Testing ECMAScript Modules with Jest

This note describes how to test ECMAScript modules with Jest.

Takahiro Iwasa

2024-02-24

4 min read

When testing ECMAScript Modules (ESM) using Jest, you might encounter the error:

SyntaxError: Cannot use import statement outside a module

This issue arises because the target modules use the import keyword. Fortunately, Jest provides experimental support for ESM to resolve such errors.

Creating ESM Package

First, create a new ESM package with the following commands:

mkdir jest-esm && cd jest-esm
npm init -y

Install the necessary development dependencies:

npm i -D typescript jest @types/jest ts-node ts-jest

Add "type": "module" to the package.json.

Your package.json should look like this:

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{
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  "name": "jest-esm",
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  "version": "1.0.0",
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  "description": "",
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  "main": "index.js",
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  "type": "module",
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  "scripts": {
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    "test": "echo \"Error: no test specified\" && exit 1"
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  },
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  "author": "",
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  "license": "ISC",
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  "devDependencies": {
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    "@types/jest": "^29.5.12",
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    "jest": "^29.7.0",
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    "ts-jest": "^29.1.2",
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    "ts-node": "^10.9.2",
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    "typescript": "^5.3.3"
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  }
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}

Configuring TypeScript

Generate a tsconfig.json file with the following command:

npx tsc --init

Update the tsconfig.json file:

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@@ -14 +14 @@
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-    "target": "es2016",                                  /* Set the JavaScript language version for emitted JavaScript and include compatible library declarations. */
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+    "target": "es6",                                     /* Set the JavaScript language version for emitted JavaScript and include compatible library declarations. */
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@@ -28 +28 @@
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-    "module": "commonjs",                                /* Specify what module code is generated. */
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+    "module": "es6",                                     /* Specify what module code is generated. */
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@@ -30 +30 @@
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-    // "moduleResolution": "node10",                     /* Specify how TypeScript looks up a file from a given module specifier. */
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+    "moduleResolution": "node",                          /* Specify how TypeScript looks up a file from a given module specifier. */

Configuring Jest

Run the following command to set up Jest configuration:

Use Jest for the “test” script: Yes
Use TypeScript for the configuration: Yes
Test environment: jsdom
Add coverage reports: No
Provider for coverage: v8
Automatically clear mock calls…: No

npm init jest@latest

Edit the generated jest.config.ts to include:

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// preset: undefined,
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preset: 'ts-jest',

Supporting ESM

Update the test script in package.json to enable Node’s experimental VM modules:

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"scripts": {
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   "test": "jest"
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   "test": "node --experimental-vm-modules node_modules/jest/bin/jest.js"
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}

Add the extensionsToTreatAsEsm configuration in jest.config.ts to indicate which file types Jest should treat as ESM:

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extensionsToTreatAsEsm: ['.ts'],

To configure ESM in ts-jest, refer to the official documentation.

Update jest.config.ts with moduleNameMapper to handle .js extensions:

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// moduleNameMapper: {},
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moduleNameMapper: {
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  '^(\\.{1,2}/.*)\\.js$': '$1',
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},

Update the transform setting to support ESM:

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// transform: undefined,
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transform: {
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  '^.+\\.tsx?$': [
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    'ts-jest',
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    {
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      useESM: true,
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    },
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  ],
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},

Creating ESM

For demonstration, install the hast-util-from-html package:

npm i hast-util-from-html

The module hast-util-from-html is an ECMAScript Module (ESM) and uses the export keyword for its exports. This can be confirmed by examining the file located at node_modules/hast-util-from-html/index.js. Here’s a snippet of the module’s export structure:

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/**
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 * @typedef {import('hast-util-from-parse5')} DoNotTouchItRegistersData
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 *
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 * @typedef {import('./lib/index.js').ErrorCode} ErrorCode
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 * @typedef {import('./lib/index.js').ErrorSeverity} ErrorSeverity
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 * @typedef {import('./lib/index.js').OnError} OnError
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 * @typedef {import('./lib/index.js').Options} Options
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 */
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export { fromHtml } from './lib/index.js';

Create index.ts:

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import { fromHtml } from 'hast-util-from-html';
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export default function JestEsm(): void {
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  const root = fromHtml(
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    '<span><a href="https://github.com">GitHub</a></span>',
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    { fragment: true }
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  );
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  console.info(root);
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}

Testing ESM

Create index.spec.ts:

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import JestEsm from './index';
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test('case1', () => {
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  JestEsm();
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});

Run the tests:

npm run test

If you see the error:

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● Validation Error:
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  Test environment jest-environment-jsdom cannot be found. Make sure the testEnvironment configuration option points to an existing node module.
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  Configuration Documentation:
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  https://jestjs.io/docs/configuration
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As of Jest 28 "jest-environment-jsdom" is no longer shipped by default, make sure to install it separately.

Install the missing package:

npm i -D jest-environment-jsdom

Run the tests again:

npm run test

Expected output:

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 PASS  ./index.spec.ts
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  ✓ case1 (20 ms)
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Test Suites: 1 passed, 1 total
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Tests:       1 passed, 1 total
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Snapshots:   0 total
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Time:        1 s
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Ran all test suites.