Simulate a timed async call

Question

I'm trying to simulate an async callback, that does something in a set number of seconds. I want these to all log at the same time, 3 seconds from when they are triggered. Right now they log consecutively 3 seconds after each other. The sleep functions are blocking the whole script from running. Any idea why?

function sleep(delay) {
  var start = new Date().getTime();
  while (new Date().getTime() < start + delay);
}

var same = function(string, callback) {
  new sleep(3000);
  return callback(string);
}

same("same1", function(string) {
  console.log(string);
});
same("same2", function(string) {
  console.log(string);
});
same("same3", function(string) {
  console.log(string);
});

https://developer.mozilla.org/en-US/docs/Web/API/Window.setTimeout — Felix Kling, Dec 02 '13 at 17:42
*"Any idea why?"* Because JavaScript is single threaded and you are continuously running the loop. — Felix Kling, Dec 02 '13 at 17:43
possible duplicate of [Is there some way to introduce a delay in javascript?](http://stackoverflow.com/questions/24849/is-there-some-way-to-introduce-a-delay-in-javascript) — Felix Kling, Dec 02 '13 at 17:43

jfriend00 · Accepted Answer · 2013-12-02T18:15:26.477

15

Use setTimeout() to schedule something for a future time.

Also, setTimeout() is async, your looping is not.

var same = function(str, callback){
    setTimeout(function() {
        callback(str);
    }, 3000);
}

Note: you cannot return a value from the async callback because it's async. The function same() completes and returns long before the callback is actually called.

edited Dec 02 '13 at 18:15

answered Dec 02 '13 at 17:52

jfriend00

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Roman · Answer 2 · 2021-08-04T19:26:26.477

10

With ES6, you can use the following technique based on an helper delay function:

const delay = async (delay = 1000, callback = () => {}) => {        

  const delayPromise = ms => new Promise(res => setTimeout(res, ms))
  await delayPromise(delay)

  callback()
}

const funcCallback = () => { console.info('msg WITH delay > 2') }

delay(5000, funcCallback)
console.info('Instant msg > 1')

edited Aug 04 '21 at 19:26

answered Nov 20 '18 at 00:33

Roman

19,236
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Why would I want to make use of a promise here instead of simply using setTimeout()? What are the advantages of using a promise here? – Rishab Gupta Mar 03 '23 at 00:27

Simulate a timed async call

2 Answers2