fix(jest-runtime): make sure a module can never be its own parent

Fixes jestjs#5235

integration_tests/__tests__/module_parent_null_in_test.js

@@ -0,0 +1,17 @@
+/**
+ * Copyright (c) 2014-present, Facebook, Inc. All rights reserved.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ *
+ * @flow
+ */
+'use strict';
+
+const runJest = require('../runJest');
+
+test('module.parent should be null in test files', () => {
+  const {status} = runJest('module_parent_null_in_test');
+
+  expect(status).toBe(0);
+});

integration_tests/module_parent_null_in_test/__tests__/index.js

@@ -0,0 +1,13 @@
+/**
+ * Copyright (c) 2014-present, Facebook, Inc. All rights reserved.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ */
+
+'use strict';
+
+test('moduleNameMapping wrong configuration', () => {
+  expect(module).not.toBe(module.parent);
+  expect(module.parent).toBeNull();
+});

integration_tests/module_parent_null_in_test/package.json

@@ -0,0 +1,4 @@
+{
+  "jest": {
+  }
+}

integration_tests/runtime-internal-module-registry/__tests__/runtime_internal_module_registry.test.js

@@ -13,7 +13,7 @@ describe('Runtime internal module registry', () => {
   it('behaves correctly when requiring a module that is used by jest internals', () => {
     const fs = require('fs');
 
-    // We require from this crazy path so that we can mimick Jest (and it's
+    // We require from this crazy path so that we can mimick Jest (and its
     // transitive deps) being installed along side a projects deps (e.g. with an
     // NPM3 flat dep tree)
     const jestUtil = require('../../../packages/jest-util');

packages/jest-runtime/src/index.js

@@ -509,6 +509,10 @@ class Runtime {
       ({
         enumerable: true,
         get() {
+          if (localModule.filename === from) {
+            return null;
+          }
+
           return moduleRegistry[from] || null;
         },
       }: Object),