fix: require.main is undefined on using jest.resetModules

e2e/__tests__/requireMainResetModules.test.ts

@@ -0,0 +1,15 @@
+/**
+ * Copyright (c) Facebook, Inc. and its affiliates. 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.
+ */
+
+import runJest from '../runJest';
+
+test('`require.main` on using `jest.resetModules` should not be undefined', () => {
+  const {exitCode} = runJest('require-main-reset-modules', [
+    `--resetModules='true'`,
+  ]);
+  expect(exitCode).toBe(0);
+});

e2e/require-main-reset-modules/__tests__/index.test.js

@@ -0,0 +1,15 @@
+/**
+ * Copyright (c) Facebook, Inc. and its affiliates. 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.
+ */
+test('require.main is set', () => {
+  const {getMain} = require('../index.js');
+  expect(getMain()).toBeTruthy();
+});
+
+test('require from main works', () => {
+  const {requireFromMain} = require('../index.js');
+  expect(requireFromMain('../package.json')).toBeTruthy();
+});

e2e/require-main-reset-modules/index.js

@@ -0,0 +1,15 @@
+/**
+ * Copyright (c) Facebook, Inc. and its affiliates. 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.
+ */
+Object.assign(exports, {getMain, requireFromMain});
+
+function getMain() {
+  return require.main;
+}
+
+function requireFromMain(pkg) {
+  return getMain().require(pkg);
+}

e2e/require-main-reset-modules/package.json

@@ -0,0 +1,5 @@
+{
+  "jest": {
+    "testEnvironment": "node"
+  }
+}

packages/jest-runtime/src/index.ts

@@ -1061,10 +1061,6 @@ class Runtime {
       }),
     ];
 
-    if (!this._mainModule && filename === this._testPath) {
-      this._mainModule = module;
-    }
-
     Object.defineProperty(module, 'main', {
       enumerable: true,
       value: this._mainModule,
@@ -1398,9 +1394,13 @@ class Runtime {
       });
     })();
 
+    if (!this._mainModule && from.filename === this._testPath) {
+      this._mainModule = module;
+    }
+
     Object.defineProperty(moduleRequire, 'main', {
       enumerable: true,
-      value: this._testPath ? this._moduleRegistry.get(this._testPath) : null,
+      value: this._mainModule,
     });
     return moduleRequire;
   }