Tips on Setting up Karma Testing with Webpack

Despite the plethora of examples on using Karma with Webpack, I struggled to get things working. Afterwards, I realized there were a few key bits of information that were either glossed over or not emphasized enough (for me to notice).

Once I did figure it out, my karma.config.js file became smaller and easier to understand. For reference, here’s my final karma.config.js file:

var webpackConfig = require('./webpack.config.js');
webpackConfig.entry = {};

module.exports = function(config) {
    basePath: '',
    frameworks: ['jasmine', 'chai'],

    reporters: ['progress'],
    port: 9876,
    colors: false,
    logLevel: config.LOG_INFO,
    autoWatch: true,
    browsers: ['Chrome'],
    singleRun: false,
    autoWatchBatchDelay: 300,

    files: [

    preprocessors: {
      './app/bundle.js': ['webpack'],      
      './tests/**/*.spec.js': ['babel']

    webpack: webpackConfig,

    webpackMiddleware: {
      noInfo: true

First, make sure karma-webpack is installed in the project.

npm install karma-webpack --save-dev

Tip 1 - Reference webpack.config.js, don’t repeat it.

Most of the examples I could find repeat the webpack.config.js in the karma.config.js file. I don’t like having the same configuration in different files. However, Karma is running in Node so it’s quite acceptable to simply import the configuration as I did in the first two lines.

var webpackConfig = require('./webpack.config.js');
webpackConfig.entry = {};

But here’s a gotcha. The entry point from the referenced Webpack configuration has to be removed or tests will fail in weird and inscrutable ways. Easy enough, just define an empty entry object (null won’t work).

Finally, reference the Webpack configuration in the Karma configuration:

webpack: webpackConfig,

Tip 2 - Specify the entry point in Karma

preprocessors: {
  './app/bundle.js': ['webpack'],      
  './tests/**/*.spec.js': ['babel']

In the preprocessors portion, add the application’s entry point. Here './app/bundle.js' is the entry point to my Web application. It’s the same one I used in Webpack.config.js. The karama-webpack plugin requires this. It’s documented but the part about removing the webpack.config.js entry point is not as evident.

Secondly, you have to tell Karma that you want the karma-webpack plugin to process these files. That’s what ['webpack'] does.

Tip 3 - Do not enumerate plugins

The plugins section is missing from my karma.config.js file. When missing, Karma will load any plugins it can find in the node-modules folder. Much simpler in my opinion.

Tip - Write tests in ES6

Writing tests in ES6 is better. Or at least if feels that way to me (it’s also feels better in application code as well). You’ll need to install the Babel preprocessor.

npm install karma-babel-preprocessor --save-dev

The karma-babel-preprocessor has additional configuration sections that I was instructed to add. In my case, I didn’t need it and simply referenced the ['babel'] preprocessor as above.

To use ES6 in my application code, I configured it in my webpack.config.js (a subject of another post perhaps).

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