setTimeout() inside JavaScript Class using "this"

Question

I am trying to use setTimeout() inside a class function in JavaScript. The setTimeout() is supposed to trigger another method in the same Class, so the function I am passing it is written as window.setTimeout("this.anotherMethod", 4000). That bring the problem: this references the calling Object, in the case of setTimeout() it is window. How can I use enclosures to return a reference to the Class Object itself?

myObject = function(){

this.move = function(){
    alert(this + " is running");
}
this.turn = function(){
    alert(this + " is turning");
}
this.wait = function(){
    window.setTimeout("this.run" ,(1000 * randomNumber(1,5)));
}

this.run = function(){
    switch(randomNumber(0,2)){
        case 0:
            this.move();
        break;
        case 1:
            this.turn();
        break;
        case 2:
            this.wait();
    }
}

}

I set that = this, and used that for all the methods, works like magic, +1 for everyone who answered that, thanks a lot folks. — Dean, May 07 '11 at 09:28

Tikhon Jelvis · Accepted Answer · 2015-05-08T21:18:55.720

103

You can do this:

 var that = this;
 setTimeout(function () {
     that.doStuff();
 }, 4000);

You can also bind for more succinct code (as originally pointed out by @Raynos):

setTimeout(this.doStuff.bind(this), 4000);

bind is a standard library function for exactly this coding pattern (ie capturing this lexically).

edited May 08 '15 at 21:18

answered May 06 '11 at 12:17

Tikhon Jelvis

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@David Dorward: I realized that just after hitting submit. I shouldn't post at 5 in the morning. – Tikhon Jelvis May 06 '11 at 12:19
@Dorward commented before the answer was edited. Now is correct – Riccardo Galli May 06 '11 at 12:20
I used this answer in a class with a singleton pattern implemented an it works! – Oscar Ardila Mar 30 '13 at 17:52
I find the syntax of @Raynos answer much cleaner (see below). ES 5 .bind() precisely solves this problem. – antoine May 08 '15 at 19:14
1

@Antoine: Fair enough, and ES5 support is probably widespread enough by now that using it is a complete non-issue. I'll edit in a note about that. – Tikhon Jelvis May 08 '15 at 21:16
what does the syntax look like for passing parameters to doStuff? setTimeout(this.doStuff.bind(this), 4000, param); – Gerry Sep 26 '18 at 15:38
`bind` can take the additional arguments: `setTimeout(this.doStuff.bind(this, param), 4000)`. – Tikhon Jelvis Sep 27 '18 at 08:33
1

Is it just me or do I find this hilarious? Good answer though! – Static Spaghetti 13 Jul 15 '23 at 21:28

score 13 · Answer 2 · answered May 06 '11 at 12:21

13

You can also bind a function to scope.

setTimeout(this.run.bind(this) ,(1000 * randomNumber(1,5)));

Be warned Function.prototype.bind is ES5

answered May 06 '11 at 12:21

Raynos

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you can just get bind to work on any js project: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function/bind :) – Paul Scheltema Oct 01 '13 at 22:41

GordonM · Answer 3 · 2011-05-06T12:26:10.107

8

this can be problematic in javascript, as you've discovered.

I usually work around this by aliasing this inside the object so that I can use the alias whenever I need a reference back to the containing object.

MyObject = function ()
{
    var self = this;

    // The rest of the code goes here

    self.wait = function(){
        window.setTimeout(self.run ,(1000 * randomNumber(1,5)));
    }
}

edited May 06 '11 at 12:26

answered May 06 '11 at 12:20

GordonM

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I find `self` to be a most misleading identifier, not least because it has very similar connotations to `this` which, as you point out, means something else. – Lightness Races in Orbit May 06 '11 at 12:31
1

Well it is just an example, you could call it MyObjRef, or anything else you think it should be. The point is that it gives you a way of getting the object reference. – GordonM May 06 '11 at 12:56

score 5 · Answer 4 · edited Jan 31 '20 at 14:34

5

class A{

   setTimeout(()=>{

       // here this != undefined because of arrow function

  },500);

}

edited Jan 31 '20 at 14:34

ChrisMM

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answered Jan 31 '20 at 14:11

M Ahmed Mushtaq

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When answering an old question, your answer would be much more useful to other StackOverflow users if you included some context to explain how your answer helps, particularly for a question that already has an accepted answer. See: [How do I write a good answer](https://stackoverflow.com/help/how-to-answer). – David Buck Jan 31 '20 at 14:40

score 4 · Answer 5 · answered May 06 '11 at 12:20

4

this.wait = function(){
    var self = this;
    window.setTimeout(function() { self.run() } ,(1000 * randomNumber(1,5)));
}

So you store the reference to the object you're calling .run on in a local variable ('self').

answered May 06 '11 at 12:20

Gijs

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score 3 · Answer 6 · answered May 06 '11 at 12:20

this is sensitive to the context in which it is called. When you pass a string to setTimeout then that is evaled in a completely different context.

You need to preserve the current value of this (by copying it to a different variable) and maintain the scope (by not using (implied) eval).

this.wait = function(){
    var self = this;
    setTimeout(function () { self.run() },
              (1000 * randomNumber(1,5))
              );
}

score 3 · Answer 7 · answered May 06 '11 at 12:20

At the top of your main myObject make a new reference to the current value of this:

var self = this;

and then create a closure for your timer callback that uses that new reference instead of the global object that setTimeout will use as the default context in callbacks:

setTimeout(function() {
    self.run();
}, 4000);

score 2 · Answer 8 · answered Aug 21 '19 at 08:03

2

you can just use the arrow function syntax:

setTimeout(() => {
     this.doStuff();
 }, 4000);

answered Aug 21 '19 at 08:03

Mioe Galaxy

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score 2 · Answer 9 · answered May 06 '11 at 15:40

var timeoutID = window.setTimeout(func, delay, [param1, param2, ...]);

inside func, this always refer to the global object. you can pass in the current object into func,

var timeoutID = window.setTimeout(func, delay, this);
function func(that) {...}

unfortunately it does NOT work in IE

Note that passing additional parameters to the function in the first syntax does not work in Internet Explorer.

score 1 · Answer 10 · answered May 06 '11 at 12:21

1

Have you tried;

window.setTimeout("myObject.run" ,(1000 * randomNumber(1,5)));

answered May 06 '11 at 12:21

BusterLuke

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score 0 · Answer 11 · answered Jul 05 '15 at 18:57

0

Shorter way. Without anonymous func.

    var self = this;
    setTimeout(self.method, 1000);

answered Jul 05 '15 at 18:57

Track83

59
4

score 0 · Answer 12 · answered Mar 24 '17 at 16:59

0

It is not recommended to use setTimeout or setInterval using strings

setTimeout("myFunction()", 5000);

//this is the same as 

setTimeout(function(){ eval("myFunction()"); }, 5000)); //<-- eval == BAD

answered Mar 24 '17 at 16:59

Vitim.us

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score 0 · Answer 13 · answered Oct 25 '17 at 20:46

Ran into a more complex situation...class A has a member of type B and a method that calls setTimeout which calls a method on class B. Solved as follows:

class A {
    constructor(b) {
        this.b = b;
    }
    setTimer(interval) {
        setTimeout(this.b.tick.bind(this.b), interval);
    }
}
class B {
    constructor(name){
        this.name = name;
        this.ele = window.document.getElementById('B');
    }
    tick() {
        console.log(this);
        this.ele.innerText += ' ' + this.name;
    }
}

Which bound A.b to this within B.tick and worked.

Here's a fiddle with bind: https://jsfiddle.net/jrme9hyh/

And one without bind which fails: https://jsfiddle.net/2jde8tq3/

score 0 · Answer 14 · answered Mar 27 '12 at 15:18

0

You can use this code instead, which works in all modern browsers -

setTimeout(function(thisObj) {thisObj.run();},1000,this);

Ref: http://klevo.sk/javascript/javascripts-settimeout-and-how-to-use-it-with-your-methods/

answered Mar 27 '12 at 15:18

KalEl

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setTimeout() inside JavaScript Class using "this"

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