util: add fast internal array join method

PR-URL: #14881
Reviewed-By: Refael Ackermann <refack@gmail.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>

lib/internal/util.js

@@ -252,6 +252,20 @@ function promisify(orig) {
 
 promisify.custom = kCustomPromisifiedSymbol;
 
+// The build-in Array#join is slower in v8 6.0
+function join(output, separator) {
+  var str = '';
+  if (output.length !== 0) {
+    for (var i = 0; i < output.length - 1; i++) {
+      // It is faster not to use a template string here
+      str += output[i];
+      str += separator;
+    }
+    str += output[i];
+  }
+  return str;
+}
+
 module.exports = {
   assertCrypto,
   cachedResult,
@@ -265,6 +279,7 @@ module.exports = {
   normalizeEncoding,
   objectToString,
   promisify,
+  join,
 
   // Symbol used to customize promisify conversion
   customPromisifyArgs: kCustomPromisifyArgsSymbol,