Can setTimeout ever return 0 as the id?

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 开发者_开发问答a will be 0. In that case I would use a !== null

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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.

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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 =.

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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

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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