How to test React components using Karma and webpack

I’m working on a project at Twitter that uses React and webpack. After a few conversations with @sokra last year, this is the setup I put in place for testing React components (authored using JSX and ES6) using Karma.


You’ll need to install various packages. It looks like a lot of dependencies, but all the non-Karma packages will be necessary for general module bundling during development.

Full set of required packages:

webpack entry file

If you use webpack-specific features in your modules (e.g., loaders, plugins) you will need to use webpack to build a test bundle. The fastest and simplest approach is to create a single, test-specific entry file.

Create a file named tests.bundle.js. Within this file, you create a webpack context to match all the files that conform to a naming pattern – in this case *.spec.js(x).

var context = require.context('.', true, /.+\.spec\.jsx?$/);
module.exports = context;

Next, you point Karma to this file.

Karma config

Karma is configured using a karma.conf.js file. The browsers, plugins, and frameworks are specified in the standard way.

Point Karma at the tests.bundle.js file, and run it through the relevant preprocessor plugins (see example below).

The karma-webpack plugin relies on 2 custom properties of the Karma config: webpack and webpackMiddleware. The value of the former must be a webpack config object.

module.exports = function (config) {
    browsers: [ 'Chrome' ],
    // karma only needs to know about the test bundle
    files: [
    frameworks: [ 'chai', 'mocha' ],
    plugins: [
    // run the bundle through the webpack and sourcemap plugins
    preprocessors: {
      'tests.bundle.js': [ 'webpack', 'sourcemap' ]
    reporters: [ 'dots' ],
    singleRun: true,
    // webpack config object
    webpack: {
      devtool: 'inline-source-map',
      module: {
        loaders: [
            exclude: /node_modules/,
            loader: 'babel-loader,
            test: /\.jsx?$/
    webpackMiddleware: {
      noInfo: true,

Rather than duplicating your webpack config, you can require it in the Karma config file and override the devtool value to get sourcemaps working.

var webpackConfig = require('./webpack.config');
webpackConfig.devtool = 'inline-source-map';

module.exports = function (config) {
    webpack: webpackConfig

That’s all you need to do to configure Karma to use webpack to load your JSX, ES6 React components.