Creating custom setTimeout() function

Question

var td = {
setTimeout: function(callback, ms){
var dt = new Date();
    var i = dt.getTime(), future = i+ms;
    while(i<=future){
      if(i === future){
        callback();
      }
      // i = dt.getTime();
      i++;
  }
 }
};
td.setTimeout(function(){
    console.log("Hello"); // executes immediately
}, 3000);

The problem with this approach is that, at the end of the while block I am setting the value of i to i + 1 which I think, should be set to the current timestamp (dt.getTime()), that way, future (a timestamp value at which the callback function should execute) can be compared with the current timestamp, which is the value of i.

But when I comment out i++ and uncomment this line:

i = dt.getTime();

and run the script, the browser freezes. Is there a way to create a setTimout() method that would serve the same purpose as window.setTimeout()?

It's is not really possible to simulate setTimeout in user land code because JavaScript doesn't expose any APIs to directly add functions to the event queue. Anyways... what's actually your question? — Felix Kling
– Felix Kling, Commented Jun 11, 2017 at 15:54
You should understand that even if you fix the endless loop issue, the browser will still lock up. That's because the loop will prevent anything else from being executed until it terminates. — Felix Kling
– Felix Kling, Commented Jun 11, 2017 at 16:05
Take a look at this answer of a more-or-less simillar question. — ibrahim mahrir
– ibrahim mahrir, Commented Jun 11, 2017 at 16:21

Maurice · Accepted Answer · 2017-06-11 15:57:11Z

1

The problem with your code is, that dt.getTime() always returns the same value (namely the time when the dt object was created. To make this working, you need to do

dt = new Date();

before you set

i = dt.getTime();

answered Jun 11, 2017 at 15:57

Maurice

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17 Comments

ibrahim mahrir Over a year ago

That is exactly what he is doing.

Felix Kling Over a year ago

@ibrahimmahrir: no, not in the loop.

Felix Kling Over a year ago

One of the reasons why I rather use Date.now()... no reason to hold on to date objects.

Felix Kling Over a year ago

@ibrahimmahrir: you are mostly right. But dt.getTime() holds the loop forever! Imagine it returns 5. Then i++ makes it 6. In the next iteration i = dt.getTime() resets it to 5... i never really increases. Getting the current time would at least fix the infinite loop. But as you can see from my comments on the question, I already pointed out that this is not the right approach.

Felix Kling Over a year ago

@ibrahimmahrir I imagine everybody encounters something like this at least once when venturing into the mysterious lands of JavaScript ;)

|

The Bomb Squad · Accepted Answer · 2021-06-16 00:20:26Z

0

Behold.. setTimeout that isn't synchronous >:D

function timeout(fn,n){
  (async()=>{
    n=n||0; var time=Date.now()+n
    await new Promise(r=>r()) //asynchronous even if n is 0
    while(Date.now()<time){
      await new Promise(r=>r())
    }
    fn()
  })()
  return true
}

//demonstration
timeout(()=>console.log(3),200)
timeout(()=>console.log(2),20)
console.log(1)

edited Jun 16, 2021 at 0:20

answered Jun 15, 2021 at 23:14

The Bomb Squad

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3 Comments

Dave Newton Over a year ago

It... doesn't really do a timeout, though. codepen.io/davelnewton/pen/poeqYyK

The Bomb Squad Over a year ago

ty i misplaced a few lines ;-;

Dave Newton Over a year ago

Fix "works" in the sense it delays, but as with any busy loop, this will peg the CPU.

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