var timer = function(length, resolve) {
    setTimeout(function () {
      console.log('Timeout done.');
      resolve();
    }, length);
}

timer(4000, () => alert('All done!'));

var timer = function(length) {
  return new Promise(function (resolve, reject) {
    setTimeout(function () {
      console.log('Timeout done.');
      resolve();
    }, length);
  });
}

timer(4000).then(() => alert('All done!'));