Do not include from information when its not valid

CHANGELOG.md

@@ -2,6 +2,8 @@
 
 ### Features
 
+* `[jest-runtime]` Prevent modules from marking themselves as thier own parent
+  ([#5235](https://github.com/facebook/jest/issues/5235))
 * `[expect]` Add support for async matchers
   ([#5836](https://github.com/facebook/jest/pull/5919))
 * `[expect]` Suggest toContainEqual

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 +1,3 @@
+{
+  "jest": {}
+}

packages/jest-runtime/src/__tests__/runtime_require_mock.test.js

@@ -173,10 +173,10 @@ describe('Runtime', () => {
         expect(exports.isManualMockModule).toBe(true);
       });
     });
+
     it('provides `require.main` in mock', () =>
       createRuntime(__filename).then(runtime => {
-        runtime._moduleRegistry[__filename] = module;
-        runtime.setMock(__filename, 'export_main', () => require.main, {
+        runtime.setMock(__filename, 'export_main', () => module, {
           virtual: true,
         });
         const mainModule = runtime.requireMock(__filename, 'export_main');

packages/jest-runtime/src/index.js

@@ -327,7 +327,9 @@ class Runtime {
         // $FlowFixMe
         localModule.exports = require(modulePath);
       } else {
-        this._execModule(localModule, options, moduleRegistry, from);
+        // Only include the fromPath if a moduleName is given. Else treat as root.
+        const fromPath = moduleName ? from : null;
+        this._execModule(localModule, options, moduleRegistry, fromPath);
       }
 
       localModule.loaded = true;
@@ -392,7 +394,10 @@ class Runtime {
         id: modulePath,
         loaded: false,
       };
-      this._execModule(localModule, undefined, this._mockRegistry, from);
+
+      // Only include the fromPath if a moduleName is given. Else treat as root.
+      const fromPath = moduleName ? from : null;
+      this._execModule(localModule, undefined, this._mockRegistry, fromPath);
       this._mockRegistry[moduleID] = localModule.exports;
       localModule.loaded = true;
     } else {
@@ -493,7 +498,7 @@ class Runtime {
     localModule: Module,
     options: ?InternalModuleOptions,
     moduleRegistry: ModuleRegistry,
-    from: Path,
+    from: ?Path,
   ) {
     // If the environment was disposed, prevent this module from being executed.
     if (!this._environment.global) {
@@ -517,7 +522,8 @@ class Runtime {
       ({
         enumerable: true,
         get() {
-          return moduleRegistry[from] || null;
+          const key = from || '';
+          return moduleRegistry[key] || null;
         },
       }: Object),
     );