gulpfile.babel.js
function _browserifyBundle() {
    let bundler = browserifyBundler();
    // Watch the bundler, and re-bundle it whenever files change
    bundler = watchify(bundler);
    bundler.on('update', () => _runBrowserifyBundle(bundler));

    return _runBrowserifyBundle(bundler);
}
function _browserifyBundle() {
    let bundler = browserifyBundler();
    // Watch the bundler, and re-bundle it whenever files change
    bundler = watchify(bundler);
    bundler.on('update', () => _runBrowserifyBundle(bundler));

    return _runBrowserifyBundle(bundler);
}
Build the unit test suite for running tests in the browser