Setting up Karma with Mocha, Chai, Babel, CommonJS. Everything else in karma.conf.js can be left at standard settings.

gistfile1.txt

// Karma configuration

module.exports = function(config) {
  config.set({

    // base path that will be used to resolve all patterns (eg. files, exclude)
    basePath: '',

    // frameworks to use
    // available frameworks: https://npmjs.org/browse/keyword/karma-adapter
    frameworks: ['chai', 'mocha', 'commonjs'],

    // list of files / patterns to load in the browser
    files: [
      'path/to/code/**/*.js'
      ],

    // list of files to exclude
    exclude: [
    ],

    // preprocess matching files before serving them to the browser
    // available preprocessors: https://npmjs.org/browse/keyword/karma-preprocessor
    preprocessors: {
      'path/to/code/**/*.js': ['babel', 'sourcemap', 'commonjs']
    },

    babelPreprocessor: {
      options: {
        sourceMap: 'inline'
      }
    },

// [....]

    plugins: [
              "karma-mocha",
              "karma-chai",
              "karma-commonjs",
              "karma-babel-preprocessor",
              "karma-sourcemap-loader"
             ],

// [....]   

  });
};