Export Jest's default options (#5948)

* Export Jest's default options

* Improve require('jest-config')

* Add "Export Jest's default options" to changelog

CHANGELOG.md

@@ -2,6 +2,8 @@
 
 ### Features
 
+* `[jest-config]` Export Jest's default options
+  ([#5948](https://github.com/facebook/jest/pull/5948))
 * `[jest-editor-support]` Move `coverage` to `ProjectWorkspace.collectCoverage`
   ([#5929](https://github.com/facebook/jest/pull/5929))
 * `[jest-editor-support]` Add `coverage` option to runner

docs/Configuration.md

@@ -44,6 +44,20 @@ These options let you control Jest's behavior in your `package.json` file. The
 Jest philosophy is to work great by default, but sometimes you just need more
 configuration power.
 
+### Defaults
+
+You can retrieve Jest's default options to expand them if needed:
+
+```js
+// jest.config.js
+const {defaults} = require('jest-config');
+module.exports = {
+  // ...
+  moduleFileExtensions: [...defaults.moduleFileExtensions, 'ts', 'tsx'],
+  // ...
+};
+```
+
 <AUTOGENERATED_TABLE_OF_CONTENTS>
 
 ---

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

@@ -0,0 +1,5 @@
+import {defaults} from '../index';
+
+test('get configuration defaults', () => {
+  expect(defaults).toBeDefined();
+});

packages/jest-config/src/index.js

@@ -25,6 +25,7 @@ export {getTestEnvironment, isJSONString} from './utils';
 export {default as normalize} from './normalize';
 export {default as deprecationEntries} from './deprecated';
 export {replaceRootDirInPath} from './utils';
+export {default as defaults} from './defaults';
 
 export function readConfig(
   argv: Argv,