karma.config.js

var argv = require('yargs').argv;
var path = require('path');

module.exports = function(config) {
  config.set({
    browsers: ['PhantomJS'],
    singleRun: !argv.watch, // just run once by default
    frameworks: ['mocha'],
    // npm i karma-spec-reporter --save-dev
    // displays tests in a nice readable format
    reporters: ['spec'],

    // include some polyfills for babel and phantomjs
    files: [
      'tests.webpack.js'
    ],

    preprocessors: {
      // these files we want to be precompiled with webpack
      // also run tests throug sourcemap for easier debugging
      'tests.webpack.js': ['webpack', 'sourcemap']
    },

    webpack: {
       devtool: 'inline-source-map',
       resolve: {
        // allow us to import components in tests like:
        // import Example from 'components/Example';
        root: path.resolve(__dirname, './src'),

        // allow us to avoid including extension name
        extensions: ['', '.js'],
      },
      module: {
        // run babel loader for our tests
        loaders: [
          { test: /\.js?$/, exclude: /(node_modules|bower_components)\//, loader: 'babel' },
        ],
      },
    },

    webpackMiddleware: {
      noInfo: true
    },
    // tell karma all the plugins we're going to be using
    plugins: [
      'karma-mocha',
      'karma-webpack',
      'karma-phantomjs-launcher',
      'karma-spec-reporter',
      'karma-sourcemap-loader'
    ]
  });
};