Karma plugin for testing with es modules

Usage no npm install needed!

<script type="module">
  import openWcKarmaEsm from 'https://cdn.skypack.dev/@open-wc/karma-esm';


permalink: 'testing/karma-esm.html' title: karma-esm section: guides tags:

  • guides


We have stopped new development of this package.

We will continue to support security patches and bug fixes, but we recommend web test runner for testing web component projects.


Karma plugin for testing with real es modules without any bundling.

Out the box es modules don't work with karma because they import their dependencies at runtime from the browser. Karma doesn't allow requesting any files it doesn't know about upfront.

The karma-esm plugin fixes this and spins up es-dev-server behind the scenes. This lets you write tests using es modules, modern javascript syntax, and features, and have karma run them on all modern browsers and IE11.

es-dev-server takes care of loading the correct polyfills, and runs babel for older browsers if necessary. On browsers which don't support es modules, dynamic imports and/or import.meta.url, systemjs is used as a module polyfill.

See the es-dev-server docs for full details on how it works.


We recommend the testing-karma configuration for a good default karma setup which includes karma-esm and many other good defaults.

Manual setup

To manually set up this plugin, add it as a karma framework:

  1. Install the plugin
npm i -D @open-wc/karma-esm
  1. Add to your karma config
module.exports = {
  // define where your test files are, make sure to set type to module
  files: [
    { pattern: 'test/**/*.test.js', type: 'module' }

  plugins: [
    // load plugin

    // fallback: resolve any karma- plugins

  frameworks: ['esm'],

  esm: {
    // if you are using 'bare module imports' you will need this option
    nodeResolve: true,


karma-esm can be configured with these options:

name type description
nodeResolve boolean Transforms bare module imports using node resolve.
dedupe boolean/array Deduplicates all modules, or modules from specified packages if the value is an array
coverage boolean Whether to report test code coverage.
importMap string Path to import map used for testing.
compatibility string Compatibility level to run the es-dev-server with.
coverageExclude array Extra glob patterns of tests to exclude from coverage.
babelConfig object Custom babel configuration file to run on served code.
moduleDirs array Directories to resolve modules from. Defaults to node_modules
babel boolean Whether to pick up a babel configuration file in your project.
fileExtensions array Custom file extensions to serve as es modules.
plugins array es-dev-server plugins to load
polyfillsLoader object Configuration for the polyfills loader
devServerPort number Port of server that serves the modules. Note that this is not the karma port. Picks a random port if not set.
preserveSymlinks boolean Run the es-dev-server with the --preserve-symlinks option.


Node resolve is necessary when you have 'bare imports' in your code and are not using import maps to resolve them.

It transforms: import foo from 'bar' to: import foo from './node_modules/bar/bar.js.

See the node-resolve documentation of es-dev-server for more information.


Due to a bug in karma, the test coverage reporter causes browser logs to appear twice which can be annoying


Allows controlling the behavior of ES imports according to the (in progress) spec. Since this feature is not enabled by default, is necessary to launch Chrome with --enable-experimental-web-platform-features flag.

In karma.config.js add:

customLaunchers: {
  ChromeHeadlessNoSandbox: {
    base: 'ChromeHeadless',
    flags: [
      '--no-sandbox', //default karma-esm configuration
      '--disable-setuid-sandbox', //default karma-esm configuration
      '--enable-experimental-web-platform-features' // necessary when using importMap option


The compatibility option makes your code compatible with older browsers. It loads polyfills and transforms modern syntax where needed.

See the compatibility documentation of es-dev-server for more information.


The es-dev-server by default resolves the symlinks in the dependency directory. This can cause a problem when you're using npm link command or other tools which rely on them. This option will make es-dev-server preserve symlinks.

Karma preprocessors

Unfortunately, to make karma work with es modules regular karma preprocessors no longer work. You can, however, configure the es-dev-server to do code transformations if needed.

Custom babel plugins

You can configure karma-esm to pick up the babel configuration files in your project:

  esm: {
    babel: true

Testing typescript

Because karma-esm doesn't do any bundling, it's easy to integrate it with typescript and doesn't require any extra tooling or plugins. Just run tsc on your code, and test the compiled output with karma-esm. You can run both tsc and karma in watch mode, changes will be picked up automatically.

Make sure to configure tsc to output real ES modules.