watchify

gulpfile.js

var customOpts = {
  entries: [config.jsES6MainFile],
  debug: true,
  transform: ['babelify']
};
var opts = _.assign({}, watchify.args, customOpts);
var b = watchify(browserify(opts)); 

gulp.task('scripts', bundle);
b.on('update', bundle);

function bundle () {
  return b.bundle()
      .on('error', function(e) { 
        $.util.log(e.toString());
        this.emit('end');
      })
      .pipe(source(config.jsES5DestFileName))
      .pipe(gulp.dest(config.tmp));  
}