feat: support config files exporting (async) functions (#10001)

CHANGELOG.md

@@ -2,6 +2,8 @@
 
 ### Features
 
+- `[jest-config]` Support config files exporting (`async`) `function`s ([#10001](https://github.com/facebook/jest/pull/10001))
+
 ### Fixes
 
 - `[jest-jasmine2]` Stop adding `:` after an error that has no message ([#9990](https://github.com/facebook/jest/pull/9990))

docs/Configuration.md

@@ -18,9 +18,17 @@ Or through JavaScript:
 
 ```js
 // jest.config.js
+//Sync object
 module.exports = {
   verbose: true,
 };
+
+//Or async function
+module.exports = async () => {
+  return {
+    verbose: true,
+  };
+};
 ```
 
 Please keep in mind that the resulting configuration must be JSON-serializable.

packages/jest-config/src/__tests__/readConfigs.test.ts

@@ -4,12 +4,50 @@
  * This source code is licensed under the MIT license found in the
  * LICENSE file in the root directory of this source tree.
  */
-
+import {Config} from '@jest/types';
 import {readConfigs} from '../index';
 
+let mockResult;
+jest.mock('graceful-fs', () => ({
+  ...jest.requireActual('fs'),
+  existsSync: jest.fn(() => true),
+  lstatSync: jest.fn(() => ({
+    isDirectory: () => false,
+  })),
+}));
+jest.mock('../readConfigFileAndSetRootDir', () => jest.fn(() => mockResult));
+
 test('readConfigs() throws when called without project paths', async () => {
   await expect(
     // @ts-ignore
     readConfigs(null /* argv */, [] /* projectPaths */),
   ).rejects.toThrowError('jest: No configuration found for any project.');
 });
+
+test('readConfigs() loads async config file', async () => {
+  mockResult = jest.fn(async () => ({
+    rootDir: './',
+  }));
+  await expect(
+    // @ts-ignore
+    readConfigs(
+      <Config.Argv>{} /* argv */,
+      ['./some-jest-config-file.js'] /* projectPaths */,
+    ),
+  ).resolves.toHaveProperty('configs');
+  expect(mockResult).toHaveBeenCalled();
+});
+
+test('readConfigs() reject if async was rejected', async () => {
+  mockResult = jest.fn(async () => {
+    throw new Error('Some error');
+  });
+  await expect(
+    // @ts-ignore
+    readConfigs(
+      <Config.Argv>{} /* argv */,
+      ['./some-jest-config-file.js'] /* projectPaths */,
+    ),
+  ).rejects.toBeTruthy();
+  expect(mockResult).toHaveBeenCalled();
+});

packages/jest-config/src/index.ts

@@ -41,7 +41,9 @@ export async function readConfig(
   parentConfigPath?: Config.Path | null,
   projectIndex: number = Infinity,
 ): Promise<ReadConfig> {
-  let rawOptions;
+  let rawOptions:
+    | Config.InitialOptions
+    | (() => Config.InitialOptions | Promise<Config.InitialOptions>);
   let configPath = null;
 
   if (typeof packageRootOrConfig !== 'string') {
@@ -82,6 +84,10 @@ export async function readConfig(
     rawOptions = await readConfigFileAndSetRootDir(configPath);
   }
 
+  if (typeof rawOptions === 'function') {
+    rawOptions = await rawOptions();
+  }
+
   const {options, hasDeprecationWarnings} = normalize(
     rawOptions,
     argv,