Add support for the "module" package.json field

This is used in modern es module codebases to designate the entry
point for code that makes use of import/export instead of commonjs.

Author: Evan Jacobs

examples/module-field/.babelrc

@@ -0,0 +1,3 @@
+{
+  "presets": ["es2015"]
+}

examples/module-field/__tests__/index.js

@@ -0,0 +1,13 @@
+/**
+ * Copyright (c) 2017-present, Facebook, Inc. All rights reserved.
+ * This source code is licensed under the BSD-style license found in the
+ * LICENSE file in the root directory of this source tree. An additional grant
+ * of patent rights can be found in the PATENTS file in the same directory.
+ */
+import Contents from '../sample';
+
+describe('module field resolution', () => {
+  it('should return the correct file exports', () => {
+    expect(Contents).toBe('👏');
+  });
+});

examples/module-field/package.json

@@ -0,0 +1,13 @@
+{
+  "devDependencies": {
+    "babel-jest": "*",
+    "babel-preset-es2015": "*",
+    "jest": "*"
+  },
+  "jest": {
+    "module": true
+  },
+  "scripts": {
+    "test": "jest"
+  }
+}

examples/module-field/sample/correct.js

@@ -0,0 +1,7 @@
+/**
+ * Copyright (c) 2017-present, Facebook, Inc. All rights reserved.
+ * This source code is licensed under the BSD-style license found in the
+ * LICENSE file in the root directory of this source tree. An additional grant
+ * of patent rights can be found in the PATENTS file in the same directory.
+ */
+export default '👏';

examples/module-field/sample/package.json

@@ -0,0 +1,4 @@
+{
+    "main": "wrong.js",
+    "module": "correct.js"
+}

examples/module-field/sample/wrong.js

@@ -0,0 +1,9 @@
+/**
+ * Copyright (c) 2017-present, Facebook, Inc. All rights reserved.
+ * This source code is licensed under the BSD-style license found in the
+ * LICENSE file in the root directory of this source tree. An additional grant
+ * of patent rights can be found in the PATENTS file in the same directory.
+ */
+'use strict';
+
+module.exports = '😒';

packages/jest-config/src/__tests__/normalize-test.js

@@ -358,6 +358,17 @@ describe('testPathIgnorePatterns', () => {
   });
 });
 
+describe('module', () => {
+  it('falsy module is not overwritten', () => {
+    const config = normalize({
+      module: true,
+      rootDir: '/root/path/foo',
+    });
+
+    expect(config.module).toBe(true);
+  });
+});
+
 describe('modulePathIgnorePatterns', () => {
   it('does not normalize paths relative to rootDir', () => {
     // This is a list of patterns, so we can't assume any of them are

packages/jest-config/src/defaults.js

@@ -32,6 +32,7 @@ module.exports = ({
     providesModuleNodeModules: [],
   },
   mocksPattern: '__mocks__',
+  module: false,
   moduleDirectories: ['node_modules'],
   moduleFileExtensions: [
     'js',

packages/jest-config/src/normalize.js

@@ -331,6 +331,7 @@ function normalize(config: InitialConfig, argv: Object = {}) {
       case 'haste':
       case 'logHeapUsage':
       case 'logTransformErrors':
+      case 'module':
       case 'mocksPattern':
       case 'moduleDirectories':
       case 'moduleFileExtensions':

packages/jest-config/src/validConfig.js

@@ -45,6 +45,7 @@ module.exports = ({
   logHeapUsage: true,
   logTransformErrors: true,
   mocksPattern: '__mocks__',
+  module: false,
   moduleDirectories: ['node_modules'],
   moduleFileExtensions: ['js', 'json', 'jsx', 'node'],
   moduleLoader: '<rootDir>',