ECMAScript Modules

caution

Jest ships with experimental support for ECMAScript Modules (ESM).

The implementation may have bugs and lack features. For the latest status check out the issue and the label on the issue tracker.

Also note that the APIs Jest uses to implement ESM support are still considered experimental by Node (as of version 18.8.0).

With the warnings out of the way, this is how you activate ESM support in your tests.

1. Ensure you either disable code transforms by passing transform: {} or otherwise configure your transformer to emit ESM rather than the default CommonJS (CJS).

2. Execute node with --experimental-vm-modules, e.g. node --experimental-vm-modules node_modules/jest/bin/jest.js or NODE_OPTIONS=--experimental-vm-modules npx jest etc.

   On Windows, you can use cross-env to be able to set environment variables.

   If you use Yarn, you can use yarn node --experimental-vm-modules $(yarn bin jest). This command will also work if you use Yarn Plug'n'Play.

3. Beyond that, we attempt to follow node's logic for activating "ESM mode" (such as looking at type in package.json or mjs files), see their docs for details

Differences between ESM and CommonJS

Most of the differences are explained in Node's documentation, but in addition to the things mentioned there, Jest injects a special variable into all executed files - the jest object. To access this object in ESM, you need to import it from the @jest/globals module.

import {jest} from '@jest/globals';

jest.useFakeTimers();

// etc.

Additionally, since ESM evaluates static import statements before looking at the code, the hoisting of jest.mock calls that happens in CJS won't work in ESM. To mock modules in ESM, you need to use require or dynamic import() after jest.mock calls to load the mocked modules - the same applies to modules which load the mocked modules.

main.cjs

const {BrowserWindow, app} = require('electron');

// etc.

module.exports = {example};

main.test.cjs

import {createRequire} from 'node:module';
import {jest} from '@jest/globals';

const require = createRequire(import.meta.url);

jest.mock('electron', () => ({
  app: {
    on: jest.fn(),
    whenReady: jest.fn(() => Promise.resolve()),
  },
  BrowserWindow: jest.fn().mockImplementation(() => ({
    // partial mocks.
  })),
}));

const {BrowserWindow} = require('electron');
const exported = require('./main.cjs');

// alternatively
const {BrowserWindow} = (await import('electron')).default;
const exported = await import('./main.cjs');

// etc.

Please note that we currently don't support jest.mock in a clean way in ESM, but that is something we intend to add proper support for in the future. Follow this issue for updates.