Tilted sphere with longitudinal stripes Modern Web Guides Docs Blog

Writing Tests: Mocking

Mocking functions

For stubbing and mocking functions, we recommend sinon.

Sinon ships an es module, you can import it in your tests like this:

import { stub, spy, useFakeTimers } from 'sinon';

Mocking es modules

Es modules are immutable, it's not possible to mock or stub imports:

import { stub } from 'sinon';
import * as myModule from './my-module.js';

// not possible
stub(myModule, 'myImport');

We can use Import Maps in our tests to resolve module imports to a mocked version.

Using Import Maps to tests

To use Import Maps in your tests, you can add @web/dev-server-import-maps to your web-test-runner.config.mjs:

import { importMapsPlugin } from '@web/dev-server-import-maps';

export default {
  plugins: [
      inject: {
        importMap: {
          imports: {
            // mock a dependeny
            'package-a': '/mocks/package-a.js',
            // mock a module in your own code
            '/src/my-module.js': '/mocks/my-module.js',

This will inject the import map to every HTML file in your tests. Go to the Import Maps Plugin page to learn more about how import maps work and how to configure more options.

Import Map per test

Import Maps are defined in HTML. We need to write a HTML test to use a specific Import Map per test.

This is an example using Mocha. We want to test if a function works correctly without actually causing side effects, like posting data to a server. We mock the module that causes side effects in our test.

Source code:


export function postData(endpoint, data) {
  return fetch(`/api/${endpoint}`, { method: 'POST', body: JSON.stringify(data) });


import { postData } from './postData.js';

export function sendMessage(message) {
  return postData('message', { message });

Mocked module:


import { stub } from 'sinon';

export const postData = stub();

Test file:

    <!-- the import map to use in our test -->
    <script type="importmap">
        "imports": {
          "./postData.js": "./mocks/postData.js"

    <script type="module">
      import { runTests } from '@web/test-runner-mocha';
      import { expect } from '@esm-bundle/chai';
      // import inside will resolve to ./mocks/postData.js
      import { sendMessage } from '../src/sendMessage.js';
      // resolves to ./mocks/postData.js
      import { postData } from './postData.js';

      runTests(() => {
        beforeEach(() => {

        describe('postMessage()', () => {
          it('calls postData', () => {
            await sendMessage('foo');
            expect(postData.getCall(0).args).to.eql(['message', { message: 'foo' }]);