npm i -D karma-webpack

// Karma configuration
module.exports = function(config) {
  config.set({
    // ... 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'
    }
  });
};

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 testcases this can result in many big files. The alterative configuration creates a single bundle with all testcases.

files: [
  // only specify one entry point
  // and require all tests in there
  'test/test_index.js'
],

preprocessors: {
  // add webpack as preprocessor
  'test/test_index.js': ['webpack']
},

// test/test_index.js

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

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

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

npm install --save-dev 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 config.

webpack

Webpack configuration.

webpackMiddleware

Configuration for webpack-dev-middleware.

beforeMiddleware

beforeMiddleware is a webpack option that allows injecting middleware before karma's own middleware are run. 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.

Note that the beforeMiddleware option is only supported in karma with version >1.0.

The MIT License (MIT)

Copyright (c) 2014 - 2016 Tobias Koppers

Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.