expect: Test more precisely for class instance getters

CHANGELOG.md

@@ -79,6 +79,7 @@
 - `[jest-haste-map]` Remove legacy condition for duplicate module detection ([#7333](https://github.com/facebook/jest/pull/7333))
 - `[jest-haste-map]` Fix `require` detection with trailing commas and ignore `import typeof` modules ([#7385](https://github.com/facebook/jest/pull/7385))
 - `[jest-cli]` Fix to set prettierPath via config file ([#7412](https://github.com/facebook/jest/pull/7412))
+- `[expect]` Test more precisely for class instance getters ([#7477](https://github.com/facebook/jest/pull/7477))
 
 ### Chore & Maintenance
 

packages/expect/src/__tests__/utils.test.js

@@ -13,6 +13,7 @@ const {
   emptyObject,
   getObjectSubset,
   getPath,
+  hasOwnProperty,
   subsetEquality,
 } = require('../utils');
 
@@ -94,6 +95,40 @@ describe('getPath()', () => {
   });
 });
 
+describe('hasOwnProperty', () => {
+  it('does inherit getter from class', () => {
+    class MyClass {
+      get key() {
+        return 'value';
+      }
+    }
+    expect(hasOwnProperty(new MyClass(), 'key')).toBe(true);
+  });
+  it('does not inherit setter from class', () => {
+    class MyClass {
+      set key(value) {}
+    }
+    expect(hasOwnProperty(new MyClass(), 'key')).toBe(false);
+  });
+  it('does not inherit method from class', () => {
+    class MyClass {
+      key() {}
+    }
+    expect(hasOwnProperty(new MyClass(), 'key')).toBe(false);
+  });
+  it('does not inherit property from constructor prototype', () => {
+    function MyClass() {}
+    MyClass.prototype.key = 'value';
+    expect(hasOwnProperty(new MyClass(), 'key')).toBe(false);
+  });
+  it('does not inherit __proto__ getter from Object', () => {
+    expect(hasOwnProperty({}, '__proto__')).toBe(false);
+  });
+  it('does not inherit toString method from Object', () => {
+    expect(hasOwnProperty({}, 'toString')).toBe(false);
+  });
+});
+
 describe('getObjectSubset()', () => {
   [
     [{a: 'b', c: 'd'}, {a: 'd'}, {a: 'b'}],

packages/expect/src/utils.js

@@ -21,18 +21,33 @@ type GetPath = {
   value?: any,
 };
 
-export const hasOwnProperty = (object: Object, value: string) => {
-  // Account for objects created using unconventional means such as
-  // `Object.create(null)`, in which case the `object.constructor` is undefined
-  // See: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/create#Custom_and_Null_objects
-  const objectConstructor = object.constructor || Object;
-
-  return (
-    Object.prototype.hasOwnProperty.call(object, value) ||
-    Object.prototype.hasOwnProperty.call(objectConstructor.prototype, value)
+// Return whether object instance inherits getter from its class.
+const hasGetterFromConstructor = (object: Object, key: string) => {
+  const constructor = object.constructor;
+  if (constructor === Object) {
+    // A literal object has Object as constructor.
+    // Therefore, it cannot inherit application-specific getters.
+    // Furthermore, Object has __proto__ getter which is not relevant.
+    // Array, Boolean, Number, String constructors don’t have any getters.
+    return false;
+  }
+  if (typeof constructor !== 'function') {
+    // Object.create(null) constructs object with no constructor nor prototype.
+    // https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/create#Custom_and_Null_objects
+    return false;
+  }
+
+  const descriptor = Object.getOwnPropertyDescriptor(
+    constructor.prototype,
+    key,
   );
+  return descriptor !== undefined && typeof descriptor.get === 'function';
 };
 
+export const hasOwnProperty = (object: Object, key: string) =>
+  Object.prototype.hasOwnProperty.call(object, key) ||
+  hasGetterFromConstructor(object, key);
+
 export const getPath = (
   object: Object,
   propertyPath: string | Array<string>,