Example of how to set up a testing environment for Electron backend (main) and front-end (renderer) modules with 1) source files being processed by Webpack 2) tests run by Mocha 3) tests launched by Karma.

karma-electron-bidi.conf.js

const path = require('path');

module.exports = function (config) {
  config.set({
    plugins: [
      'karma-electron',
      'karma-mocha',
      'karma-spec-reporter',
      'karma-sourcemap-loader',
      'karma-webpack',
    ],

    browsers: [ 'VisibleElectron' ],
    browserConsoleLogOptions: {
      level: 'debug',
      terminal: true,
    },
    browserDisconnectTolerance: 0,
    browserNoActivityTimeout: 2000,
    concurrency: 1,
    customLaunchers: {
      VisibleElectron: {
        base: 'Electron',
        flags: [ '--show' ],
        devTools: true,
        entry: path.resolve(__dirname, 'ui/index--test-core-entry.js'),
        browserWindowOptions: {
          webPreferences: {
            webSecurity: false,
            allowRunningInsecureContent: true,
          }
        }
      }
    },

    frameworks: [
      'mocha'
    ],

    files: [
      {
        pattern: 'ui/index--test.js',
        watched: false,
        included: true,
        served: true,
      },
      {
        pattern: 'ui/index--test-entry.js',
        watched: false,
        included: true,
        served: true,
      },
    ],

    preprocessors: {
      'ui/index--test.js': [ 'electron' ],
      'ui/index--test-entry.js': [ 'webpack', 'sourcemap' ],
    },

    reporters: [ 'spec' ],

    webpack: require('./webpack/test.js'),

    client: {
      captureConsole: true,
      clearContext: true,
      useIframe: false,
      runInParent: true,
      loadScriptsViaRequire: true,
    },

    webpackServer: {
      noInfo: true
    }
  });
};

// @file: ui/index--test-core-entry.js
// Here we'll load our main-process files. In my case, I have only a single
// module that responds to messages on `electron.ipcMain`.
require(require('path').resolve(__dirname, '../main/messageHandlers'));

// @file: ui/index--test.js
// Before we get webpack into play, we must rewrite Electron's window.require
// to another variable so that it doesn't conflict with webpack's. This assumes
// our application source files (that get processed by webpack) are using this
// identifier to include Electron modules.
if (!window.hasOwnProperty('electronRequire')) {
  window.electronRequire = require;
  delete window.require;
  delete window.exports;
  delete window.module;
}

// Now, window.require can be clobbered by webpack just fine,
// our app sources will use electronRequire() to get to Electron's.

// @file: index--test-entry.js
// Here we actually load the tests through Webpack.
const tap = (x, fn) => fn(x);
const loadTestFile = require.context('./', true, /\.test\.js$/);

loadTestFile.keys().forEach(loadTestFile);

process.on('uncaughtException', function handleUncaughtException (err) {
  console.warn('uncaughtException in karma process!')
  throw err;
});

window.addEventListener('unhandledrejection', function(data) {
  console.warn('Throwing unhandled promise rejection!');

  throw data.reason;
});