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Use webpack to preprocess files in karma.

Getting Started

To begin, you'll need to install karma-webpack:

npm i --save-dev karma-webpack

Then add config:


module.exports = (config) => {
    // ... normal karma configuration
    files: [
      // all files ending in "_test"
      { pattern: 'test/*_test.js', watched: false },
      { pattern: 'test/**/*_test.js', watched: false },
      // each file acts as entry point for the webpack configuration

    preprocessors: {
      // add webpack as preprocessor
      'test/*_test.js': ['webpack'],
      'test/**/*_test.js': ['webpack'],

    webpack: {
      // karma watches the test entry points
      // (you don't need to specify the entry option)
      // webpack watches dependencies
      // webpack configuration

    webpackMiddleware: {
      // webpack-dev-middleware configuration
      // i. e.
      stats: 'errors-only',

Alternative Usage

This configuration is more performant, but you cannot run single test anymore (only the complete suite).

The above configuration generates a webpack bundle for each test. For many test cases this can result in many big files. The alternative configuration creates a single bundle with all test cases.


files: [
  // only specify one entry point
  // and require all tests in there

preprocessors: {
  // add webpack as preprocessor
  'test/index_test.js': [ 'webpack' ]


// require all modules ending in "_test" from the
// current directory and all subdirectories
const testsContext = require.context('.', true, /_test$/);


Every test file is required using the require.context and compiled with webpack into one test bundle.

Source Maps

You can use the karma-sourcemap-loader to get the source maps generated for your test bundle.

npm i -D karma-sourcemap-loader

And then add it to your preprocessors.


preprocessors: {
  'test/test_index.js': [ 'webpack', 'sourcemap' ]

And tell webpack to generate sourcemaps.


webpack: {
  // ...
  devtool: 'inline-source-map';


This is the full list of options you can specify in your karma.conf.js

Name Type Default Description
webpack {Object} {} Pass webpack.config.js to karma
webpackMiddleware {Object} {} Pass webpack-dev-middleware configuration to karma
beforeMiddleware {Object} {} Pass custom middleware configuration to karma, before any karma middleware runs


webpack configuration (webpack.config.js).


Configuration for webpack-dev-middleware.


beforeMiddleware is a webpack option that allows injecting middleware before karma's own middleware runs. This loader provides a webpackBlocker middleware that will block tests from running until code recompiles. That is, given this scenario

  1. Have a browser open on the karma debug page (http://localhost:9876/debug.html)
  2. Make a code change
  3. Refresh

Without the webpackBlocker middleware karma will serve files from before the code change. With the webpackBlocker middleware the loader will not serve the files until the code has finished recompiling.

⚠️ The beforeMiddleware option is only supported in karma >= v1.0.0


Please take a moment to read our contributing guidelines if you haven't yet done so.




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