Test TypeScript ESM

This guide sets up Jest for a TypeScript package that has "type": "module" in package.json. The ts-esm preset transforms TypeScript with @swc/jest and runs it as native ESM.

Steps

Install Jest, the preset package, and SWC:

pnpm add -D jest @repobuddy/jest @swc/jest @swc/core cross-env

Add the watch plugins if you use jest --watch:

pnpm add -D jest-watch-suspend jest-watch-toggle-config jest-watch-typeahead

Create jest.config.mjs:
jest.config.mjs
```
export default {
  preset: '@repobuddy/jest/presets/ts-esm-watch',
}
```
Use ts-esm instead if you skipped step 2. ts-watch also works: it picks ts-esm-watch because of "type": "module".

Run Jest with ESM support. Add scripts to package.json:

{
  "scripts": {
    "test": "cross-env NODE_OPTIONS=--experimental-vm-modules jest",
    "test:watch": "cross-env NODE_OPTIONS=--experimental-vm-modules jest --watch"
  }
}

Without the flag, test files fail with SyntaxError: Cannot use import statement outside a module.

Put tests in the source folder (src, source, ts, or js) and name them <name>.spec.ts or another test file name.

Import local files with the .js extension, as Node.js ESM requires. The preset maps ./foo.js to ./foo.ts:

import { expect, it } from '@jest/globals'
import { foo } from './foo.js'

it('works', () => {
  expect(foo).toBe(1)
})

Finished config

export default {
  preset: '@repobuddy/jest/presets/ts-esm-watch',
}

Verify

List the test files Jest found:

pnpm exec cross-env NODE_OPTIONS=--experimental-vm-modules jest --listTests

Run the tests:
Terminal window
```
pnpm test
```
The ts-esm fixture in this repository runs 15 suites this way, including .mts, .cts, .tsx, and .node<major>.ts files.

Use ts-jest instead of SWC

The ESM presets use SWC only. To run TypeScript ESM through ts-jest, compose the config from configs and fields (see customize a preset).

Test TypeScript ESM

Steps

Finished config

Verify

Use ts-jest instead of SWC

Related