Can setTimeout ever return 0 as the id?

Question

I am writing a check to see if a timeout is active. I was thinking of doing this:

var a = setTimeout(fn, 10);
// ... Other code ... where clearTimeout(a) can be called and set to null
if (a != null)
{
   // do soemthing
}

I was wondering if it would ever be possible that a will be 0. In that case I would use a !== null

score 40 · Answer 1 · answered Oct 15 '10 at 15:19

40

It shouldn't, given this:

handle = window . setTimeout( handler [, timeout [, arguments ] ] )

Let handle be a user-agent-defined integer that is greater than zero that will identify the timeout to be set by this call.

answered Oct 15 '10 at 15:19

Felix Kling

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@Alin: I came to this site via https://developer.mozilla.org/en/DOM/window.setTimeout, where it says that it is part of DOM Level 0. So I thought this actually older than HTML5, but maybe I misinterpreting something. – Felix Kling Oct 15 '10 at 15:24
+1 Thanks for the link. You did answer my question, though the accepted answer addressed my concern. – Aishwar Oct 15 '10 at 16:21
@Aishwar, but null is not a number, it is not good to mix types, even in js – 4esn0k Oct 12 '12 at 13:42

Alin Purcaru · Answer 2 · 2014-01-12T01:00:52.233

15

The specifications from Microsoft, Sun and Mozilla just say that it will return an integer. So 0 may be included.

It may be possible (and probable) that some implementations exclude 0 but you shouldn't rely on that. You should go with the !==.

To summarize: Although probably all browser exclude 0 from the IDs returned by setTimeout, you should not write your code with that in mind especially when all your have to do is add an extra =.

edited Jan 12 '14 at 01:00

answered Oct 15 '10 at 15:18

Alin Purcaru

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+1 Thanks. You did answer my question, though the accepted answer addressed my concern. – Aishwar Oct 15 '10 at 16:20

score 2 · Answer 3 · edited May 23 '17 at 12:09

2

Most browsers will return an int starting at 1 and incrementing for each call of setTimeout so in theory it could never be 0.

But keep in mind that this is not really a requirement of the spec, just a convention browsers tend to follow. See the accepted answer here for more details: setInterval/setTimeout return value

edited May 23 '17 at 12:09

Community

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answered Oct 15 '10 at 15:19

brendan

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It's apparently part of the HTML5 spec (http://www.w3.org/TR/html5/timers.html). Pre-HTML5 it was just a convention. – mhenry1384 Aug 31 '11 at 19:25

score 2 · Accepted Answer · answered Oct 15 '10 at 15:21

2

First: 0 isn't the same as null, (0 == null) would be false in every case';

if you want to test 'a' against something: define 'a' first and later assign the settimeout to 'a'. then check against the type of 'a'. If its 'undefined', the timer hasn't triggered yet

answered Oct 15 '10 at 15:21

DoXicK

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My mistake :) I didn't check, but yes `0 != null` always. I expected them to be equal in Javascript, considering equalities like '' and 0 - but this is not the case. – Aishwar Oct 15 '10 at 16:17
2

This does not answer the question. – Grant Gryczan Oct 12 '18 at 19:23
@GrantGryczan and yet, back in the days when the dinosaurs were still walking among us, Aishwar accepted it as the answer. While i indeed agree that my answer doesn't answer the title, it did fix the problem. The answer of Felix Kling is however the correct answer. – DoXicK Oct 15 '18 at 13:01

Can setTimeout ever return 0 as the id?

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