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

Answer 1

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.

Answer 2

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 =.