ES6 + Babel + Browserify + Mocha + Chai + Karma + Istanbul + PhantomJS

karma.conf.js

var istanbul = require('browserify-istanbul');

module.exports = function (config) {
    config.set({
        basePath: '../',
        files: ['src/scripts/**/*.js', 'test/unit/**/*.js'],

        frameworks: ['browserify', 'mocha', 'chai'],
        browsers: ['PhantomJS'], // 'Chrome'

        preprocessors: {
            'src/scripts/**/*.js': ['browserify'],
            'test/unit/**/*.js': ['browserify']
        },
        browserify: {
            debug: true,
            bundleDelay: 1000,
            transform: [['babelify', {
                ignore: /node_modules/
            }], istanbul({
                ignore: ['test/**', '**/node_modules/**']
            })],
            extensions: ['.js']
        },

        reporters: ['coverage', 'mocha'],
        coverageReporter: {
            reporters: [{type: 'text'}, {
                type: 'html',
                dir: 'coverage',
                subdir: 'html'
            }, {
                type: 'lcovonly',
                dir: 'coverage',
                subdir: 'lcov'
            }]
        },

        logLevel: config.LOG_DISABLE,
        singleRun: true,
        autoWatch: false
    });
};

package.json

{
  "scripts": {
    "test": "./node_modules/.bin/karma start ./test/karma.conf.js"
  },
  "devDependencies": {
    "istanbul": "gotwarlost/istanbul#source-map",
    "babelify": "^6.3.0",
    "browserify": "^11.0.1",
    "browserify-istanbul": "^0.2.1",
    "chai": "^3.2.0",
    "karma": "^0.13.9",
    "karma-browserify": "^4.3.0",
    "karma-chai": "^0.1.0",
    "karma-chrome-launcher": "^0.2.0",
    "karma-mocha": "^0.2.0",
    "karma-mocha-reporter": "^1.1.1",
    "karma-phantomjs-launcher": "^0.2.1",
    "mocha": "^2.3.0",
    "phantomjs": "^1.9.18",
    "karma-coverage": "^0.5.1"
  },
}