', function() {
+  describe('list', () => {
+    it('Sends the correct request', () => {
       stripe.paymentIntents.list();
       expect(stripe.LAST_REQUEST).to.deep.equal({
         method: 'GET',
@@ -35,62 +35,62 @@ describe('Payment Intents Resource', function() {
     });
   });
 
-  describe('retrieve', function() {
-    it('Sends the correct request', function() {
+  describe('retrieve', () => {
+    it('Sends the correct request', () => {
       stripe.paymentIntents.retrieve(PAYMENT_INTENT_TEST_ID);
       expect(stripe.LAST_REQUEST).to.deep.equal({
         method: 'GET',
-        url: '/v1/payment_intents/' + PAYMENT_INTENT_TEST_ID,
+        url: `/v1/payment_intents/${PAYMENT_INTENT_TEST_ID}`,
         headers: {},
         data: {},
       });
     });
   });
 
-  describe('update', function() {
-    it('Sends the correct request', function() {
+  describe('update', () => {
+    it('Sends the correct request', () => {
       stripe.paymentIntents.update(PAYMENT_INTENT_TEST_ID, {
         metadata: {key: 'value'},
       });
       expect(stripe.LAST_REQUEST).to.deep.equal({
         method: 'POST',
-        url: '/v1/payment_intents/' + PAYMENT_INTENT_TEST_ID,
+        url: `/v1/payment_intents/${PAYMENT_INTENT_TEST_ID}`,
         headers: {},
         data: {metadata: {key: 'value'}},
       });
     });
   });
 
-  describe('cancel', function() {
-    it('Sends the correct request', function() {
+  describe('cancel', () => {
+    it('Sends the correct request', () => {
       stripe.paymentIntents.cancel(PAYMENT_INTENT_TEST_ID);
       expect(stripe.LAST_REQUEST).to.deep.equal({
         method: 'POST',
-        url: '/v1/payment_intents/' + PAYMENT_INTENT_TEST_ID + '/cancel',
+        url: `/v1/payment_intents/${PAYMENT_INTENT_TEST_ID}/cancel`,
         headers: {},
         data: {},
       });
     });
   });
 
-  describe('capture', function() {
-    it('Sends the correct request', function() {
+  describe('capture', () => {
+    it('Sends the correct request', () => {
       stripe.paymentIntents.capture(PAYMENT_INTENT_TEST_ID);
       expect(stripe.LAST_REQUEST).to.deep.equal({
         method: 'POST',
-        url: '/v1/payment_intents/' + PAYMENT_INTENT_TEST_ID + '/capture',
+        url: `/v1/payment_intents/${PAYMENT_INTENT_TEST_ID}/capture`,
         headers: {},
         data: {},
       });
     });
   });
 
-  describe('confirm', function() {
-    it('Sends the correct request', function() {
+  describe('confirm', () => {
+    it('Sends the correct request', () => {
       stripe.paymentIntents.confirm(PAYMENT_INTENT_TEST_ID);
       expect(stripe.LAST_REQUEST).to.deep.equal({
         method: 'POST',
-        url: '/v1/payment_intents/' + PAYMENT_INTENT_TEST_ID + '/confirm',
+        url: `/v1/payment_intents/${PAYMENT_INTENT_TEST_ID}/confirm`,
         headers: {},
         data: {},
       });
diff --git a/test/resources/PaymentMethods.spec.js b/test/resources/PaymentMethods.spec.js
index bc395521db..0860c1cb45 100644
--- a/test/resources/PaymentMethods.spec.js
+++ b/test/resources/PaymentMethods.spec.js
@@ -1,11 +1,11 @@
 'use strict';
 
-var stripe = require('../../testUtils').getSpyableStripe();
-var expect = require('chai').expect;
+const stripe = require('../../testUtils').getSpyableStripe();
+const expect = require('chai').expect;
 
-describe('PaymentMethods Resource', function() {
-  describe('retrieve', function() {
-    it('Sends the correct request', function() {
+describe('PaymentMethods Resource', () => {
+  describe('retrieve', () => {
+    it('Sends the correct request', () => {
       stripe.paymentMethods.retrieve('pm_123');
       expect(stripe.LAST_REQUEST).to.deep.equal({
         method: 'GET',
@@ -16,9 +16,9 @@ describe('PaymentMethods Resource', function() {
     });
   });
 
-  describe('create', function() {
-    it('Sends the correct request', function() {
-      var data = {
+  describe('create', () => {
+    it('Sends the correct request', () => {
+      const data = {
         type: 'card',
       };
       stripe.paymentMethods.create(data);
@@ -26,14 +26,14 @@ describe('PaymentMethods Resource', function() {
         method: 'POST',
         url: '/v1/payment_methods',
         headers: {},
-        data: data,
+        data,
       });
     });
   });
 
-  describe('list', function() {
-    it('Sends the correct request', function() {
-      var data = {
+  describe('list', () => {
+    it('Sends the correct request', () => {
+      const data = {
         customer: 'cus_123',
         type: 'card',
       };
@@ -42,14 +42,14 @@ describe('PaymentMethods Resource', function() {
         method: 'GET',
         url: '/v1/payment_methods',
         headers: {},
-        data: data,
+        data,
       });
     });
   });
 
-  describe('update', function() {
-    it('Sends the correct request', function() {
-      var data = {
+  describe('update', () => {
+    it('Sends the correct request', () => {
+      const data = {
         metadata: {key: 'value'},
       };
       stripe.paymentMethods.update('pm_123', data);
@@ -57,13 +57,13 @@ describe('PaymentMethods Resource', function() {
         method: 'POST',
         url: '/v1/payment_methods/pm_123',
         headers: {},
-        data: data,
+        data,
       });
     });
   });
 
-  describe('attach', function() {
-    it('Sends the correct request', function() {
+  describe('attach', () => {
+    it('Sends the correct request', () => {
       stripe.paymentMethods.attach('pm_123', {customer: 'cus_123'});
       expect(stripe.LAST_REQUEST).to.deep.equal({
         method: 'POST',
@@ -74,8 +74,8 @@ describe('PaymentMethods Resource', function() {
     });
   });
 
-  describe('detach', function() {
-    it('Sends the correct request', function() {
+  describe('detach', () => {
+    it('Sends the correct request', () => {
       stripe.paymentMethods.detach('pm_123');
       expect(stripe.LAST_REQUEST).to.deep.equal({
         method: 'POST',
diff --git a/test/resources/Payouts.spec.js b/test/resources/Payouts.spec.js
index 8b19252a64..1f1678acae 100644
--- a/test/resources/Payouts.spec.js
+++ b/test/resources/Payouts.spec.js
@@ -1,25 +1,25 @@
 'use strict';
 
-var stripe = require('../../testUtils').getSpyableStripe();
-var expect = require('chai').expect;
+const stripe = require('../../testUtils').getSpyableStripe();
+const expect = require('chai').expect;
 
-var PAYOUT_TEST_ID = 'po_testid1';
+const PAYOUT_TEST_ID = 'po_testid1';
 
-describe('Payouts Resource', function() {
-  describe('retrieve', function() {
-    it('Sends the correct request', function() {
+describe('Payouts Resource', () => {
+  describe('retrieve', () => {
+    it('Sends the correct request', () => {
       stripe.payouts.retrieve(PAYOUT_TEST_ID);
       expect(stripe.LAST_REQUEST).to.deep.equal({
         method: 'GET',
-        url: '/v1/payouts/' + PAYOUT_TEST_ID,
+        url: `/v1/payouts/${PAYOUT_TEST_ID}`,
         headers: {},
         data: {},
       });
     });
   });
 
-  describe('create', function() {
-    it('Sends the correct request', function() {
+  describe('create', () => {
+    it('Sends the correct request', () => {
       stripe.payouts.create({
         amount: 200,
         currency: 'usd',
@@ -33,34 +33,34 @@ describe('Payouts Resource', function() {
     });
   });
 
-  describe('update', function() {
-    it('Sends the correct request', function() {
+  describe('update', () => {
+    it('Sends the correct request', () => {
       stripe.payouts.update(PAYOUT_TEST_ID, {
         metadata: {key: 'value'},
       });
       expect(stripe.LAST_REQUEST).to.deep.equal({
         method: 'POST',
-        url: '/v1/payouts/' + PAYOUT_TEST_ID,
+        url: `/v1/payouts/${PAYOUT_TEST_ID}`,
         headers: {},
         data: {metadata: {key: 'value'}},
       });
     });
   });
 
-  describe('cancel', function() {
-    it('Sends the correct request', function() {
+  describe('cancel', () => {
+    it('Sends the correct request', () => {
       stripe.payouts.cancel(PAYOUT_TEST_ID);
       expect(stripe.LAST_REQUEST).to.deep.equal({
         method: 'POST',
-        url: '/v1/payouts/' + PAYOUT_TEST_ID + '/cancel',
+        url: `/v1/payouts/${PAYOUT_TEST_ID}/cancel`,
         headers: {},
         data: {},
       });
     });
   });
 
-  describe('list', function() {
-    it('Sends the correct request', function() {
+  describe('list', () => {
+    it('Sends the correct request', () => {
       stripe.payouts.list();
       expect(stripe.LAST_REQUEST).to.deep.equal({
         method: 'GET',
@@ -71,12 +71,12 @@ describe('Payouts Resource', function() {
     });
   });
 
-  describe('listTransactions', function() {
-    it('Sends the correct request', function() {
+  describe('listTransactions', () => {
+    it('Sends the correct request', () => {
       stripe.payouts.listTransactions(PAYOUT_TEST_ID);
       expect(stripe.LAST_REQUEST).to.deep.equal({
         method: 'GET',
-        url: '/v1/payouts/' + PAYOUT_TEST_ID + '/transactions',
+        url: `/v1/payouts/${PAYOUT_TEST_ID}/transactions`,
         headers: {},
         data: {},
       });
diff --git a/test/resources/Persons.spec.js b/test/resources/Persons.spec.js
index 8d7112dbce..0b199f6715 100644
--- a/test/resources/Persons.spec.js
+++ b/test/resources/Persons.spec.js
@@ -1,77 +1,77 @@
 'use strict';
 
-var resources = require('../../lib/stripe').resources;
-var stripe = require('../../testUtils').getSpyableStripe();
-var expect = require('chai').expect;
+const resources = require('../../lib/stripe').resources;
+const stripe = require('../../testUtils').getSpyableStripe();
+const expect = require('chai').expect;
 
-var ACCOUNT_TEST_ID = 'acct_123';
-var PERSON_TEST_ID = 'person_123';
+const ACCOUNT_TEST_ID = 'acct_123';
+const PERSON_TEST_ID = 'person_123';
 
 // Create new Person instance with pre-filled accountId:
-var person = new resources.Persons(stripe, {accountId: ACCOUNT_TEST_ID});
+const person = new resources.Persons(stripe, {accountId: ACCOUNT_TEST_ID});
 
 // Use spy from existing resource:
 person._request = stripe.customers._request;
 
-describe('Person Resource', function() {
-  describe('create', function() {
-    it('Sends the correct request', function() {
+describe('Person Resource', () => {
+  describe('create', () => {
+    it('Sends the correct request', () => {
       person.create({
         first_name: 'John',
       });
       expect(stripe.LAST_REQUEST).to.deep.equal({
         method: 'POST',
-        url: '/v1/accounts/' + ACCOUNT_TEST_ID + '/persons',
+        url: `/v1/accounts/${ACCOUNT_TEST_ID}/persons`,
         data: {first_name: 'John'},
         headers: {},
       });
     });
   });
 
-  describe('delete', function() {
-    it('Sends the correct request', function() {
+  describe('delete', () => {
+    it('Sends the correct request', () => {
       person.del(PERSON_TEST_ID);
       expect(stripe.LAST_REQUEST).to.deep.equal({
         method: 'DELETE',
-        url: '/v1/accounts/' + ACCOUNT_TEST_ID + '/persons/' + PERSON_TEST_ID,
+        url: `/v1/accounts/${ACCOUNT_TEST_ID}/persons/${PERSON_TEST_ID}`,
         data: {},
         headers: {},
       });
     });
   });
 
-  describe('list', function() {
-    it('Sends the correct request', function() {
+  describe('list', () => {
+    it('Sends the correct request', () => {
       person.list();
       expect(stripe.LAST_REQUEST).to.deep.equal({
         method: 'GET',
-        url: '/v1/accounts/' + ACCOUNT_TEST_ID + '/persons',
+        url: `/v1/accounts/${ACCOUNT_TEST_ID}/persons`,
         data: {},
         headers: {},
       });
     });
   });
 
-  describe('retrieve', function() {
-    it('Sends the correct request', function() {
+  describe('retrieve', () => {
+    it('Sends the correct request', () => {
       person.retrieve(PERSON_TEST_ID);
       expect(stripe.LAST_REQUEST).to.deep.equal({
         method: 'GET',
-        url: '/v1/accounts/' + ACCOUNT_TEST_ID + '/persons/' + PERSON_TEST_ID,
+        url: `/v1/accounts/${ACCOUNT_TEST_ID}/persons/${PERSON_TEST_ID}`,
         data: {},
         headers: {},
       });
     });
   });
 
-  describe('update', function() {
-    it('Sends the correct request', function() {
+  describe('update', () => {
+    it('Sends the correct request', () => {
       person.update(PERSON_TEST_ID, {
         first_name: 'John',
       });
       expect(stripe.LAST_REQUEST).to.deep.equal({
         method: 'POST',
-        url: '/v1/accounts/' + ACCOUNT_TEST_ID + '/persons/' + PERSON_TEST_ID,
+        url: `/v1/accounts/${ACCOUNT_TEST_ID}/persons/${PERSON_TEST_ID}`,
         data: {first_name: 'John'},
         headers: {},
       });
diff --git a/test/resources/Plans.spec.js b/test/resources/Plans.spec.js
index 6468c95f46..85f9226fdc 100644
--- a/test/resources/Plans.spec.js
+++ b/test/resources/Plans.spec.js
@@ -1,11 +1,11 @@
 'use strict';
 
-var stripe = require('../../testUtils').getSpyableStripe();
-var expect = require('chai').expect;
+const stripe = require('../../testUtils').getSpyableStripe();
+const expect = require('chai').expect;
 
-describe('Plans Resource', function() {
-  describe('retrieve', function() {
-    it('Sends the correct request', function() {
+describe('Plans Resource', () => {
+  describe('retrieve', () => {
+    it('Sends the correct request', () => {
       stripe.plans.retrieve('planId1');
       expect(stripe.LAST_REQUEST).to.deep.equal({
         method: 'GET',
@@ -16,8 +16,8 @@ describe('Plans Resource', function() {
     });
   });
 
-  describe('create', function() {
-    it('Sends the correct request', function() {
+  describe('create', () => {
+    it('Sends the correct request', () => {
       stripe.plans.create({
         amount: 200,
         currency: 'usd',
@@ -30,7 +30,7 @@ describe('Plans Resource', function() {
       });
     });
 
-    it('Sends the correct request for metered', function() {
+    it('Sends the correct request for metered', () => {
       stripe.plans.create({
         amount: 200,
         currency: 'usd',
@@ -44,7 +44,7 @@ describe('Plans Resource', function() {
       });
     });
 
-    it('Sends the correct request for tiered plans', function() {
+    it('Sends the correct request for tiered plans', () => {
       stripe.plans.create({
         currency: 'usd',
         billing_scheme: 'tiered',
@@ -64,7 +64,7 @@ describe('Plans Resource', function() {
       });
     });
 
-    it('Sends the correct request for transform usage plans', function() {
+    it('Sends the correct request for transform usage plans', () => {
       stripe.plans.create({
         amount: 200,
         currency: 'usd',
@@ -83,8 +83,8 @@ describe('Plans Resource', function() {
     });
   });
 
-  describe('update', function() {
-    it('Sends the correct request', function() {
+  describe('update', () => {
+    it('Sends the correct request', () => {
       stripe.plans.update('planId3', {
         amount: 1900,
         currency: 'usd',
@@ -98,8 +98,8 @@ describe('Plans Resource', function() {
     });
   });
 
-  describe('del', function() {
-    it('Sends the correct request', function() {
+  describe('del', () => {
+    it('Sends the correct request', () => {
       stripe.plans.del('planId4');
       expect(stripe.LAST_REQUEST).to.deep.equal({
         method: 'DELETE',
@@ -110,8 +110,8 @@ describe('Plans Resource', function() {
     });
   });
 
-  describe('list', function() {
-    it('Sends the correct request', function() {
+  describe('list', () => {
+    it('Sends the correct request', () => {
       stripe.plans.list();
       expect(stripe.LAST_REQUEST).to.deep.equal({
         method: 'GET',
diff --git a/test/resources/Products.spec.js b/test/resources/Products.spec.js
index 9efb4ec49e..21c6193521 100644
--- a/test/resources/Products.spec.js
+++ b/test/resources/Products.spec.js
@@ -1,11 +1,11 @@
 'use strict';
 
-var stripe = require('../../testUtils').getSpyableStripe();
-var expect = require('chai').expect;
+const stripe = require('../../testUtils').getSpyableStripe();
+const expect = require('chai').expect;
 
-describe('Product Resource', function() {
-  describe('retrieve', function() {
-    it('Sends the correct request', function() {
+describe('Product Resource', () => {
+  describe('retrieve', () => {
+    it('Sends the correct request', () => {
       stripe.products.retrieve('productIdFoo123');
       expect(stripe.LAST_REQUEST).to.deep.equal({
         method: 'GET',
@@ -16,8 +16,8 @@ describe('Product Resource', function() {
     });
   });
 
-  describe('create', function() {
-    it('Sends the correct request', function() {
+  describe('create', () => {
+    it('Sends the correct request', () => {
       stripe.products.create({
         name: 'Llamas',
         active: true,
@@ -36,8 +36,8 @@ describe('Product Resource', function() {
     });
   });
 
-  describe('list', function() {
-    it('Sends the correct request', function() {
+  describe('list', () => {
+    it('Sends the correct request', () => {
       stripe.products.list({
         limit: 3,
       });
@@ -51,7 +51,7 @@ describe('Product Resource', function() {
       });
     });
 
-    it('Supports filtering by shippable', function() {
+    it('Supports filtering by shippable', () => {
       stripe.products.list({
         shippable: true,
       });
@@ -66,8 +66,8 @@ describe('Product Resource', function() {
     });
   });
 
-  describe('update', function() {
-    it('Sends the correct request', function() {
+  describe('update', () => {
+    it('Sends the correct request', () => {
       stripe.products.update('productIdFoo3242', {caption: 'test'});
       expect(stripe.LAST_REQUEST).to.deep.equal({
         method: 'POST',
@@ -78,8 +78,8 @@ describe('Product Resource', function() {
     });
   });
 
-  describe('del', function() {
-    it('Sends the correct request', function() {
+  describe('del', () => {
+    it('Sends the correct request', () => {
       stripe.products.del('productIdFoo3242');
       expect(stripe.LAST_REQUEST).to.deep.equal({
         method: 'DELETE',
diff --git a/test/resources/Radar/ValueListItems.spec.js b/test/resources/Radar/ValueListItems.spec.js
index ee414a1e77..286ef956ba 100644
--- a/test/resources/Radar/ValueListItems.spec.js
+++ b/test/resources/Radar/ValueListItems.spec.js
@@ -1,13 +1,13 @@
 'use strict';
 
-var stripe = require('../../../testUtils').getSpyableStripe();
+const stripe = require('../../../testUtils').getSpyableStripe();
 
-var expect = require('chai').expect;
+const expect = require('chai').expect;
 
-describe('Radar', function() {
-  describe('ValueLists Resource', function() {
-    describe('retrieve', function() {
-      it('Sends the correct request', function() {
+describe('Radar', () => {
+  describe('ValueLists Resource', () => {
+    describe('retrieve', () => {
+      it('Sends the correct request', () => {
         stripe.radar.valueListItems.retrieve('rsli_123');
 
         expect(stripe.LAST_REQUEST).to.deep.equal({
@@ -19,8 +19,8 @@ describe('Radar', function() {
       });
     });
 
-    describe('create', function() {
-      it('Sends the correct request', function() {
+    describe('create', () => {
+      it('Sends the correct request', () => {
         stripe.radar.valueListItems.create({
           value_list: 'rsl_123',
           value: 'value',
@@ -37,8 +37,8 @@ describe('Radar', function() {
       });
     });
 
-    describe('list', function() {
-      it('Sends the correct request', function() {
+    describe('list', () => {
+      it('Sends the correct request', () => {
         stripe.radar.valueListItems.list({
           value_list: 'rsl_123',
         });
@@ -53,8 +53,8 @@ describe('Radar', function() {
       });
     });
 
-    describe('del', function() {
-      it('Sends the correct request', function() {
+    describe('del', () => {
+      it('Sends the correct request', () => {
         stripe.radar.valueListItems.del('rsli_123');
         expect(stripe.LAST_REQUEST).to.deep.equal({
           method: 'DELETE',
diff --git a/test/resources/Radar/ValueLists.spec.js b/test/resources/Radar/ValueLists.spec.js
index 1acaf6dc53..57c778257e 100644
--- a/test/resources/Radar/ValueLists.spec.js
+++ b/test/resources/Radar/ValueLists.spec.js
@@ -1,13 +1,13 @@
 'use strict';
 
-var stripe = require('../../../testUtils').getSpyableStripe();
+const stripe = require('../../../testUtils').getSpyableStripe();
 
-var expect = require('chai').expect;
+const expect = require('chai').expect;
 
-describe('Radar', function() {
-  describe('ValueLists Resource', function() {
-    describe('retrieve', function() {
-      it('Sends the correct request', function() {
+describe('Radar', () => {
+  describe('ValueLists Resource', () => {
+    describe('retrieve', () => {
+      it('Sends the correct request', () => {
         stripe.radar.valueLists.retrieve('rsl_123');
 
         expect(stripe.LAST_REQUEST).to.deep.equal({
@@ -19,8 +19,8 @@ describe('Radar', function() {
       });
     });
 
-    describe('create', function() {
-      it('Sends the correct request', function() {
+    describe('create', () => {
+      it('Sends the correct request', () => {
         stripe.radar.valueLists.create({
           alias: 'alias',
           name: 'name',
@@ -37,8 +37,8 @@ describe('Radar', function() {
       });
     });
 
-    describe('list', function() {
-      it('Sends the correct request', function() {
+    describe('list', () => {
+      it('Sends the correct request', () => {
         stripe.radar.valueLists.list();
         expect(stripe.LAST_REQUEST).to.deep.equal({
           method: 'GET',
@@ -49,8 +49,8 @@ describe('Radar', function() {
       });
     });
 
-    describe('del', function() {
-      it('Sends the correct request', function() {
+    describe('del', () => {
+      it('Sends the correct request', () => {
         stripe.radar.valueLists.del('rsl_123');
         expect(stripe.LAST_REQUEST).to.deep.equal({
           method: 'DELETE',
@@ -61,8 +61,8 @@ describe('Radar', function() {
       });
     });
 
-    describe('update', function() {
-      it('Sends the correct request', function() {
+    describe('update', () => {
+      it('Sends the correct request', () => {
         stripe.radar.valueLists.update('rsl_123', {
           metadata: {a: '1234'},
         });
diff --git a/test/resources/RecipientCards.spec.js b/test/resources/RecipientCards.spec.js
index abd4007442..885b974912 100644
--- a/test/resources/RecipientCards.spec.js
+++ b/test/resources/RecipientCards.spec.js
@@ -1,79 +1,79 @@
 'use strict';
 
-var resources = require('../../lib/stripe').resources;
-var stripe = require('../../testUtils').getSpyableStripe();
-var expect = require('chai').expect;
+const resources = require('../../lib/stripe').resources;
+const stripe = require('../../testUtils').getSpyableStripe();
+const expect = require('chai').expect;
 
-var RECIPIENT_TEST_ID = 'recipientIdTest999';
+const RECIPIENT_TEST_ID = 'recipientIdTest999';
 
 // Create new recipientCard instance with pre-filled recipientId:
-var recipientCard = new resources.RecipientCards(stripe, {
+const recipientCard = new resources.RecipientCards(stripe, {
   recipientId: RECIPIENT_TEST_ID,
 });
 
 // Use spy from existing resource:
 recipientCard._request = stripe.recipients._request;
 
-describe('RecipientCard Resource', function() {
-  describe('retrieve', function() {
-    it('Sends the correct request', function() {
+describe('RecipientCard Resource', () => {
+  describe('retrieve', () => {
+    it('Sends the correct request', () => {
       recipientCard.retrieve('cardIdFoo456');
       expect(stripe.LAST_REQUEST).to.deep.equal({
         method: 'GET',
-        url: '/v1/recipients/' + RECIPIENT_TEST_ID + '/cards/cardIdFoo456',
+        url: `/v1/recipients/${RECIPIENT_TEST_ID}/cards/cardIdFoo456`,
         headers: {},
         data: {},
       });
     });
   });
 
-  describe('create', function() {
-    it('Sends the correct request', function() {
+  describe('create', () => {
+    it('Sends the correct request', () => {
       recipientCard.create({
         number: '123456',
         exp_month: '12',
       });
       expect(stripe.LAST_REQUEST).to.deep.equal({
         method: 'POST',
-        url: '/v1/recipients/' + RECIPIENT_TEST_ID + '/cards',
+        url: `/v1/recipients/${RECIPIENT_TEST_ID}/cards`,
         headers: {},
         data: {number: '123456', exp_month: '12'},
       });
     });
   });
 
-  describe('update', function() {
-    it('Sends the correct request', function() {
+  describe('update', () => {
+    it('Sends the correct request', () => {
       recipientCard.update('cardIdFoo456', {
         name: 'Bob M. Baz',
       });
       expect(stripe.LAST_REQUEST).to.deep.equal({
         method: 'POST',
-        url: '/v1/recipients/' + RECIPIENT_TEST_ID + '/cards/cardIdFoo456',
+        url: `/v1/recipients/${RECIPIENT_TEST_ID}/cards/cardIdFoo456`,
         headers: {},
         data: {name: 'Bob M. Baz'},
       });
     });
   });
 
-  describe('del', function() {
-    it('Sends the correct request', function() {
+  describe('del', () => {
+    it('Sends the correct request', () => {
       recipientCard.del('cardIdFoo456');
       expect(stripe.LAST_REQUEST).to.deep.equal({
         method: 'DELETE',
-        url: '/v1/recipients/' + RECIPIENT_TEST_ID + '/cards/cardIdFoo456',
+        url: `/v1/recipients/${RECIPIENT_TEST_ID}/cards/cardIdFoo456`,
         headers: {},
         data: {},
       });
     });
   });
 
-  describe('list', function() {
-    it('Sends the correct request', function() {
+  describe('list', () => {
+    it('Sends the correct request', () => {
       recipientCard.list();
       expect(stripe.LAST_REQUEST).to.deep.equal({
         method: 'GET',
-        url: '/v1/recipients/' + RECIPIENT_TEST_ID + '/cards',
+        url: `/v1/recipients/${RECIPIENT_TEST_ID}/cards`,
         headers: {},
         data: {},
       });
diff --git a/test/resources/Recipients.spec.js b/test/resources/Recipients.spec.js
index 3c428b1ff5..8ee3f33fe1 100644
--- a/test/resources/Recipients.spec.js
+++ b/test/resources/Recipients.spec.js
@@ -1,13 +1,13 @@
 'use strict';
 
-var stripe = require('../../testUtils').getSpyableStripe();
-var expect = require('chai').expect;
+const stripe = require('../../testUtils').getSpyableStripe();
+const expect = require('chai').expect;
 
-var TEST_AUTH_KEY = 'aGN0bIwXnHdw5645VABjPdSn8nWY7G11';
+const TEST_AUTH_KEY = 'aGN0bIwXnHdw5645VABjPdSn8nWY7G11';
 
-describe('Recipients Resource', function() {
-  describe('retrieve', function() {
-    it('Sends the correct request', function() {
+describe('Recipients Resource', () => {
+  describe('retrieve', () => {
+    it('Sends the correct request', () => {
       stripe.recipients.retrieve('recipientId1');
       expect(stripe.LAST_REQUEST).to.deep.equal({
         method: 'GET',
@@ -18,8 +18,8 @@ describe('Recipients Resource', function() {
     });
   });
 
-  describe('create', function() {
-    it('Sends the correct request', function() {
+  describe('create', () => {
+    it('Sends the correct request', () => {
       stripe.recipients.create({
         name: 'Bob',
         type: 'individual',
@@ -33,8 +33,8 @@ describe('Recipients Resource', function() {
     });
   });
 
-  describe('update', function() {
-    it('Sends the correct request', function() {
+  describe('update', () => {
+    it('Sends the correct request', () => {
       stripe.recipients.update('recipientId3', {
         name: 'Bob Smith',
       });
@@ -47,8 +47,8 @@ describe('Recipients Resource', function() {
     });
   });
 
-  describe('del', function() {
-    it('Sends the correct request', function() {
+  describe('del', () => {
+    it('Sends the correct request', () => {
       stripe.recipients.del('recipientId4');
       expect(stripe.LAST_REQUEST).to.deep.equal({
         method: 'DELETE',
@@ -59,8 +59,8 @@ describe('Recipients Resource', function() {
     });
   });
 
-  describe('list', function() {
-    it('Sends the correct request', function() {
+  describe('list', () => {
+    it('Sends the correct request', () => {
       stripe.recipients.list();
       expect(stripe.LAST_REQUEST).to.deep.equal({
         method: 'GET',
@@ -71,9 +71,9 @@ describe('Recipients Resource', function() {
     });
   });
 
-  describe('Card methods', function() {
-    describe('retrieveCard', function() {
-      it('Sends the correct request', function() {
+  describe('Card methods', () => {
+    describe('retrieveCard', () => {
+      it('Sends the correct request', () => {
         stripe.recipients.retrieveCard('recipientIdFoo321', 'cardIdFoo456');
         expect(stripe.LAST_REQUEST).to.deep.equal({
           method: 'GET',
@@ -83,7 +83,7 @@ describe('Recipients Resource', function() {
         });
       });
 
-      it('Sends the correct request [with specified auth]', function() {
+      it('Sends the correct request [with specified auth]', () => {
         stripe.recipients.retrieveCard(
           'recipientIdFoo321',
           'cardIdFoo456',
@@ -99,8 +99,8 @@ describe('Recipients Resource', function() {
       });
     });
 
-    describe('createCard', function() {
-      it('Sends the correct request', function() {
+    describe('createCard', () => {
+      it('Sends the correct request', () => {
         stripe.recipients.createCard('recipientIdFoo321', {
           number: '123456',
           exp_month: '12',
@@ -113,7 +113,7 @@ describe('Recipients Resource', function() {
         });
       });
 
-      it('Sends the correct request [with specified auth]', function() {
+      it('Sends the correct request [with specified auth]', () => {
         stripe.recipients.createCard(
           'recipientIdFoo321',
           {
@@ -132,8 +132,8 @@ describe('Recipients Resource', function() {
       });
     });
 
-    describe('updateCard', function() {
-      it('Sends the correct request', function() {
+    describe('updateCard', () => {
+      it('Sends the correct request', () => {
         stripe.recipients.updateCard('recipientIdFoo321', 'cardIdFoo456', {
           name: 'Bob M. Baz',
         });
@@ -146,8 +146,8 @@ describe('Recipients Resource', function() {
       });
     });
 
-    describe('deleteCard', function() {
-      it('Sends the correct request', function() {
+    describe('deleteCard', () => {
+      it('Sends the correct request', () => {
         stripe.recipients.deleteCard('recipientIdFoo321', 'cardIdFoo456');
         expect(stripe.LAST_REQUEST).to.deep.equal({
           method: 'DELETE',
@@ -157,7 +157,7 @@ describe('Recipients Resource', function() {
         });
       });
 
-      it('Sends the correct request [with specified auth]', function() {
+      it('Sends the correct request [with specified auth]', () => {
         stripe.recipients.deleteCard(
           'recipientIdFoo321',
           'cardIdFoo456',
@@ -173,8 +173,8 @@ describe('Recipients Resource', function() {
       });
     });
 
-    describe('listCards', function() {
-      it('Sends the correct request', function() {
+    describe('listCards', () => {
+      it('Sends the correct request', () => {
         stripe.recipients.listCards('recipientIdFoo321');
         expect(stripe.LAST_REQUEST).to.deep.equal({
           method: 'GET',
@@ -184,7 +184,7 @@ describe('Recipients Resource', function() {
         });
       });
 
-      it('Sends the correct request [with specified auth]', function() {
+      it('Sends the correct request [with specified auth]', () => {
         stripe.recipients.listCards('recipientIdFoo321', TEST_AUTH_KEY);
         expect(stripe.LAST_REQUEST).to.deep.equal({
           method: 'GET',
diff --git a/test/resources/Refunds.spec.js b/test/resources/Refunds.spec.js
index fd8a32b104..843477d149 100644
--- a/test/resources/Refunds.spec.js
+++ b/test/resources/Refunds.spec.js
@@ -1,11 +1,11 @@
 'use strict';
 
-var stripe = require('../../testUtils').getSpyableStripe();
-var expect = require('chai').expect;
+const stripe = require('../../testUtils').getSpyableStripe();
+const expect = require('chai').expect;
 
-describe('Refund Resource', function() {
-  describe('create', function() {
-    it('Sends the correct request', function() {
+describe('Refund Resource', () => {
+  describe('create', () => {
+    it('Sends the correct request', () => {
       stripe.refunds.create({
         amount: '300',
         charge: 'ch_123',
@@ -23,8 +23,8 @@ describe('Refund Resource', function() {
     });
   });
 
-  describe('retrieve', function() {
-    it('Sends the correct request', function() {
+  describe('retrieve', () => {
+    it('Sends the correct request', () => {
       stripe.refunds.retrieve('re_123');
       expect(stripe.LAST_REQUEST).to.deep.equal({
         method: 'GET',
@@ -35,8 +35,8 @@ describe('Refund Resource', function() {
     });
   });
 
-  describe('list', function() {
-    it('Sends the correct request', function() {
+  describe('list', () => {
+    it('Sends the correct request', () => {
       stripe.refunds.list();
       expect(stripe.LAST_REQUEST).to.deep.equal({
         method: 'GET',
@@ -47,8 +47,8 @@ describe('Refund Resource', function() {
     });
   });
 
-  describe('update', function() {
-    it('Sends the correct request', function() {
+  describe('update', () => {
+    it('Sends the correct request', () => {
       stripe.refunds.update('re_123', {metadata: {key: 'abcd'}});
       expect(stripe.LAST_REQUEST).to.deep.equal({
         method: 'POST',
diff --git a/test/resources/Reporting/ReportRuns.spec.js b/test/resources/Reporting/ReportRuns.spec.js
index a9f04c6692..dd83a00854 100644
--- a/test/resources/Reporting/ReportRuns.spec.js
+++ b/test/resources/Reporting/ReportRuns.spec.js
@@ -1,13 +1,13 @@
 'use strict';
 
-var stripe = require('../../../testUtils').getSpyableStripe();
+const stripe = require('../../../testUtils').getSpyableStripe();
 
-var expect = require('chai').expect;
+const expect = require('chai').expect;
 
-describe('Reporting', function() {
-  describe('ReportRuns Resource', function() {
-    describe('retrieve', function() {
-      it('Sends the correct request', function() {
+describe('Reporting', () => {
+  describe('ReportRuns Resource', () => {
+    describe('retrieve', () => {
+      it('Sends the correct request', () => {
         stripe.reporting.reportRuns.retrieve('frr_123');
 
         expect(stripe.LAST_REQUEST).to.deep.equal({
@@ -19,8 +19,8 @@ describe('Reporting', function() {
       });
     });
 
-    describe('create', function() {
-      it('Sends the correct request', function() {
+    describe('create', () => {
+      it('Sends the correct request', () => {
         stripe.reporting.reportRuns.create({
           parameters: {
             connected_account: 'acct_123',
@@ -41,8 +41,8 @@ describe('Reporting', function() {
       });
     });
 
-    describe('list', function() {
-      it('Sends the correct request', function() {
+    describe('list', () => {
+      it('Sends the correct request', () => {
         stripe.reporting.reportRuns.list();
         expect(stripe.LAST_REQUEST).to.deep.equal({
           method: 'GET',
diff --git a/test/resources/Reporting/ReportTypes.spec.js b/test/resources/Reporting/ReportTypes.spec.js
index a13c2db4b3..37e7bd3134 100644
--- a/test/resources/Reporting/ReportTypes.spec.js
+++ b/test/resources/Reporting/ReportTypes.spec.js
@@ -1,13 +1,13 @@
 'use strict';
 
-var stripe = require('../../../testUtils').getSpyableStripe();
+const stripe = require('../../../testUtils').getSpyableStripe();
 
-var expect = require('chai').expect;
+const expect = require('chai').expect;
 
-describe('Reporting', function() {
-  describe('ReportTypes Resource', function() {
-    describe('retrieve', function() {
-      it('Sends the correct request', function() {
+describe('Reporting', () => {
+  describe('ReportTypes Resource', () => {
+    describe('retrieve', () => {
+      it('Sends the correct request', () => {
         stripe.reporting.reportTypes.retrieve('activity.summary.1');
 
         expect(stripe.LAST_REQUEST).to.deep.equal({
@@ -19,8 +19,8 @@ describe('Reporting', function() {
       });
     });
 
-    describe('list', function() {
-      it('Sends the correct request', function() {
+    describe('list', () => {
+      it('Sends the correct request', () => {
         stripe.reporting.reportTypes.list();
         expect(stripe.LAST_REQUEST).to.deep.equal({
           method: 'GET',
diff --git a/test/resources/Reviews.spec.js b/test/resources/Reviews.spec.js
index 850473afec..03296088aa 100644
--- a/test/resources/Reviews.spec.js
+++ b/test/resources/Reviews.spec.js
@@ -1,11 +1,11 @@
 'use strict';
 
-var stripe = require('../../testUtils').getSpyableStripe();
-var expect = require('chai').expect;
+const stripe = require('../../testUtils').getSpyableStripe();
+const expect = require('chai').expect;
 
-describe('Review Resource', function() {
-  describe('retrieve', function() {
-    it('Sends the correct request', function() {
+describe('Review Resource', () => {
+  describe('retrieve', () => {
+    it('Sends the correct request', () => {
       stripe.reviews.retrieve('prv_123');
       expect(stripe.LAST_REQUEST).to.deep.equal({
         method: 'GET',
@@ -16,8 +16,8 @@ describe('Review Resource', function() {
     });
   });
 
-  describe('list', function() {
-    it('Sends the correct request', function() {
+  describe('list', () => {
+    it('Sends the correct request', () => {
       stripe.reviews.list();
       expect(stripe.LAST_REQUEST).to.deep.equal({
         method: 'GET',
@@ -28,8 +28,8 @@ describe('Review Resource', function() {
     });
   });
 
-  describe('approve', function() {
-    it('Sends the correct request', function() {
+  describe('approve', () => {
+    it('Sends the correct request', () => {
       stripe.reviews.approve('prv_123', {amount: 23});
       expect(stripe.LAST_REQUEST).to.deep.equal({
         method: 'POST',
diff --git a/test/resources/SKUs.spec.js b/test/resources/SKUs.spec.js
index fc9d1f27e7..5155d8fec8 100644
--- a/test/resources/SKUs.spec.js
+++ b/test/resources/SKUs.spec.js
@@ -1,11 +1,11 @@
 'use strict';
 
-var stripe = require('../../testUtils').getSpyableStripe();
-var expect = require('chai').expect;
+const stripe = require('../../testUtils').getSpyableStripe();
+const expect = require('chai').expect;
 
-describe('SKU Resource', function() {
-  describe('retrieve', function() {
-    it('Sends the correct request', function() {
+describe('SKU Resource', () => {
+  describe('retrieve', () => {
+    it('Sends the correct request', () => {
       stripe.skus.retrieve('skuIdFoo123');
       expect(stripe.LAST_REQUEST).to.deep.equal({
         method: 'GET',
@@ -16,8 +16,8 @@ describe('SKU Resource', function() {
     });
   });
 
-  describe('create', function() {
-    it('Sends the correct request', function() {
+  describe('create', () => {
+    it('Sends the correct request', () => {
       stripe.skus.create({
         currency: 'usd',
         inventory: {type: 'finite', quantity: 500},
@@ -40,8 +40,8 @@ describe('SKU Resource', function() {
     });
   });
 
-  describe('list', function() {
-    it('Sends the correct request', function() {
+  describe('list', () => {
+    it('Sends the correct request', () => {
       stripe.skus.list({
         limit: 3,
       });
@@ -55,7 +55,7 @@ describe('SKU Resource', function() {
       });
     });
 
-    it('Supports filtering by product', function() {
+    it('Supports filtering by product', () => {
       stripe.skus.list({
         product: 'prodId123',
       });
@@ -70,8 +70,8 @@ describe('SKU Resource', function() {
     });
   });
 
-  describe('update', function() {
-    it('Sends the correct request', function() {
+  describe('update', () => {
+    it('Sends the correct request', () => {
       stripe.skus.update('skuIdFoo3242', {caption: 'test'});
       expect(stripe.LAST_REQUEST).to.deep.equal({
         method: 'POST',
@@ -82,8 +82,8 @@ describe('SKU Resource', function() {
     });
   });
 
-  describe('del', function() {
-    it('Sends the correct request', function() {
+  describe('del', () => {
+    it('Sends the correct request', () => {
       stripe.skus.del('skuIdFoo3242');
       expect(stripe.LAST_REQUEST).to.deep.equal({
         method: 'DELETE',
diff --git a/test/resources/Sigma/ScheduledQueryRuns.spec.js b/test/resources/Sigma/ScheduledQueryRuns.spec.js
index 857fee694e..710762d8e0 100644
--- a/test/resources/Sigma/ScheduledQueryRuns.spec.js
+++ b/test/resources/Sigma/ScheduledQueryRuns.spec.js
@@ -1,12 +1,12 @@
 'use strict';
 
-var stripe = require('../../../testUtils').getSpyableStripe();
-var expect = require('chai').expect;
+const stripe = require('../../../testUtils').getSpyableStripe();
+const expect = require('chai').expect;
 
-describe('Sigma', function() {
-  describe('ScheduledQueryRun Resource', function() {
-    describe('retrieve', function() {
-      it('Sends the correct request', function() {
+describe('Sigma', () => {
+  describe('ScheduledQueryRun Resource', () => {
+    describe('retrieve', () => {
+      it('Sends the correct request', () => {
         stripe.sigma.scheduledQueryRuns.retrieve('sqr_123');
         expect(stripe.LAST_REQUEST).to.deep.equal({
           method: 'GET',
@@ -17,8 +17,8 @@ describe('Sigma', function() {
       });
     });
 
-    describe('list', function() {
-      it('Sends the correct request', function() {
+    describe('list', () => {
+      it('Sends the correct request', () => {
         stripe.sigma.scheduledQueryRuns.list();
         expect(stripe.LAST_REQUEST).to.deep.equal({
           method: 'GET',
diff --git a/test/resources/Sources.spec.js b/test/resources/Sources.spec.js
index 51d7111539..89da485560 100644
--- a/test/resources/Sources.spec.js
+++ b/test/resources/Sources.spec.js
@@ -1,11 +1,11 @@
 'use strict';
 
-var stripe = require('../../testUtils').getSpyableStripe();
-var expect = require('chai').expect;
+const stripe = require('../../testUtils').getSpyableStripe();
+const expect = require('chai').expect;
 
-describe('Sources Resource', function() {
-  describe('retrieve', function() {
-    it('Sends the correct request', function() {
+describe('Sources Resource', () => {
+  describe('retrieve', () => {
+    it('Sends the correct request', () => {
       stripe.sources.retrieve('sourceId1');
       expect(stripe.LAST_REQUEST).to.deep.equal({
         method: 'GET',
@@ -16,8 +16,8 @@ describe('Sources Resource', function() {
     });
   });
 
-  describe('create', function() {
-    it('Sends the correct request', function() {
+  describe('create', () => {
+    it('Sends the correct request', () => {
       stripe.sources.create({
         amount: 200,
         currency: 'usd',
@@ -46,8 +46,8 @@ describe('Sources Resource', function() {
     });
   });
 
-  describe('update', function() {
-    it('Sends the correct request', function() {
+  describe('update', () => {
+    it('Sends the correct request', () => {
       stripe.sources.update('src_foo', {
         metadata: {foo: 'bar'},
       });
@@ -60,8 +60,8 @@ describe('Sources Resource', function() {
     });
   });
 
-  describe('listSourceTransactions', function() {
-    it('Sends the correct request', function() {
+  describe('listSourceTransactions', () => {
+    it('Sends the correct request', () => {
       stripe.sources.listSourceTransactions('src_foo');
       expect(stripe.LAST_REQUEST).to.deep.equal({
         method: 'GET',
@@ -72,8 +72,8 @@ describe('Sources Resource', function() {
     });
   });
 
-  describe('verify', function() {
-    it('Sends the correct request', function() {
+  describe('verify', () => {
+    it('Sends the correct request', () => {
       stripe.sources.verify('src_foo', {values: [32, 45]});
       expect(stripe.LAST_REQUEST).to.deep.equal({
         method: 'POST',
diff --git a/test/resources/SubscriptionItems.spec.js b/test/resources/SubscriptionItems.spec.js
index 76b1c598b3..73e59d58c2 100644
--- a/test/resources/SubscriptionItems.spec.js
+++ b/test/resources/SubscriptionItems.spec.js
@@ -1,11 +1,11 @@
 'use strict';
 
-var stripe = require('../../testUtils').getSpyableStripe();
-var expect = require('chai').expect;
+const stripe = require('../../testUtils').getSpyableStripe();
+const expect = require('chai').expect;
 
-describe('SubscriptionItems Resource', function() {
-  describe('retrieve', function() {
-    it('Sends the correct request', function() {
+describe('SubscriptionItems Resource', () => {
+  describe('retrieve', () => {
+    it('Sends the correct request', () => {
       stripe.subscriptionItems.retrieve('test_sub_item');
       expect(stripe.LAST_REQUEST).to.deep.equal({
         method: 'GET',
@@ -16,8 +16,8 @@ describe('SubscriptionItems Resource', function() {
     });
   });
 
-  describe('del', function() {
-    it('Sends the correct request', function() {
+  describe('del', () => {
+    it('Sends the correct request', () => {
       stripe.subscriptionItems.del('test_sub_item');
       expect(stripe.LAST_REQUEST).to.deep.equal({
         method: 'DELETE',
@@ -28,8 +28,8 @@ describe('SubscriptionItems Resource', function() {
     });
   });
 
-  describe('update', function() {
-    it('Sends the correct request', function() {
+  describe('update', () => {
+    it('Sends the correct request', () => {
       stripe.subscriptionItems.update('test_sub_item', {
         plan: 'gold',
       });
@@ -44,8 +44,8 @@ describe('SubscriptionItems Resource', function() {
     });
   });
 
-  describe('create', function() {
-    it('Sends the correct request', function() {
+  describe('create', () => {
+    it('Sends the correct request', () => {
       stripe.subscriptionItems.create({
         subscription: 'test_sub',
         plan: 'gold',
@@ -63,8 +63,8 @@ describe('SubscriptionItems Resource', function() {
     });
   });
 
-  describe('list', function() {
-    it('Sends the correct request', function() {
+  describe('list', () => {
+    it('Sends the correct request', () => {
       stripe.subscriptionItems.list({
         limit: 3,
         subscription: 'test_sub',
diff --git a/test/resources/SubscriptionSchedule.spec.js b/test/resources/SubscriptionSchedule.spec.js
index 9e07b182a2..68107b3382 100644
--- a/test/resources/SubscriptionSchedule.spec.js
+++ b/test/resources/SubscriptionSchedule.spec.js
@@ -1,44 +1,44 @@
 'use strict';
 
-var stripe = require('../../testUtils').getSpyableStripe();
-var expect = require('chai').expect;
+const stripe = require('../../testUtils').getSpyableStripe();
+const expect = require('chai').expect;
 
-var SCHEDULE_TEST_ID = 'sub_sched_123';
-var REVISION_TEST_ID = 'sub_sched_rev_123';
+const SCHEDULE_TEST_ID = 'sub_sched_123';
+const REVISION_TEST_ID = 'sub_sched_rev_123';
 
-describe('Subscription Schedule Resource', function() {
-  describe('cancel', function() {
-    it('Sends the correct request', function() {
-      var data = {
+describe('Subscription Schedule Resource', () => {
+  describe('cancel', () => {
+    it('Sends the correct request', () => {
+      const data = {
         invoice_now: true,
       };
       stripe.subscriptionSchedules.cancel(SCHEDULE_TEST_ID, data);
       expect(stripe.LAST_REQUEST).to.deep.equal({
         method: 'POST',
-        url: '/v1/subscription_schedules/' + SCHEDULE_TEST_ID + '/cancel',
-        data: data,
+        url: `/v1/subscription_schedules/${SCHEDULE_TEST_ID}/cancel`,
+        data,
         headers: {},
       });
     });
   });
 
-  describe('create', function() {
-    it('Sends the correct request', function() {
-      var data = {
+  describe('create', () => {
+    it('Sends the correct request', () => {
+      const data = {
         customer: 'cus_123',
       };
       stripe.subscriptionSchedules.create(data);
       expect(stripe.LAST_REQUEST).to.deep.equal({
         method: 'POST',
         url: '/v1/subscription_schedules',
-        data: data,
+        data,
         headers: {},
       });
     });
   });
 
-  describe('list', function() {
-    it('Sends the correct request', function() {
+  describe('list', () => {
+    it('Sends the correct request', () => {
       stripe.subscriptionSchedules.list();
       expect(stripe.LAST_REQUEST).to.deep.equal({
         method: 'GET',
@@ -49,23 +49,23 @@ describe('Subscription Schedule Resource', function() {
     });
   });
 
-  describe('release', function() {
-    it('Sends the correct request', function() {
-      var data = {
+  describe('release', () => {
+    it('Sends the correct request', () => {
+      const data = {
         preserve_cancel_date: true,
       };
       stripe.subscriptionSchedules.release(SCHEDULE_TEST_ID, data);
       expect(stripe.LAST_REQUEST).to.deep.equal({
         method: 'POST',
-        url: '/v1/subscription_schedules/' + SCHEDULE_TEST_ID + '/release',
-        data: data,
+        url: `/v1/subscription_schedules/${SCHEDULE_TEST_ID}/release`,
+        data,
         headers: {},
       });
     });
   });
 
-  describe('retrieve', function() {
-    it('Sends the correct request', function() {
+  describe('retrieve', () => {
+    it('Sends the correct request', () => {
       stripe.subscriptionSchedules.retrieve(SCHEDULE_TEST_ID);
       expect(stripe.LAST_REQUEST).to.deep.equal({
         method: 'GET',
@@ -76,45 +76,41 @@ describe('Subscription Schedule Resource', function() {
     });
   });
 
-  describe('update', function() {
-    it('Sends the correct request', function() {
-      var data = {metadata: {key: 'value'}};
+  describe('update', () => {
+    it('Sends the correct request', () => {
+      const data = {metadata: {key: 'value'}};
       stripe.subscriptionSchedules.update(SCHEDULE_TEST_ID, data);
       expect(stripe.LAST_REQUEST).to.deep.equal({
         method: 'POST',
-        url: '/v1/subscription_schedules/' + SCHEDULE_TEST_ID,
-        data: data,
+        url: `/v1/subscription_schedules/${SCHEDULE_TEST_ID}`,
+        data,
         headers: {},
       });
     });
   });
 
-  describe('Revision methods', function() {
-    describe('retrieveRevision', function() {
-      it('Sends the correct request', function() {
+  describe('Revision methods', () => {
+    describe('retrieveRevision', () => {
+      it('Sends the correct request', () => {
         stripe.subscriptionSchedules.retrieveRevision(
           SCHEDULE_TEST_ID,
           REVISION_TEST_ID
         );
         expect(stripe.LAST_REQUEST).to.deep.equal({
           method: 'GET',
-          url:
-            '/v1/subscription_schedules/' +
-            SCHEDULE_TEST_ID +
-            '/revisions/' +
-            REVISION_TEST_ID,
+          url: `/v1/subscription_schedules/${SCHEDULE_TEST_ID}/revisions/${REVISION_TEST_ID}`,
           headers: {},
           data: {},
         });
       });
     });
 
-    describe('listRevisions', function() {
-      it('Sends the correct request', function() {
+    describe('listRevisions', () => {
+      it('Sends the correct request', () => {
         stripe.subscriptionSchedules.listRevisions(SCHEDULE_TEST_ID);
         expect(stripe.LAST_REQUEST).to.deep.equal({
           method: 'GET',
-          url: '/v1/subscription_schedules/' + SCHEDULE_TEST_ID + '/revisions',
+          url: `/v1/subscription_schedules/${SCHEDULE_TEST_ID}/revisions`,
           headers: {},
           data: {},
         });
diff --git a/test/resources/SubscriptionScheduleRevision.spec.js b/test/resources/SubscriptionScheduleRevision.spec.js
index 58adf867cf..4c15320d7a 100644
--- a/test/resources/SubscriptionScheduleRevision.spec.js
+++ b/test/resources/SubscriptionScheduleRevision.spec.js
@@ -1,43 +1,39 @@
 'use strict';
 
-var resources = require('../../lib/stripe').resources;
-var stripe = require('../../testUtils').getSpyableStripe();
-var expect = require('chai').expect;
+const resources = require('../../lib/stripe').resources;
+const stripe = require('../../testUtils').getSpyableStripe();
+const expect = require('chai').expect;
 
-var SCHEDULE_TEST_ID = 'sub_sched_123';
-var REVISION_TEST_ID = 'sub_sched_rev_123';
+const SCHEDULE_TEST_ID = 'sub_sched_123';
+const REVISION_TEST_ID = 'sub_sched_rev_123';
 
 // Create new SubscriptionScheduleRevision instance with pre-filled scheduleId:
-var revision = new resources.SubscriptionScheduleRevisions(stripe, {
+const revision = new resources.SubscriptionScheduleRevisions(stripe, {
   scheduleId: SCHEDULE_TEST_ID,
 });
 
 // Use spy from existing resource:
 revision._request = stripe.customers._request;
 
-describe('SubscriptionScheduleRevision Resource', function() {
-  describe('list', function() {
-    it('Sends the correct request', function() {
+describe('SubscriptionScheduleRevision Resource', () => {
+  describe('list', () => {
+    it('Sends the correct request', () => {
       revision.list();
       expect(stripe.LAST_REQUEST).to.deep.equal({
         method: 'GET',
-        url: '/v1/subscription_schedules/' + SCHEDULE_TEST_ID + '/revisions',
+        url: `/v1/subscription_schedules/${SCHEDULE_TEST_ID}/revisions`,
         data: {},
         headers: {},
       });
     });
   });
 
-  describe('retrieve', function() {
-    it('Sends the correct request', function() {
+  describe('retrieve', () => {
+    it('Sends the correct request', () => {
       revision.retrieve(REVISION_TEST_ID);
       expect(stripe.LAST_REQUEST).to.deep.equal({
         method: 'GET',
-        url:
-          '/v1/subscription_schedules/' +
-          SCHEDULE_TEST_ID +
-          '/revisions/' +
-          REVISION_TEST_ID,
+        url: `/v1/subscription_schedules/${SCHEDULE_TEST_ID}/revisions/${REVISION_TEST_ID}`,
         data: {},
         headers: {},
       });
diff --git a/test/resources/Subscriptions.spec.js b/test/resources/Subscriptions.spec.js
index 31b1e876f5..d1f7631295 100644
--- a/test/resources/Subscriptions.spec.js
+++ b/test/resources/Subscriptions.spec.js
@@ -1,11 +1,11 @@
 'use strict';
 
-var stripe = require('../../testUtils').getSpyableStripe();
-var expect = require('chai').expect;
+const stripe = require('../../testUtils').getSpyableStripe();
+const expect = require('chai').expect;
 
-describe('subscriptions Resource', function() {
-  describe('retrieve', function() {
-    it('Sends the correct request', function() {
+describe('subscriptions Resource', () => {
+  describe('retrieve', () => {
+    it('Sends the correct request', () => {
       stripe.subscriptions.retrieve('test_sub');
       expect(stripe.LAST_REQUEST).to.deep.equal({
         method: 'GET',
@@ -16,8 +16,8 @@ describe('subscriptions Resource', function() {
     });
   });
 
-  describe('del', function() {
-    it('Sends the correct request', function() {
+  describe('del', () => {
+    it('Sends the correct request', () => {
       stripe.subscriptions.del('test_sub');
       expect(stripe.LAST_REQUEST).to.deep.equal({
         method: 'DELETE',
@@ -28,8 +28,8 @@ describe('subscriptions Resource', function() {
     });
   });
 
-  describe('update', function() {
-    it('Sends the correct request', function() {
+  describe('update', () => {
+    it('Sends the correct request', () => {
       stripe.subscriptions.update('test_sub', {
         metadata: {a: '1234'},
       });
@@ -44,8 +44,8 @@ describe('subscriptions Resource', function() {
     });
   });
 
-  describe('create', function() {
-    it('Sends the correct request', function() {
+  describe('create', () => {
+    it('Sends the correct request', () => {
       stripe.subscriptions.create({
         customer: 'test_cus',
         plan: 'gold',
@@ -63,8 +63,8 @@ describe('subscriptions Resource', function() {
     });
   });
 
-  describe('update with items array', function() {
-    it('Sends the correct request', function() {
+  describe('update with items array', () => {
+    it('Sends the correct request', () => {
       stripe.subscriptions.update('test_sub', {
         items: [
           {
@@ -89,8 +89,8 @@ describe('subscriptions Resource', function() {
     });
   });
 
-  describe('create with items array', function() {
-    it('Sends the correct request', function() {
+  describe('create with items array', () => {
+    it('Sends the correct request', () => {
       stripe.subscriptions.create({
         items: [
           {
@@ -116,8 +116,8 @@ describe('subscriptions Resource', function() {
     });
   });
 
-  describe('update with items object', function() {
-    it('Sends the correct request', function() {
+  describe('update with items object', () => {
+    it('Sends the correct request', () => {
       stripe.subscriptions.update('test_sub', {
         items: {
           '0': {
@@ -142,8 +142,8 @@ describe('subscriptions Resource', function() {
     });
   });
 
-  describe('create with items object', function() {
-    it('Sends the correct request', function() {
+  describe('create with items object', () => {
+    it('Sends the correct request', () => {
       stripe.subscriptions.create({
         items: {
           '0': {
@@ -169,8 +169,8 @@ describe('subscriptions Resource', function() {
     });
   });
 
-  describe('list', function() {
-    it('Sends the correct request', function() {
+  describe('list', () => {
+    it('Sends the correct request', () => {
       stripe.subscriptions.list({
         limit: 3,
         customer: 'test_cus',
@@ -189,9 +189,9 @@ describe('subscriptions Resource', function() {
     });
   });
 
-  describe('Discount methods', function() {
-    describe('deleteDiscount', function() {
-      it('Sends the correct request', function() {
+  describe('Discount methods', () => {
+    describe('deleteDiscount', () => {
+      it('Sends the correct request', () => {
         stripe.subscriptions.deleteDiscount('test_sub');
         expect(stripe.LAST_REQUEST).to.deep.equal({
           method: 'DELETE',
diff --git a/test/resources/TaxIds.spec.js b/test/resources/TaxIds.spec.js
index 773400e030..fda30f9faa 100644
--- a/test/resources/TaxIds.spec.js
+++ b/test/resources/TaxIds.spec.js
@@ -1,64 +1,64 @@
 'use strict';
 
-var resources = require('../../lib/stripe').resources;
-var stripe = require('../../testUtils').getSpyableStripe();
-var expect = require('chai').expect;
+const resources = require('../../lib/stripe').resources;
+const stripe = require('../../testUtils').getSpyableStripe();
+const expect = require('chai').expect;
 
-var CUSTOMER_TEST_ID = 'cus_123';
-var TAX_ID_TEST_ID = 'txi_123';
+const CUSTOMER_TEST_ID = 'cus_123';
+const TAX_ID_TEST_ID = 'txi_123';
 
-var taxId = new resources.TaxIds(stripe, {customerId: CUSTOMER_TEST_ID});
+const taxId = new resources.TaxIds(stripe, {customerId: CUSTOMER_TEST_ID});
 
 // Use spy from existing resource:
 taxId._request = stripe.customers._request;
 
-describe('TaxId Resource', function() {
-  describe('create', function() {
-    it('Sends the correct request', function() {
-      var data = {
+describe('TaxId Resource', () => {
+  describe('create', () => {
+    it('Sends the correct request', () => {
+      const data = {
         type: 'eu_vat',
         value: '11111',
       };
       taxId.create(data);
       expect(stripe.LAST_REQUEST).to.deep.equal({
         method: 'POST',
-        url: '/v1/customers/' + CUSTOMER_TEST_ID + '/tax_ids',
-        data: data,
+        url: `/v1/customers/${CUSTOMER_TEST_ID}/tax_ids`,
+        data,
         headers: {},
       });
     });
   });
 
-  describe('delete', function() {
-    it('Sends the correct request', function() {
+  describe('delete', () => {
+    it('Sends the correct request', () => {
       taxId.del(TAX_ID_TEST_ID);
       expect(stripe.LAST_REQUEST).to.deep.equal({
         method: 'DELETE',
-        url: '/v1/customers/' + CUSTOMER_TEST_ID + '/tax_ids/' + TAX_ID_TEST_ID,
+        url: `/v1/customers/${CUSTOMER_TEST_ID}/tax_ids/${TAX_ID_TEST_ID}`,
         data: {},
         headers: {},
       });
     });
   });
 
-  describe('list', function() {
-    it('Sends the correct request', function() {
+  describe('list', () => {
+    it('Sends the correct request', () => {
       taxId.list();
       expect(stripe.LAST_REQUEST).to.deep.equal({
         method: 'GET',
-        url: '/v1/customers/' + CUSTOMER_TEST_ID + '/tax_ids',
+        url: `/v1/customers/${CUSTOMER_TEST_ID}/tax_ids`,
         data: {},
         headers: {},
       });
     });
   });
 
-  describe('retrieve', function() {
-    it('Sends the correct request', function() {
+  describe('retrieve', () => {
+    it('Sends the correct request', () => {
       taxId.retrieve(TAX_ID_TEST_ID);
       expect(stripe.LAST_REQUEST).to.deep.equal({
         method: 'GET',
-        url: '/v1/customers/' + CUSTOMER_TEST_ID + '/tax_ids/' + TAX_ID_TEST_ID,
+        url: `/v1/customers/${CUSTOMER_TEST_ID}/tax_ids/${TAX_ID_TEST_ID}`,
         data: {},
         headers: {},
       });
diff --git a/test/resources/TaxRates.spec.js b/test/resources/TaxRates.spec.js
index 74e931b29b..65afc211b3 100644
--- a/test/resources/TaxRates.spec.js
+++ b/test/resources/TaxRates.spec.js
@@ -1,11 +1,11 @@
 'use strict';
 
-var stripe = require('../../testUtils').getSpyableStripe();
-var expect = require('chai').expect;
+const stripe = require('../../testUtils').getSpyableStripe();
+const expect = require('chai').expect;
 
-describe('TaxRates Resource', function() {
-  describe('retrieve', function() {
-    it('Sends the correct request', function() {
+describe('TaxRates Resource', () => {
+  describe('retrieve', () => {
+    it('Sends the correct request', () => {
       stripe.taxRates.retrieve('txr_123');
       expect(stripe.LAST_REQUEST).to.deep.equal({
         method: 'GET',
@@ -16,9 +16,9 @@ describe('TaxRates Resource', function() {
     });
   });
 
-  describe('update', function() {
-    it('Sends the correct request', function() {
-      var data = {
+  describe('update', () => {
+    it('Sends the correct request', () => {
+      const data = {
         metadata: {a: '1234'},
       };
       stripe.taxRates.update('txr_123', data);
@@ -26,14 +26,14 @@ describe('TaxRates Resource', function() {
         method: 'POST',
         url: '/v1/tax_rates/txr_123',
         headers: {},
-        data: data,
+        data,
       });
     });
   });
 
-  describe('create', function() {
-    it('Sends the correct request', function() {
-      var data = {
+  describe('create', () => {
+    it('Sends the correct request', () => {
+      const data = {
         display_name: 'name',
         inclusive: false,
         percentage: 10.15,
@@ -44,13 +44,13 @@ describe('TaxRates Resource', function() {
         method: 'POST',
         url: '/v1/tax_rates',
         headers: {},
-        data: data,
+        data,
       });
     });
   });
 
-  describe('list', function() {
-    it('Sends the correct request', function() {
+  describe('list', () => {
+    it('Sends the correct request', () => {
       stripe.taxRates.list();
       expect(stripe.LAST_REQUEST).to.deep.equal({
         method: 'GET',
diff --git a/test/resources/Terminal/ConnectionTokens.spec.js b/test/resources/Terminal/ConnectionTokens.spec.js
index 75dd800361..2fa3a23845 100644
--- a/test/resources/Terminal/ConnectionTokens.spec.js
+++ b/test/resources/Terminal/ConnectionTokens.spec.js
@@ -1,13 +1,13 @@
 'use strict';
 
-var stripe = require('../../../testUtils').getSpyableStripe();
+const stripe = require('../../../testUtils').getSpyableStripe();
 
-var expect = require('chai').expect;
+const expect = require('chai').expect;
 
-describe('Terminal', function() {
-  describe('ConnectionToken Resource', function() {
-    describe('create', function() {
-      it('Sends the correct request', function() {
+describe('Terminal', () => {
+  describe('ConnectionToken Resource', () => {
+    describe('create', () => {
+      it('Sends the correct request', () => {
         stripe.terminal.connectionTokens.create({});
         expect(stripe.LAST_REQUEST).to.deep.equal({
           method: 'POST',
diff --git a/test/resources/Terminal/Locations.spec.js b/test/resources/Terminal/Locations.spec.js
index 335beb55b9..9ed65d5f8b 100644
--- a/test/resources/Terminal/Locations.spec.js
+++ b/test/resources/Terminal/Locations.spec.js
@@ -1,13 +1,13 @@
 'use strict';
 
-var stripe = require('../../../testUtils').getSpyableStripe();
+const stripe = require('../../../testUtils').getSpyableStripe();
 
-var expect = require('chai').expect;
+const expect = require('chai').expect;
 
-describe('Terminal', function() {
-  describe('Locations Resource', function() {
-    describe('retrieve', function() {
-      it('Sends the correct request', function() {
+describe('Terminal', () => {
+  describe('Locations Resource', () => {
+    describe('retrieve', () => {
+      it('Sends the correct request', () => {
         stripe.terminal.locations.retrieve('loc_123');
 
         expect(stripe.LAST_REQUEST).to.deep.equal({
@@ -19,8 +19,8 @@ describe('Terminal', function() {
       });
     });
 
-    describe('create', function() {
-      it('Sends the correct request', function() {
+    describe('create', () => {
+      it('Sends the correct request', () => {
         stripe.terminal.locations.create({
           display_name: 'name',
           address: {
@@ -49,8 +49,8 @@ describe('Terminal', function() {
       });
     });
 
-    describe('del', function() {
-      it('Sends the correct request', function() {
+    describe('del', () => {
+      it('Sends the correct request', () => {
         stripe.terminal.locations.del('loc_123');
         expect(stripe.LAST_REQUEST).to.deep.equal({
           method: 'DELETE',
@@ -61,8 +61,8 @@ describe('Terminal', function() {
       });
     });
 
-    describe('update', function() {
-      it('Sends the correct request', function() {
+    describe('update', () => {
+      it('Sends the correct request', () => {
         stripe.terminal.locations.update('loc_123', {
           display_name: 'name',
         });
@@ -77,8 +77,8 @@ describe('Terminal', function() {
       });
     });
 
-    describe('list', function() {
-      it('Sends the correct request', function() {
+    describe('list', () => {
+      it('Sends the correct request', () => {
         stripe.terminal.locations.list();
         expect(stripe.LAST_REQUEST).to.deep.equal({
           method: 'GET',
diff --git a/test/resources/Terminal/Readers.spec.js b/test/resources/Terminal/Readers.spec.js
index ab6f60451f..f4bd88fb90 100644
--- a/test/resources/Terminal/Readers.spec.js
+++ b/test/resources/Terminal/Readers.spec.js
@@ -1,13 +1,13 @@
 'use strict';
 
-var stripe = require('../../../testUtils').getSpyableStripe();
+const stripe = require('../../../testUtils').getSpyableStripe();
 
-var expect = require('chai').expect;
+const expect = require('chai').expect;
 
-describe('Terminal', function() {
-  describe('Readers Resource', function() {
-    describe('retrieve', function() {
-      it('Sends the correct request', function() {
+describe('Terminal', () => {
+  describe('Readers Resource', () => {
+    describe('retrieve', () => {
+      it('Sends the correct request', () => {
         stripe.terminal.readers.retrieve('rdr_123');
 
         expect(stripe.LAST_REQUEST).to.deep.equal({
@@ -19,8 +19,8 @@ describe('Terminal', function() {
       });
     });
 
-    describe('create', function() {
-      it('Sends the correct request', function() {
+    describe('create', () => {
+      it('Sends the correct request', () => {
         stripe.terminal.readers.create({
           registration_code: 'a-b-c',
           label: 'name',
@@ -37,8 +37,8 @@ describe('Terminal', function() {
       });
     });
 
-    describe('del', function() {
-      it('Sends the correct request', function() {
+    describe('del', () => {
+      it('Sends the correct request', () => {
         stripe.terminal.readers.del('rdr_123');
         expect(stripe.LAST_REQUEST).to.deep.equal({
           method: 'DELETE',
@@ -49,8 +49,8 @@ describe('Terminal', function() {
       });
     });
 
-    describe('update', function() {
-      it('Sends the correct request', function() {
+    describe('update', () => {
+      it('Sends the correct request', () => {
         stripe.terminal.readers.update('rdr_123', {
           label: 'name',
         });
@@ -65,8 +65,8 @@ describe('Terminal', function() {
       });
     });
 
-    describe('list', function() {
-      it('Sends the correct request', function() {
+    describe('list', () => {
+      it('Sends the correct request', () => {
         stripe.terminal.readers.list();
         expect(stripe.LAST_REQUEST).to.deep.equal({
           method: 'GET',
diff --git a/test/resources/ThreeDSecure.spec.js b/test/resources/ThreeDSecure.spec.js
index 9efc67b1e6..7ff3aad648 100644
--- a/test/resources/ThreeDSecure.spec.js
+++ b/test/resources/ThreeDSecure.spec.js
@@ -1,11 +1,11 @@
 'use strict';
 
-var stripe = require('../../testUtils').getSpyableStripe();
-var expect = require('chai').expect;
+const stripe = require('../../testUtils').getSpyableStripe();
+const expect = require('chai').expect;
 
-describe('ThreeDSecure Resource', function() {
-  describe('retrieve', function() {
-    it('Sends the correct request', function() {
+describe('ThreeDSecure Resource', () => {
+  describe('retrieve', () => {
+    it('Sends the correct request', () => {
       stripe.threeDSecure.retrieve('tdsrc_id');
       expect(stripe.LAST_REQUEST).to.deep.equal({
         method: 'GET',
@@ -16,8 +16,8 @@ describe('ThreeDSecure Resource', function() {
     });
   });
 
-  describe('create', function() {
-    it('Sends the correct request', function() {
+  describe('create', () => {
+    it('Sends the correct request', () => {
       stripe.threeDSecure.create({
         card: 'tok_test',
         amount: 1500,
diff --git a/test/resources/Tokens.spec.js b/test/resources/Tokens.spec.js
index 397168f002..a5dd59708a 100644
--- a/test/resources/Tokens.spec.js
+++ b/test/resources/Tokens.spec.js
@@ -1,11 +1,11 @@
 'use strict';
 
-var stripe = require('../../testUtils').getSpyableStripe();
-var expect = require('chai').expect;
+const stripe = require('../../testUtils').getSpyableStripe();
+const expect = require('chai').expect;
 
-describe('Tokens Resource', function() {
-  describe('create', function() {
-    it('Sends the correct request', function() {
+describe('Tokens Resource', () => {
+  describe('create', () => {
+    it('Sends the correct request', () => {
       stripe.tokens.create({
         card: {number: 123},
       });
@@ -18,8 +18,8 @@ describe('Tokens Resource', function() {
     });
   });
 
-  describe('retrieve', function() {
-    it('Sends the correct request', function() {
+  describe('retrieve', () => {
+    it('Sends the correct request', () => {
       stripe.tokens.retrieve('tokenId1');
       expect(stripe.LAST_REQUEST).to.deep.equal({
         method: 'GET',
diff --git a/test/resources/Topups.spec.js b/test/resources/Topups.spec.js
index 7c1d1c2b94..dc77c0d2d0 100644
--- a/test/resources/Topups.spec.js
+++ b/test/resources/Topups.spec.js
@@ -1,11 +1,11 @@
 'use strict';
 
-var stripe = require('../../testUtils').getSpyableStripe();
-var expect = require('chai').expect;
+const stripe = require('../../testUtils').getSpyableStripe();
+const expect = require('chai').expect;
 
-describe('Topup Resource', function() {
-  describe('retrieve', function() {
-    it('Sends the correct request', function() {
+describe('Topup Resource', () => {
+  describe('retrieve', () => {
+    it('Sends the correct request', () => {
       stripe.topups.retrieve('tu_123');
       expect(stripe.LAST_REQUEST).to.deep.equal({
         method: 'GET',
@@ -16,8 +16,8 @@ describe('Topup Resource', function() {
     });
   });
 
-  describe('create', function() {
-    it('Sends the correct request', function() {
+  describe('create', () => {
+    it('Sends the correct request', () => {
       stripe.topups.create({
         source: 'src_123',
         amount: '1500',
@@ -40,8 +40,8 @@ describe('Topup Resource', function() {
     });
   });
 
-  describe('list', function() {
-    it('Sends the correct request', function() {
+  describe('list', () => {
+    it('Sends the correct request', () => {
       stripe.topups.list();
       expect(stripe.LAST_REQUEST).to.deep.equal({
         method: 'GET',
@@ -52,8 +52,8 @@ describe('Topup Resource', function() {
     });
   });
 
-  describe('cancel', function() {
-    it('Sends the correct request', function() {
+  describe('cancel', () => {
+    it('Sends the correct request', () => {
       stripe.topups.cancel('tu_123');
       expect(stripe.LAST_REQUEST).to.deep.equal({
         method: 'POST',
@@ -64,8 +64,8 @@ describe('Topup Resource', function() {
     });
   });
 
-  describe('update', function() {
-    it('Sends the correct request', function() {
+  describe('update', () => {
+    it('Sends the correct request', () => {
       stripe.topups.update('tu_123', {metadata: {key: 'value'}});
       expect(stripe.LAST_REQUEST).to.deep.equal({
         method: 'POST',
diff --git a/test/resources/TransferReversals.spec.js b/test/resources/TransferReversals.spec.js
index 5f06d8c0e9..3b0c9e67fb 100644
--- a/test/resources/TransferReversals.spec.js
+++ b/test/resources/TransferReversals.spec.js
@@ -1,75 +1,67 @@
 'use strict';
 
-var resources = require('../../lib/stripe').resources;
-var stripe = require('../../testUtils').getSpyableStripe();
-var expect = require('chai').expect;
+const resources = require('../../lib/stripe').resources;
+const stripe = require('../../testUtils').getSpyableStripe();
+const expect = require('chai').expect;
 
-var TRANSFER_TEST_ID = 'transferIdTest999';
-var REVERSAL_TEST_ID = 'reversalIdTest999';
+const TRANSFER_TEST_ID = 'transferIdTest999';
+const REVERSAL_TEST_ID = 'reversalIdTest999';
 
 // Create new CustomerCard instance with pre-filled customerId:
-var transferReversal = new resources.TransferReversals(stripe, {
+const transferReversal = new resources.TransferReversals(stripe, {
   transferId: TRANSFER_TEST_ID,
 });
 
 // Use spy from existing resource:
 transferReversal._request = stripe.customers._request;
 
-describe('TransferReversal Resource', function() {
-  describe('retrieve', function() {
-    it('Sends the correct request', function() {
+describe('TransferReversal Resource', () => {
+  describe('retrieve', () => {
+    it('Sends the correct request', () => {
       transferReversal.retrieve(REVERSAL_TEST_ID);
       expect(stripe.LAST_REQUEST).to.deep.equal({
         method: 'GET',
-        url:
-          '/v1/transfers/' +
-          TRANSFER_TEST_ID +
-          '/reversals/' +
-          REVERSAL_TEST_ID,
+        url: `/v1/transfers/${TRANSFER_TEST_ID}/reversals/${REVERSAL_TEST_ID}`,
         data: {},
         headers: {},
       });
     });
   });
 
-  describe('create', function() {
-    it('Sends the correct request', function() {
+  describe('create', () => {
+    it('Sends the correct request', () => {
       transferReversal.create({
         amount: 100,
       });
       expect(stripe.LAST_REQUEST).to.deep.equal({
         method: 'POST',
-        url: '/v1/transfers/' + TRANSFER_TEST_ID + '/reversals',
+        url: `/v1/transfers/${TRANSFER_TEST_ID}/reversals`,
         data: {amount: 100},
         headers: {},
       });
     });
   });
 
-  describe('update', function() {
-    it('Sends the correct request', function() {
+  describe('update', () => {
+    it('Sends the correct request', () => {
       transferReversal.update(REVERSAL_TEST_ID, {
         metadata: {key: 'value'},
       });
       expect(stripe.LAST_REQUEST).to.deep.equal({
         method: 'POST',
-        url:
-          '/v1/transfers/' +
-          TRANSFER_TEST_ID +
-          '/reversals/' +
-          REVERSAL_TEST_ID,
+        url: `/v1/transfers/${TRANSFER_TEST_ID}/reversals/${REVERSAL_TEST_ID}`,
         data: {metadata: {key: 'value'}},
         headers: {},
       });
     });
   });
 
-  describe('list', function() {
-    it('Sends the correct request', function() {
+  describe('list', () => {
+    it('Sends the correct request', () => {
       transferReversal.list();
       expect(stripe.LAST_REQUEST).to.deep.equal({
         method: 'GET',
-        url: '/v1/transfers/' + TRANSFER_TEST_ID + '/reversals',
+        url: `/v1/transfers/${TRANSFER_TEST_ID}/reversals`,
         data: {},
         headers: {},
       });
diff --git a/test/resources/Transfers.spec.js b/test/resources/Transfers.spec.js
index f00ba9defb..bc369d8b4d 100644
--- a/test/resources/Transfers.spec.js
+++ b/test/resources/Transfers.spec.js
@@ -1,11 +1,11 @@
 'use strict';
 
-var stripe = require('../../testUtils').getSpyableStripe();
-var expect = require('chai').expect;
+const stripe = require('../../testUtils').getSpyableStripe();
+const expect = require('chai').expect;
 
-describe('Transfers Resource', function() {
-  describe('retrieve', function() {
-    it('Sends the correct request', function() {
+describe('Transfers Resource', () => {
+  describe('retrieve', () => {
+    it('Sends the correct request', () => {
       stripe.transfers.retrieve('transferId1');
       expect(stripe.LAST_REQUEST).to.deep.equal({
         method: 'GET',
@@ -16,8 +16,8 @@ describe('Transfers Resource', function() {
     });
   });
 
-  describe('create', function() {
-    it('Sends the correct request', function() {
+  describe('create', () => {
+    it('Sends the correct request', () => {
       stripe.transfers.create({
         amount: 200,
         currency: 'usd',
@@ -32,8 +32,8 @@ describe('Transfers Resource', function() {
     });
   });
 
-  describe('update', function() {
-    it('Sends the correct request', function() {
+  describe('update', () => {
+    it('Sends the correct request', () => {
       stripe.transfers.update('transferId6654', {
         amount: 300,
       });
@@ -46,8 +46,8 @@ describe('Transfers Resource', function() {
     });
   });
 
-  describe('cancel', function() {
-    it('Sends the correct request', function() {
+  describe('cancel', () => {
+    it('Sends the correct request', () => {
       stripe.transfers.cancel('transferId4');
       expect(stripe.LAST_REQUEST).to.deep.equal({
         method: 'POST',
@@ -58,8 +58,8 @@ describe('Transfers Resource', function() {
     });
   });
 
-  describe('reverse', function() {
-    it('Sends the correct request', function() {
+  describe('reverse', () => {
+    it('Sends the correct request', () => {
       stripe.transfers.reverse('transferId4');
       expect(stripe.LAST_REQUEST).to.deep.equal({
         method: 'POST',
@@ -70,8 +70,8 @@ describe('Transfers Resource', function() {
     });
   });
 
-  describe('list', function() {
-    it('Sends the correct request', function() {
+  describe('list', () => {
+    it('Sends the correct request', () => {
       stripe.transfers.list();
       expect(stripe.LAST_REQUEST).to.deep.equal({
         method: 'GET',
@@ -82,8 +82,8 @@ describe('Transfers Resource', function() {
     });
   });
 
-  describe('listTransactions', function() {
-    it('Sends the correct request', function() {
+  describe('listTransactions', () => {
+    it('Sends the correct request', () => {
       stripe.transfers.listTransactions('tr_14222');
       expect(stripe.LAST_REQUEST).to.deep.equal({
         method: 'GET',
diff --git a/test/resources/UsageRecordSummaries.spec.js b/test/resources/UsageRecordSummaries.spec.js
index 436dd580d7..0661ca727d 100644
--- a/test/resources/UsageRecordSummaries.spec.js
+++ b/test/resources/UsageRecordSummaries.spec.js
@@ -1,11 +1,11 @@
 'use strict';
 
-var stripe = require('../../testUtils').getSpyableStripe();
-var expect = require('chai').expect;
+const stripe = require('../../testUtils').getSpyableStripe();
+const expect = require('chai').expect;
 
-describe('UsageRecordSummaries Resource', function() {
-  describe('list', function() {
-    it('Sends the correct request', function() {
+describe('UsageRecordSummaries Resource', () => {
+  describe('list', () => {
+    it('Sends the correct request', () => {
       stripe.usageRecordSummaries.list('si_123', {});
 
       expect(stripe.LAST_REQUEST).to.deep.equal({
@@ -16,7 +16,7 @@ describe('UsageRecordSummaries Resource', function() {
       });
     });
 
-    it('Includes any options that were provided', function(done) {
+    it('Includes any options that were provided', (done) => {
       stripe.usageRecordSummaries
         .list(
           'si_123',
@@ -25,7 +25,7 @@ describe('UsageRecordSummaries Resource', function() {
             stripe_account: 'acct_456',
           }
         )
-        .then(function(record) {
+        .then((record) => {
           expect(stripe.LAST_REQUEST).to.deep.equal({
             method: 'GET',
             url: '/v1/subscription_items/si_123/usage_record_summaries',
@@ -39,8 +39,8 @@ describe('UsageRecordSummaries Resource', function() {
         });
     });
 
-    it('Calls a given callback', function(done) {
-      stripe.usageRecordSummaries.list('si_123', {}, function(error, record) {
+    it('Calls a given callback', (done) => {
+      stripe.usageRecordSummaries.list('si_123', {}, (error, record) => {
         done(error);
       });
     });
diff --git a/test/resources/UsageRecords.spec.js b/test/resources/UsageRecords.spec.js
index 0bd5f0b04f..fd9e3c7afc 100644
--- a/test/resources/UsageRecords.spec.js
+++ b/test/resources/UsageRecords.spec.js
@@ -1,11 +1,11 @@
 'use strict';
 
-var stripe = require('../../testUtils').getSpyableStripe();
-var expect = require('chai').expect;
+const stripe = require('../../testUtils').getSpyableStripe();
+const expect = require('chai').expect;
 
-describe('UsageRecords Resource', function() {
-  describe('create', function() {
-    it('Sends the correct request', function() {
+describe('UsageRecords Resource', () => {
+  describe('create', () => {
+    it('Sends the correct request', () => {
       stripe.usageRecords.create('si_123', {
         quantity: 123,
         timestamp: 123321,
@@ -24,7 +24,7 @@ describe('UsageRecords Resource', function() {
       });
     });
 
-    it('Includes any options that were provided', function(done) {
+    it('Includes any options that were provided', (done) => {
       stripe.usageRecords
         .create(
           'si_123',
@@ -37,7 +37,7 @@ describe('UsageRecords Resource', function() {
             stripe_account: 'acct_456',
           }
         )
-        .then(function(record) {
+        .then((record) => {
           expect(stripe.LAST_REQUEST).to.deep.equal({
             method: 'POST',
             url: '/v1/subscription_items/si_123/usage_records',
@@ -55,7 +55,7 @@ describe('UsageRecords Resource', function() {
         });
     });
 
-    it('Calls a given callback', function(done) {
+    it('Calls a given callback', (done) => {
       stripe.usageRecords.create(
         'si_123',
         {
@@ -63,7 +63,7 @@ describe('UsageRecords Resource', function() {
           timestamp: 123321,
           action: 'increment',
         },
-        function(error, record) {
+        (error, record) => {
           done(error);
         }
       );
diff --git a/test/resources/WebhookEndpoints.spec.js b/test/resources/WebhookEndpoints.spec.js
index a2ac60b390..4c76854402 100644
--- a/test/resources/WebhookEndpoints.spec.js
+++ b/test/resources/WebhookEndpoints.spec.js
@@ -1,11 +1,11 @@
 'use strict';
 
-var stripe = require('../../testUtils').getSpyableStripe();
-var expect = require('chai').expect;
+const stripe = require('../../testUtils').getSpyableStripe();
+const expect = require('chai').expect;
 
-describe('WebhookEndpoints Resource', function() {
-  describe('retrieve', function() {
-    it('Sends the correct request', function() {
+describe('WebhookEndpoints Resource', () => {
+  describe('retrieve', () => {
+    it('Sends the correct request', () => {
       stripe.webhookEndpoints.retrieve('we_123');
       expect(stripe.LAST_REQUEST).to.deep.equal({
         method: 'GET',
@@ -16,8 +16,8 @@ describe('WebhookEndpoints Resource', function() {
     });
   });
 
-  describe('del', function() {
-    it('Sends the correct request', function() {
+  describe('del', () => {
+    it('Sends the correct request', () => {
       stripe.webhookEndpoints.del('we_123');
       expect(stripe.LAST_REQUEST).to.deep.equal({
         method: 'DELETE',
@@ -28,8 +28,8 @@ describe('WebhookEndpoints Resource', function() {
     });
   });
 
-  describe('update', function() {
-    it('Sends the correct request', function() {
+  describe('update', () => {
+    it('Sends the correct request', () => {
       stripe.webhookEndpoints.update('we_123', {
         enabled_events: ['charge.succeeded'],
       });
@@ -44,8 +44,8 @@ describe('WebhookEndpoints Resource', function() {
     });
   });
 
-  describe('create', function() {
-    it('Sends the correct request', function() {
+  describe('create', () => {
+    it('Sends the correct request', () => {
       stripe.webhookEndpoints.create({
         enabled_events: ['charge.succeeded'],
         url: 'https://stripe.com',
@@ -63,8 +63,8 @@ describe('WebhookEndpoints Resource', function() {
     });
   });
 
-  describe('list', function() {
-    it('Sends the correct request', function() {
+  describe('list', () => {
+    it('Sends the correct request', () => {
       stripe.webhookEndpoints.list();
       expect(stripe.LAST_REQUEST).to.deep.equal({
         method: 'GET',
diff --git a/test/stripe.spec.js b/test/stripe.spec.js
index a3901a3f56..7304a9d3e5 100644
--- a/test/stripe.spec.js
+++ b/test/stripe.spec.js
@@ -1,94 +1,93 @@
 'use strict';
 
-var testUtils = require('../testUtils');
-var utils = require('../lib/utils');
-var stripe = require('../lib/stripe')(testUtils.getUserStripeKey(), 'latest');
+const testUtils = require('../testUtils');
+const utils = require('../lib/utils');
+const stripe = require('../lib/stripe')(testUtils.getUserStripeKey(), 'latest');
 
-var http = require('http');
+const http = require('http');
 
-var expect = require('chai').expect;
+const expect = require('chai').expect;
 
-var CUSTOMER_DETAILS = {
+const CUSTOMER_DETAILS = {
   description: 'Some customer',
   card: 'tok_visa',
 };
 
 describe('Stripe Module', function() {
-  var cleanup = new testUtils.CleanupUtility();
+  const cleanup = new testUtils.CleanupUtility();
   this.timeout(20000);
 
-  describe('setApiKey', function() {
-    it('uses Bearer auth', function() {
+  describe('setApiKey', () => {
+    it('uses Bearer auth', () => {
       expect(stripe.getApiField('auth')).to.equal(
-        'Bearer ' + testUtils.getUserStripeKey()
+        `Bearer ${testUtils.getUserStripeKey()}`
       );
     });
   });
 
-  describe('GetClientUserAgent', function() {
-    it('Should return a user-agent serialized JSON object', function() {
-      return expect(
-        new Promise(function(resolve, reject) {
-          stripe.getClientUserAgent(function(c) {
+  describe('GetClientUserAgent', () => {
+    it('Should return a user-agent serialized JSON object', () =>
+      expect(
+        new Promise((resolve, reject) => {
+          stripe.getClientUserAgent((c) => {
             resolve(JSON.parse(c));
           });
         })
-      ).to.eventually.have.property('lang', 'node');
-    });
+      ).to.eventually.have.property('lang', 'node'));
   });
 
-  describe('GetClientUserAgentSeeded', function() {
-    it('Should return a user-agent serialized JSON object', function() {
-      var userAgent = {lang: 'node'};
+  describe('GetClientUserAgentSeeded', () => {
+    it('Should return a user-agent serialized JSON object', () => {
+      const userAgent = {lang: 'node'};
       return expect(
-        new Promise(function(resolve, reject) {
-          stripe.getClientUserAgentSeeded(userAgent, function(c) {
+        new Promise((resolve, reject) => {
+          stripe.getClientUserAgentSeeded(userAgent, (c) => {
             resolve(JSON.parse(c));
           });
         })
       ).to.eventually.have.property('lang', 'node');
     });
 
-    it('Should URI-encode user-agent fields', function() {
-      var userAgent = {lang: 'ï'};
+    it('Should URI-encode user-agent fields', () => {
+      const userAgent = {lang: 'ï'};
       return expect(
-        new Promise(function(resolve, reject) {
-          stripe.getClientUserAgentSeeded(userAgent, function(c) {
+        new Promise((resolve, reject) => {
+          stripe.getClientUserAgentSeeded(userAgent, (c) => {
             resolve(JSON.parse(c));
           });
         })
       ).to.eventually.have.property('lang', '%C3%AF');
     });
 
-    describe('uname', function() {
-      var origExec;
-      beforeEach(function() {
+    describe('uname', () => {
+      let origExec;
+      beforeEach(() => {
         origExec = utils.safeExec;
       });
-      afterEach(function() {
+      afterEach(() => {
         utils.safeExec = origExec;
       });
 
-      it('gets added to the user-agent', function() {
-        utils.safeExec = function(cmd, cb) {
+      it('gets added to the user-agent', () => {
+        utils.safeExec = (cmd, cb) => {
           cb(null, 'foøname');
         };
         return expect(
-          new Promise(function(resolve, reject) {
-            stripe.getClientUserAgentSeeded({lang: 'node'}, function(c) {
+          new Promise((resolve, reject) => {
+            stripe.getClientUserAgentSeeded({lang: 'node'}, (c) => {
               resolve(JSON.parse(c));
             });
           })
         ).to.eventually.have.property('uname', 'fo%C3%B8name');
       });
 
-      it('sets uname to UNKOWN in case of an error', function() {
-        utils.safeExec = function(cmd, cb) {
+      it('sets uname to UNKOWN in case of an error', () => {
+        utils.safeExec = (cmd, cb) => {
           cb(new Error('security'), null);
         };
         return expect(
-          new Promise(function(resolve, reject) {
-            stripe.getClientUserAgentSeeded({lang: 'node'}, function(c) {
+          new Promise((resolve, reject) => {
+            stripe.getClientUserAgentSeeded({lang: 'node'}, (c) => {
               resolve(JSON.parse(c));
             });
           })
@@ -97,17 +96,17 @@ describe('Stripe Module', function() {
     });
   });
 
-  describe('setTimeout', function() {
-    it('Should define a default equal to the node default', function() {
+  describe('setTimeout', () => {
+    it('Should define a default equal to the node default', () => {
       expect(stripe.getApiField('timeout')).to.equal(
         http.createServer().timeout
       );
     });
-    it('Should allow me to set a custom timeout', function() {
+    it('Should allow me to set a custom timeout', () => {
       stripe.setTimeout(900);
       expect(stripe.getApiField('timeout')).to.equal(900);
     });
-    it('Should allow me to set null, to reset to the default', function() {
+    it('Should allow me to set null, to reset to the default', () => {
       stripe.setTimeout(null);
       expect(stripe.getApiField('timeout')).to.equal(
         http.createServer().timeout
@@ -115,27 +114,27 @@ describe('Stripe Module', function() {
     });
   });
 
-  describe('setAppInfo', function() {
-    describe('when given nothing or an empty object', function() {
-      it('should unset stripe._appInfo', function() {
+  describe('setAppInfo', () => {
+    describe('when given nothing or an empty object', () => {
+      it('should unset stripe._appInfo', () => {
         stripe.setAppInfo();
         expect(stripe._appInfo).to.be.undefined;
       });
     });
 
-    describe('when given an object with no `name`', function() {
-      it('should throw an error', function() {
-        expect(function() {
+    describe('when given an object with no `name`', () => {
+      it('should throw an error', () => {
+        expect(() => {
           stripe.setAppInfo({});
         }).to.throw(/AppInfo.name is required/);
 
-        expect(function() {
+        expect(() => {
           stripe.setAppInfo({
             version: '1.2.3',
           });
         }).to.throw(/AppInfo.name is required/);
 
-        expect(function() {
+        expect(() => {
           stripe.setAppInfo({
             cats: '42',
           });
@@ -143,8 +142,8 @@ describe('Stripe Module', function() {
       });
     });
 
-    describe('when given at least a `name`', function() {
-      it('should set name, partner ID, url, and version of stripe._appInfo', function() {
+    describe('when given at least a `name`', () => {
+      it('should set name, partner ID, url, and version of stripe._appInfo', () => {
         stripe.setAppInfo({
           name: 'MyAwesomeApp',
         });
@@ -180,7 +179,7 @@ describe('Stripe Module', function() {
         });
       });
 
-      it('should ignore any invalid properties', function() {
+      it('should ignore any invalid properties', () => {
         stripe.setAppInfo({
           name: 'MyAwesomeApp',
           partner_id: 'partner_1234',
@@ -197,8 +196,8 @@ describe('Stripe Module', function() {
       });
     });
 
-    it('should be included in the ClientUserAgent and be added to the UserAgent String', function(done) {
-      var appInfo = {
+    it('should be included in the ClientUserAgent and be added to the UserAgent String', (done) => {
+      const appInfo = {
         name: testUtils.getRandomString(),
         version: '1.2.345',
         url: 'https://myawesomeapp.info',
@@ -206,11 +205,11 @@ describe('Stripe Module', function() {
 
       stripe.setAppInfo(appInfo);
 
-      stripe.getClientUserAgent(function(uaString) {
+      stripe.getClientUserAgent((uaString) => {
         expect(JSON.parse(uaString).application).to.eql(appInfo);
 
         expect(stripe.getAppInfoAsString()).to.eql(
-          appInfo.name + '/' + appInfo.version + ' (' + appInfo.url + ')'
+          `${appInfo.name}/${appInfo.version} (${appInfo.url})`
         );
 
         done();
@@ -218,26 +217,25 @@ describe('Stripe Module', function() {
     });
   });
 
-  describe('Callback support', function() {
-    describe('Any given endpoint', function() {
-      it('Will call a callback if successful', function() {
-        return expect(
-          new Promise(function(resolve, reject) {
-            stripe.customers.create(CUSTOMER_DETAILS, function(err, customer) {
+  describe('Callback support', () => {
+    describe('Any given endpoint', () => {
+      it('Will call a callback if successful', () =>
+        expect(
+          new Promise((resolve, reject) => {
+            stripe.customers.create(CUSTOMER_DETAILS, (err, customer) => {
               cleanup.deleteCustomer(customer.id);
               resolve('Called!');
             });
           })
-        ).to.eventually.equal('Called!');
-      });
+        ).to.eventually.equal('Called!'));
 
-      it('Will expose HTTP response object', function() {
-        return expect(
-          new Promise(function(resolve, reject) {
-            stripe.customers.create(CUSTOMER_DETAILS, function(err, customer) {
+      it('Will expose HTTP response object', () =>
+        expect(
+          new Promise((resolve, reject) => {
+            stripe.customers.create(CUSTOMER_DETAILS, (err, customer) => {
               cleanup.deleteCustomer(customer.id);
 
-              var headers = customer.lastResponse.headers;
+              const headers = customer.lastResponse.headers;
               expect(headers).to.contain.keys('request-id');
 
               expect(customer.lastResponse.requestId).to.match(/^req_/);
@@ -246,13 +244,15 @@ describe('Stripe Module', function() {
               resolve('Called!');
             });
           })
-        ).to.eventually.equal('Called!');
-        return expect(
-          new Promise(function(resolve, reject) {
+        ).to.eventually.equal('Called!'));
+
+      it('Given an error the callback will receive it', () =>
+        expect(
+          new Promise((resolve, reject) => {
             stripe.customers.createCard(
               'nonExistentCustId',
               {card: {}},
-              function(err, customer) {
+              (err, customer) => {
                 if (err) {
                   resolve('ErrorWasPassed');
                 } else {
@@ -261,14 +261,13 @@ describe('Stripe Module', function() {
               }
             );
           })
-        ).to.eventually.become('ErrorWasPassed');
-      });
+        ).to.eventually.become('ErrorWasPassed'));
     });
   });
 
-  describe('errors', function() {
-    it('Exports errors as types', function() {
-      var Stripe = require('../lib/stripe');
+  describe('errors', () => {
+    it('Exports errors as types', () => {
+      const Stripe = require('../lib/stripe');
       expect(
         new Stripe.errors.StripeInvalidRequestError({
           message: 'error',
@@ -277,14 +276,14 @@ describe('Stripe Module', function() {
     });
   });
 
-  describe('setMaxNetworkRetries', function() {
-    describe('when given an empty or non-number variable', function() {
-      it('should error', function() {
-        expect(function() {
+  describe('setMaxNetworkRetries', () => {
+    describe('when given an empty or non-number variable', () => {
+      it('should error', () => {
+        expect(() => {
           stripe.setMaxNetworkRetries('foo');
         }).to.throw(/maxNetworkRetries must be a number/);
 
-        expect(function() {
+        expect(() => {
           stripe.setMaxNetworkRetries();
         }).to.throw(/maxNetworkRetries must be a number/);
       });
diff --git a/test/telemetry.spec.js b/test/telemetry.spec.js
index cc62a61a86..355ec61de5 100644
--- a/test/telemetry.spec.js
+++ b/test/telemetry.spec.js
@@ -1,14 +1,14 @@
 'use strict';
 
 require('../testUtils');
-var http = require('http');
+const http = require('http');
 
-var expect = require('chai').expect;
-var testServer = null;
+const expect = require('chai').expect;
+let testServer = null;
 
 function createTestServer(handlerFunc, cb) {
-  var host = '127.0.0.1';
-  testServer = http.createServer(function(req, res) {
+  const host = '127.0.0.1';
+  testServer = http.createServer((req, res) => {
     try {
       handlerFunc(req, res);
     } catch (e) {
@@ -20,28 +20,28 @@ function createTestServer(handlerFunc, cb) {
       );
     }
   });
-  testServer.listen(0, host, function() {
-    var port = testServer.address().port;
+  testServer.listen(0, host, () => {
+    const port = testServer.address().port;
     cb(host, port);
   });
 }
 
-describe('Client Telemetry', function() {
-  afterEach(function() {
+describe('Client Telemetry', () => {
+  afterEach(() => {
     if (testServer) {
       testServer.close();
       testServer = null;
     }
   });
 
-  it('Does not send telemetry when disabled', function(done) {
-    var numRequests = 0;
+  it('Does not send telemetry when disabled', (done) => {
+    let numRequests = 0;
 
     createTestServer(
-      function(req, res) {
+      (req, res) => {
         numRequests += 1;
 
-        var telemetry = req.headers['x-stripe-client-telemetry'];
+        const telemetry = req.headers['x-stripe-client-telemetry'];
 
         switch (numRequests) {
           case 1:
@@ -56,7 +56,7 @@ describe('Client Telemetry', function() {
         res.writeHead(200, {'Content-Type': 'application/json'});
         res.end('{}');
       },
-      function(host, port) {
+      (host, port) => {
         const stripe = require('../lib/stripe')(
           'sk_test_FEiILxKZwnmmocJDUjUNO6pa'
         );
@@ -64,10 +64,8 @@ describe('Client Telemetry', function() {
 
         stripe.balance
           .retrieve()
-          .then(function(res) {
-            return stripe.balance.retrieve();
-          })
-          .then(function(res) {
+          .then((res) => stripe.balance.retrieve())
+          .then((res) => {
             expect(numRequests).to.equal(2);
             done();
           })
@@ -76,14 +74,14 @@ describe('Client Telemetry', function() {
     );
   });
 
-  it('Sends client telemetry on the second request when enabled', function(done) {
-    var numRequests = 0;
+  it('Sends client telemetry on the second request when enabled', (done) => {
+    let numRequests = 0;
 
     createTestServer(
-      function(req, res) {
+      (req, res) => {
         numRequests += 1;
 
-        var telemetry = req.headers['x-stripe-client-telemetry'];
+        const telemetry = req.headers['x-stripe-client-telemetry'];
 
         switch (numRequests) {
           case 1:
@@ -103,7 +101,7 @@ describe('Client Telemetry', function() {
         res.writeHead(200, {'Content-Type': 'application/json'});
         res.end('{}');
       },
-      function(host, port) {
+      (host, port) => {
         const stripe = require('../lib/stripe')(
           'sk_test_FEiILxKZwnmmocJDUjUNO6pa'
         );
@@ -112,10 +110,8 @@ describe('Client Telemetry', function() {
 
         stripe.balance
           .retrieve()
-          .then(function(res) {
-            return stripe.balance.retrieve();
-          })
-          .then(function(res) {
+          .then((res) => stripe.balance.retrieve())
+          .then((res) => {
             expect(numRequests).to.equal(2);
             done();
           })
@@ -124,14 +120,14 @@ describe('Client Telemetry', function() {
     );
   });
 
-  it('Buffers metrics on concurrent requests', function(done) {
-    var numRequests = 0;
+  it('Buffers metrics on concurrent requests', (done) => {
+    let numRequests = 0;
 
     createTestServer(
-      function(req, res) {
+      (req, res) => {
         numRequests += 1;
 
-        var telemetry = req.headers['x-stripe-client-telemetry'];
+        const telemetry = req.headers['x-stripe-client-telemetry'];
 
         switch (numRequests) {
           case 1:
@@ -153,7 +149,7 @@ describe('Client Telemetry', function() {
         res.writeHead(200, {'Content-Type': 'application/json'});
         res.end('{}');
       },
-      function(host, port) {
+      (host, port) => {
         const stripe = require('../lib/stripe')(
           'sk_test_FEiILxKZwnmmocJDUjUNO6pa'
         );
@@ -161,13 +157,10 @@ describe('Client Telemetry', function() {
         stripe.setHost(host, port, 'http');
 
         Promise.all([stripe.balance.retrieve(), stripe.balance.retrieve()])
-          .then(function() {
-            return Promise.all([
-              stripe.balance.retrieve(),
-              stripe.balance.retrieve(),
-            ]);
-          })
-          .then(function() {
+          .then(() =>
+            Promise.all([stripe.balance.retrieve(), stripe.balance.retrieve()])
+          )
+          .then(() => {
             expect(numRequests).to.equal(4);
             done();
           })
diff --git a/test/utils.spec.js b/test/utils.spec.js
index d9ae8a6d54..202da2f67c 100644
--- a/test/utils.spec.js
+++ b/test/utils.spec.js
@@ -2,14 +2,14 @@
 
 require('../testUtils');
 
-var utils = require('../lib/utils');
-var expect = require('chai').expect;
-var Buffer = require('safe-buffer').Buffer;
+const utils = require('../lib/utils');
+const expect = require('chai').expect;
+const Buffer = require('safe-buffer').Buffer;
 
-describe('utils', function() {
-  describe('makeURLInterpolator', function() {
-    it('Interpolates values into a prepared template', function() {
-      var template = utils.makeURLInterpolator('/some/url/{foo}/{baz}?ok=1');
+describe('utils', () => {
+  describe('makeURLInterpolator', () => {
+    it('Interpolates values into a prepared template', () => {
+      const template = utils.makeURLInterpolator('/some/url/{foo}/{baz}?ok=1');
 
       expect(template({foo: 1, baz: 2})).to.equal('/some/url/1/2?ok=1');
 
@@ -22,8 +22,8 @@ describe('utils', function() {
     });
   });
 
-  describe('stringifyRequestData', function() {
-    it('Handles basic types', function() {
+  describe('stringifyRequestData', () => {
+    it('Handles basic types', () => {
       expect(
         utils.stringifyRequestData({
           a: 1,
@@ -32,7 +32,7 @@ describe('utils', function() {
       ).to.equal('a=1&b=foo');
     });
 
-    it('Handles Dates', function() {
+    it('Handles Dates', () => {
       expect(
         utils.stringifyRequestData({
           date: new Date('2009-02-13T23:31:30Z'),
@@ -50,7 +50,7 @@ describe('utils', function() {
       );
     });
 
-    it('Handles deeply nested object', function() {
+    it('Handles deeply nested object', () => {
       expect(
         utils.stringifyRequestData({
           a: {
@@ -64,7 +64,7 @@ describe('utils', function() {
       ).to.equal('a[b][c][d]=2');
     });
 
-    it('Handles arrays of objects', function() {
+    it('Handles arrays of objects', () => {
       expect(
         utils.stringifyRequestData({
           a: [{b: 'c'}, {b: 'd'}],
@@ -72,7 +72,7 @@ describe('utils', function() {
       ).to.equal('a[0][b]=c&a[1][b]=d');
     });
 
-    it('Handles indexed arrays', function() {
+    it('Handles indexed arrays', () => {
       expect(
         utils.stringifyRequestData({
           a: {
@@ -83,7 +83,7 @@ describe('utils', function() {
       ).to.equal('a[0][b]=c&a[1][b]=d');
     });
 
-    it('Creates a string from an object, handling shallow nested objects', function() {
+    it('Creates a string from an object, handling shallow nested objects', () => {
       expect(
         utils.stringifyRequestData({
           test: 1,
@@ -106,11 +106,11 @@ describe('utils', function() {
     });
   });
 
-  describe('protoExtend', function() {
-    it('Provides an extension mechanism', function() {
+  describe('protoExtend', () => {
+    it('Provides an extension mechanism', () => {
       function A() {}
       A.extend = utils.protoExtend;
-      var B = A.extend({
+      const B = A.extend({
         constructor: function() {
           this.called = true;
         },
@@ -122,38 +122,38 @@ describe('utils', function() {
     });
   });
 
-  describe('getDataFromArgs', function() {
-    it('handles an empty list', function() {
+  describe('getDataFromArgs', () => {
+    it('handles an empty list', () => {
       expect(utils.getDataFromArgs([])).to.deep.equal({});
     });
-    it('handles a list with no object', function() {
-      var args = [1, 3];
+    it('handles a list with no object', () => {
+      const args = [1, 3];
       expect(utils.getDataFromArgs(args)).to.deep.equal({});
       expect(args.length).to.equal(2);
     });
-    it('ignores a hash with only options', function(done) {
-      var args = [{api_key: 'foo'}];
+    it('ignores a hash with only options', (done) => {
+      const args = [{api_key: 'foo'}];
 
       handleWarnings(
-        function() {
+        () => {
           expect(utils.getDataFromArgs(args)).to.deep.equal({});
           expect(args.length).to.equal(1);
 
           done();
         },
-        function(message) {
-          throw new Error('Should not have warned, but did: ' + message);
+        (message) => {
+          throw new Error(`Should not have warned, but did: ${message}`);
         }
       );
     });
-    it('warns if the hash contains both data and options', function(done) {
-      var args = [{foo: 'bar', api_key: 'foo', idempotency_key: 'baz'}];
+    it('warns if the hash contains both data and options', (done) => {
+      const args = [{foo: 'bar', api_key: 'foo', idempotency_key: 'baz'}];
 
       handleWarnings(
-        function() {
+        () => {
           utils.getDataFromArgs(args);
         },
-        function(message) {
+        (message) => {
           expect(message).to.equal(
             'Stripe: Options found in arguments (api_key, idempotency_key).' +
               ' Did you mean to pass an options object? See https://github.com/stripe/stripe-node/wiki/Passing-Options.'
@@ -163,62 +163,62 @@ describe('utils', function() {
         }
       );
     });
-    it('finds the data', function() {
-      var args = [{foo: 'bar'}, {api_key: 'foo'}];
+    it('finds the data', () => {
+      const args = [{foo: 'bar'}, {api_key: 'foo'}];
       expect(utils.getDataFromArgs(args)).to.deep.equal({foo: 'bar'});
       expect(args.length).to.equal(1);
     });
   });
 
-  describe('getOptsFromArgs', function() {
-    it('handles an empty list', function() {
+  describe('getOptsFromArgs', () => {
+    it('handles an empty list', () => {
       expect(utils.getOptionsFromArgs([])).to.deep.equal({
         auth: null,
         headers: {},
       });
     });
-    it('handles an list with no object', function() {
-      var args = [1, 3];
+    it('handles an list with no object', () => {
+      const args = [1, 3];
       expect(utils.getOptionsFromArgs(args)).to.deep.equal({
         auth: null,
         headers: {},
       });
       expect(args.length).to.equal(2);
     });
-    it('ignores a non-options object', function() {
-      var args = [{foo: 'bar'}];
+    it('ignores a non-options object', () => {
+      const args = [{foo: 'bar'}];
       expect(utils.getOptionsFromArgs(args)).to.deep.equal({
         auth: null,
         headers: {},
       });
       expect(args.length).to.equal(1);
     });
-    it('parses an api key', function() {
-      var args = ['sk_test_iiiiiiiiiiiiiiiiiiiiiiii'];
+    it('parses an api key', () => {
+      const args = ['sk_test_iiiiiiiiiiiiiiiiiiiiiiii'];
       expect(utils.getOptionsFromArgs(args)).to.deep.equal({
         auth: 'sk_test_iiiiiiiiiiiiiiiiiiiiiiii',
         headers: {},
       });
       expect(args.length).to.equal(0);
     });
-    it('parses an idempotency key', function() {
-      var args = [{foo: 'bar'}, {idempotency_key: 'foo'}];
+    it('parses an idempotency key', () => {
+      const args = [{foo: 'bar'}, {idempotency_key: 'foo'}];
       expect(utils.getOptionsFromArgs(args)).to.deep.equal({
         auth: null,
         headers: {'Idempotency-Key': 'foo'},
       });
       expect(args.length).to.equal(1);
     });
-    it('parses an api version', function() {
-      var args = [{foo: 'bar'}, {stripe_version: '2003-03-30'}];
+    it('parses an api version', () => {
+      const args = [{foo: 'bar'}, {stripe_version: '2003-03-30'}];
       expect(utils.getOptionsFromArgs(args)).to.deep.equal({
         auth: null,
         headers: {'Stripe-Version': '2003-03-30'},
       });
       expect(args.length).to.equal(1);
     });
-    it('parses an idempotency key and api key and api version (with data)', function() {
-      var args = [
+    it('parses an idempotency key and api key and api version (with data)', () => {
+      const args = [
         {foo: 'bar'},
         {
           api_key: 'sk_test_iiiiiiiiiiiiiiiiiiiiiiii',
@@ -235,8 +235,8 @@ describe('utils', function() {
       });
       expect(args.length).to.equal(1);
     });
-    it('parses an idempotency key and api key and api version', function() {
-      var args = [
+    it('parses an idempotency key and api key and api version', () => {
+      const args = [
         {
           api_key: 'sk_test_iiiiiiiiiiiiiiiiiiiiiiii',
           idempotency_key: 'foo',
@@ -252,8 +252,8 @@ describe('utils', function() {
       });
       expect(args.length).to.equal(0);
     });
-    it('warns if the hash contains something that does not belong', function(done) {
-      var args = [
+    it('warns if the hash contains something that does not belong', (done) => {
+      const args = [
         {foo: 'bar'},
         {
           api_key: 'sk_test_iiiiiiiiiiiiiiiiiiiiiiii',
@@ -265,10 +265,10 @@ describe('utils', function() {
       ];
 
       handleWarnings(
-        function() {
+        () => {
           utils.getOptionsFromArgs(args);
         },
-        function(message) {
+        (message) => {
           expect(message).to.equal(
             'Stripe: Invalid options found (fishsticks, custard); ignoring.'
           );
@@ -279,24 +279,24 @@ describe('utils', function() {
     });
   });
 
-  describe('secureCompare', function() {
-    it('returns true given two equal things', function() {
+  describe('secureCompare', () => {
+    it('returns true given two equal things', () => {
       expect(utils.secureCompare('potato', 'potato')).to.equal(true);
     });
 
-    it('returns false given two unequal things', function() {
+    it('returns false given two unequal things', () => {
       expect(utils.secureCompare('potato', 'tomato')).to.equal(false);
     });
 
-    it('throws an error if not given two things to compare', function() {
-      expect(function() {
+    it('throws an error if not given two things to compare', () => {
+      expect(() => {
         utils.secureCompare('potato');
       }).to.throw();
     });
   });
 
-  describe('removeEmpty', function() {
-    it('removes empty properties and leaves non-empty ones', function() {
+  describe('removeEmpty', () => {
+    it('removes empty properties and leaves non-empty ones', () => {
       expect(
         utils.removeEmpty({
           cat: 3,
@@ -310,25 +310,25 @@ describe('utils', function() {
       });
     });
 
-    it('throws an error if not given two things to compare', function() {
-      expect(function() {
+    it('throws an error if not given two things to compare', () => {
+      expect(() => {
         utils.removeEmpty('potato');
       }).to.throw();
     });
   });
 
-  describe('safeExec', function() {
-    var origExec;
-    beforeEach(function() {
+  describe('safeExec', () => {
+    let origExec;
+    beforeEach(() => {
       origExec = utils._exec;
     });
-    afterEach(function() {
+    afterEach(() => {
       utils._exec = origExec;
     });
 
-    it('runs exec', function() {
-      var calls = [];
-      utils._exec = function(cmd, cb) {
+    it('runs exec', () => {
+      const calls = [];
+      utils._exec = (cmd, cb) => {
         calls.push([cmd, cb]);
       };
 
@@ -337,13 +337,13 @@ describe('utils', function() {
       expect(calls).to.deep.equal([['hello', myCb]]);
     });
 
-    it('passes along normal errors', function() {
-      var myErr = Error('hi');
-      utils._exec = function(cmd, cb) {
+    it('passes along normal errors', () => {
+      const myErr = Error('hi');
+      utils._exec = (cmd, cb) => {
         cb(myErr, null);
       };
 
-      var calls = [];
+      const calls = [];
       function myCb(err, x) {
         calls.push([err, x]);
       }
@@ -351,13 +351,13 @@ describe('utils', function() {
       expect(calls).to.deep.equal([[myErr, null]]);
     });
 
-    it('passes along thrown errors as normal callback errors', function() {
-      var myErr = Error('hi');
-      utils._exec = function(cmd, cb) {
+    it('passes along thrown errors as normal callback errors', () => {
+      const myErr = Error('hi');
+      utils._exec = (cmd, cb) => {
         throw myErr;
       };
 
-      var calls = [];
+      const calls = [];
       function myCb(err, x) {
         calls.push([err, x]);
       }
@@ -366,8 +366,8 @@ describe('utils', function() {
     });
   });
 
-  describe('flattenAndStringify', function() {
-    it('Stringifies primitive types', function() {
+  describe('flattenAndStringify', () => {
+    it('Stringifies primitive types', () => {
       expect(
         utils.flattenAndStringify({
           a: 1,
@@ -378,7 +378,7 @@ describe('utils', function() {
       ).to.eql({a: '1', b: 'foo', c: 'true', d: 'null'});
     });
 
-    it('Flattens nested values', function() {
+    it('Flattens nested values', () => {
       expect(
         utils.flattenAndStringify({
           x: {
@@ -389,7 +389,7 @@ describe('utils', function() {
       ).to.eql({'x[a]': '1', 'x[b]': 'foo'});
     });
 
-    it('Does not flatten File objects', function() {
+    it('Does not flatten File objects', () => {
       expect(
         utils.flattenAndStringify({
           file: {
@@ -402,10 +402,10 @@ describe('utils', function() {
       ).to.eql({file: {data: 'foo'}, 'x[a]': '1'});
     });
 
-    it('Does not flatten Buffer objects', function() {
-      var buf = Buffer.from('Hi!');
-      var flattened = utils.flattenAndStringify({
-        buf: buf,
+    it('Does not flatten Buffer objects', () => {
+      const buf = Buffer.from('Hi!');
+      const flattened = utils.flattenAndStringify({
+        buf,
         x: {
           a: 1,
         },
@@ -423,7 +423,7 @@ function handleWarnings(doWithShimmedConsoleWarn, onWarn) {
     /* eslint-disable no-console */
 
     // Shim `console.warn`
-    var _warn = console.warn;
+    const _warn = console.warn;
     console.warn = onWarn;
 
     doWithShimmedConsoleWarn();
@@ -435,14 +435,14 @@ function handleWarnings(doWithShimmedConsoleWarn, onWarn) {
   } else {
     /* eslint-disable-next-line no-inner-declarations */
     function onProcessWarn(warning) {
-      onWarn(warning.name + ': ' + warning.message);
+      onWarn(`${warning.name}: ${warning.message}`);
     }
 
     process.on('warning', onProcessWarn);
 
     doWithShimmedConsoleWarn();
 
-    process.nextTick(function() {
+    process.nextTick(() => {
       process.removeListener('warning', onProcessWarn);
     });
   }
diff --git a/testUtils/index.js b/testUtils/index.js
index 443b2c4655..8215645e53 100644
--- a/testUtils/index.js
+++ b/testUtils/index.js
@@ -6,26 +6,26 @@ require('mocha');
 // Ensure we are using the 'as promised' libs before any tests are run:
 require('chai').use(require('chai-as-promised'));
 
-var ResourceNamespace = require('../lib/ResourceNamespace').ResourceNamespace;
+const ResourceNamespace = require('../lib/ResourceNamespace').ResourceNamespace;
 
-var utils = (module.exports = {
-  getUserStripeKey: function() {
-    var key =
+const utils = (module.exports = {
+  getUserStripeKey: () => {
+    const key =
       process.env.STRIPE_TEST_API_KEY || 'tGN0bIwXnHdwOa85VABjPdSn8nWY7G7I';
 
     return key;
   },
 
-  getSpyableStripe: function() {
+  getSpyableStripe: () => {
     // Provide a testable stripe instance
     // That is, with mock-requests built in and hookable
 
-    var stripe = require('../lib/stripe');
-    var stripeInstance = stripe('fakeAuthToken');
+    const stripe = require('../lib/stripe');
+    const stripeInstance = stripe('fakeAuthToken');
 
     stripeInstance.REQUESTS = [];
 
-    for (var i in stripeInstance) {
+    for (const i in stripeInstance) {
       makeInstanceSpyable(stripeInstance, stripeInstance[i]);
     }
 
@@ -33,9 +33,9 @@ var utils = (module.exports = {
       if (thisInstance instanceof stripe.StripeResource) {
         patchRequest(stripeInstance, thisInstance);
       } else if (thisInstance instanceof ResourceNamespace) {
-        var namespace = thisInstance;
+        const namespace = thisInstance;
 
-        for (var j in namespace) {
+        for (const j in namespace) {
           makeInstanceSpyable(stripeInstance, namespace[j]);
         }
       }
@@ -43,10 +43,10 @@ var utils = (module.exports = {
 
     function patchRequest(stripeInstance, instance) {
       instance._request = function(method, host, url, data, auth, options, cb) {
-        var req = (stripeInstance.LAST_REQUEST = {
-          method: method,
-          url: url,
-          data: data,
+        const req = (stripeInstance.LAST_REQUEST = {
+          method,
+          url,
+          data,
           headers: options.headers || {},
         });
         if (auth) {
@@ -68,11 +68,11 @@ var utils = (module.exports = {
    * CleanupUtility will automatically register on the mocha afterEach hook,
    * ensuring its called after each descendent-describe block.
    */
-  CleanupUtility: (function() {
+  CleanupUtility: (() => {
     CleanupUtility.DEFAULT_TIMEOUT = 20000;
 
     function CleanupUtility(timeout) {
-      var self = this;
+      const self = this;
       this._cleanupFns = [];
       this._stripe = require('../lib/stripe')(
         utils.getUserStripeKey(),
@@ -85,27 +85,27 @@ var utils = (module.exports = {
     }
 
     CleanupUtility.prototype = {
-      doCleanup: function(done) {
-        var cleanups = this._cleanupFns;
-        var total = cleanups.length;
-        var completed = 0;
-        var fn;
+      doCleanup(done) {
+        const cleanups = this._cleanupFns;
+        const total = cleanups.length;
+        let completed = 0;
+        let fn;
         while ((fn = cleanups.shift())) {
-          var promise = fn.call(this);
+          const promise = fn.call(this);
           if (!promise || !promise.then) {
             throw new Error(
               'CleanupUtility expects cleanup functions to return promises!'
             );
           }
           promise.then(
-            function() {
+            () => {
               // cleanup successful
               completed += 1;
               if (completed === total) {
                 done();
               }
             },
-            function(err) {
+            (err) => {
               // not successful
               throw err;
             }
@@ -115,25 +115,25 @@ var utils = (module.exports = {
           done();
         }
       },
-      add: function(fn) {
+      add(fn) {
         this._cleanupFns.push(fn);
       },
-      deleteCustomer: function(custId) {
+      deleteCustomer(custId) {
         this.add(function() {
           return this._stripe.customers.del(custId);
         });
       },
-      deletePlan: function(pId) {
+      deletePlan(pId) {
         this.add(function() {
           return this._stripe.plans.del(pId);
         });
       },
-      deleteCoupon: function(cId) {
+      deleteCoupon(cId) {
         this.add(function() {
           return this._stripe.coupons.del(cId);
         });
       },
-      deleteInvoiceItem: function(iiId) {
+      deleteInvoiceItem(iiId) {
         this.add(function() {
           return this._stripe.invoiceItems.del(iiId);
         });
@@ -146,17 +146,17 @@ var utils = (module.exports = {
   /**
    * Get a random string for test Object creation
    */
-  getRandomString: function() {
+  getRandomString: () => {
     return Math.random()
       .toString(36)
       .slice(2);
   },
 
-  envSupportsForAwait: function() {
+  envSupportsForAwait: () => {
     return typeof Symbol !== 'undefined' && Symbol.asyncIterator;
   },
 
-  envSupportsAwait: function() {
+  envSupportsAwait: () => {
     try {
       eval('(async function() {})'); // eslint-disable-line no-eval
       return true;