ECMAScript Modules

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

Note that due to its experimental nature there are many bugs and missing features in Jest’s implementation, both known and unknown. Note that due to its experimental nature there are many bugs and missing features in Jest’s implementation, both known and unknown. You should check out the tracking issue and the label on the issue tracker for the latest status.

Also note that the APIs Jest uses to implement ESM support is still considered experimental by Node (as of version 14.13.1).

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.

4. If you want to treat other file extensions (such as .jsx or .ts) as ESM, please use the extensionsToTreatAsEsm option.

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. To access this object in ESM, you need to import it from the @jest/globals module.

import {jest} from '@jest/globals';
jest.useFakeTimers();
// 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. Follow this issue for updates.