gulpfile.js
function series() {
  const tasks = Array.prototype.slice.call(arguments);
  var fn = cb => cb();

  if (typeof tasks[tasks.length - 1] === 'function')
    fn = tasks.pop();

  return (cb) => {
    const tasks_with_cb = tasks.concat([(err) => {
      if (err) return cb(err);
      fn(cb);
    }]);

    runSequence.apply(this, tasks_with_cb);
  }
}
function series() {
  const tasks = Array.prototype.slice.call(arguments);
  var fn = cb => cb();

  if (typeof tasks[tasks.length - 1] === 'function')
    fn = tasks.pop();

  return (cb) => {
    const tasks_with_cb = tasks.concat([(err) => {
      if (err) return cb(err);
      fn(cb);
    }]);

    runSequence.apply(this, tasks_with_cb);
  }
}
function series ( ) { const tasks = Array . prototype . slice . call ( arguments ) ; var fn = cb => cb ( ) ; if ( typeof tasks [ tasks . length - 1 ] === 'function' ) fn = tasks . pop ( ) ; return ( cb ) => { const tasks_with_cb = tasks . concat ( [ ( err ) => { if ( err ) return cb ( err ) ; fn ( cb ) ; } ] ) ; runSequence . apply ( this , tasks_with_cb ) ; } }
Approximate gulp 4.0 series

@param {...String} ...tasks
@param {Function} [fn] Function to call at end of series
@return {Function}
Approximate gulp 4 . 0 series