karma.base.config.js, Generate Coverage with Karma for New JavaScript features.

karma.base.config.js

const webpackConfig = {
  module: {
    rules: [
      {
        test: /\.js$/,
        loader: 'babel-loader',
        exclude: /node_modules/,
      },
    ],
  },
};

module.exports = {
  basePath: '../../',
  frameworks: ['mocha'],
  reporters: ['mocha', 'coverage'], // coverage as subsequent reporter(optional)
  files: [
    // should include all *.karma.js and helpers
    { pattern: 'spec/helpers/*.js' },
    { pattern: 'spec/**/*.karma.js' },
  ],
  preprocessors: {
    // preprocessing with complier if your tests are written in new JavaScript features.
    // Do not include source code files for preprocssing by coverage, because babel-plugin-istanbul
    // will deal with it.
    'spec/helpers/*.js': ['webpack'],
    'spec/**/*.karma.js': ['webpack'],
  },
  webpack: webpackConfig,
  plugins: [
    'karma-mocha',
    'karma-webpack',
  ],
  coverageReporter: {
    dir: './coverage',
    reporters: [
      { type: 'text' },
      { type: 'html' },
    ],
  },
};