Example karma config for running jasmine tests that uses webpack to support jsx.

karma.config.js

/**
 * This is the Karma configuration file. It contains information about this skeleton
 * that provides the test runner with instructions on how to run the tests and
 * generate the code coverage report.
 *
 * For more info, see: http://karma-runner.github.io/0.12/config/configuration-file.html
 */
module.exports = function(config) {
  config.set({

    /**
     * These are the files required to run the tests.
     *
     * The `Function.prototype.bind` polyfill is required by PhantomJS
     * because it uses an older version of JavaScript.
     */
    files: [
      './app/assets/javascripts/test/polyfill.js',
      './app/assets/javascripts/**/__tests__/*Spec.js'
    ],

    /**
     * The actual tests are preprocessed by the karma-webpack plugin, so that
     * their source can be properly transpiled.
     */
    preprocessors: {
      './app/assets/javascripts/**/__tests__/*Spec.js': ['webpack'],
    },

    /**
     * We want to run the tests using the PhantomJS headless browser.
     * This is especially useful for continuous integration.
     */
    browsers: ['PhantomJS'],

    /**
     * Use Mocha as the test framework, Sinon for mocking, and
     * Chai for assertions.
     */
    frameworks: ['jasmine', 'jasmine-matchers'],

    /**
     * After running the tests, return the results and generate a
     * code coverage report.
     */
    reporters: ['mocha'],

    /**
     * The configuration for the karma-webpack plugin.
     *
     * This is very similar to the main webpack.local.config.js, with the
     * exception of specifying an istanbul-transformer post loader so
     * that we can generate an accurate code coverage report.
     */
    webpack: {
      context: __dirname + '/app/assets/javascripts',
      module: {
        loaders: [
          { test: /\.jsx?$/, exclude: /node_modules/, loader: "jsx-loader?harmony"}
        ]
      },
      resolve: {
        extensions: ['', '.js', '.jsx']
      }
    },

    /**
     * Configuration option to turn off verbose logging of webpack compilation.
     */
    webpackMiddleware: {
      noInfo: true
    },

    /**
     * Once the mocha test suite returns, we want to exit from the test runner as well.
     */
    singleRun: true,

    /**
     * List of plugins
     */
    plugins: [
      'karma-webpack',
      'karma-jasmine',
      'karma-jasmine-matchers',
      'karma-phantomjs-launcher',
      'karma-mocha-reporter'
    ],
  });
}