Can you use setInterval to call functions asynchronously?

Question

The following code is taken from Project Silk (a Microsoft sample application) The publish method below loops thru an an array of event callBacks and executes each one. Instead of using a for loop setInterval is used instead.

The documentation says this allows each subscriber callback to be invoked before the previous callback finishes. Is this correct? I thought the browser would not allow the execution of the function inside the interval to run until all prior executions of it had finished.

Is this really any different than doing a for loop?

that.publish = function (eventName, data) 
{
    var context, intervalId, idx = 0;
    if (queue[eventName]) 
    {
        intervalId = setInterval(function () 
        {
            if (queue[eventName][idx]) 
            {
                context = queue[eventName][idx].context || this;
                queue[eventName][idx].callback.call(context, data);
                idx += 1;
            } 
            else { clearInterval(intervalId); }
        }, 0);
    }

Are you looking for something like this: github.com/caolan/async#parallel — Syed Aslam
– Syed Aslam, Commented Feb 17, 2012 at 6:45
I'm interested in understanding if this code does what the documentation claims it does. Because if it does there's something I don't understand about the way setInterval works. I don't understand it, especially since JS is single threaded. — C.J.
– C.J., Commented Feb 17, 2012 at 6:54
The setInterval(..., 0) callbacks won't be triggered until the browser has control again so the setInterval calls are pretty much saying "browser, once I'm done, call this function". — mu is too short
– mu is too short, Commented Feb 17, 2012 at 9:13
That was my thinking too. So why would you do this over a simple for loop? — C.J.
– C.J., Commented Feb 17, 2012 at 14:41

mWillis · Accepted Answer · 2012-02-17 18:47:29Z

4

Using setInterval here does make the execution sort of "asynchronous", because it schedules the execution of the callback for the next time the main execution thread is available.

This means that the callback executions should not block the browser because any other synchronous processing will take place before the callbacks (because the callbacks are scheduled to run only when the main execution thread has a spare millisecond) - and that's what makes this construct "better" than a regular for loop - the fact that the callbacks won't block the browser and cause the dreaded "This page has a script that's taking too long" error.

The side effect of this scheduling pattern is that the timeout is only a "suggestion" - which is why they use 0 here.

See: http://ejohn.org/blog/how-javascript-timers-work/

answered Feb 17, 2012 at 18:47

mWillis

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Gabe Moothart · Accepted Answer · 2012-02-17 18:45:27Z

setInterval(..., 0) can be used to yield control to the browser UI, to prevent it from freezing if your code takes a long time to run.

In this case, that.publish will exit almost immediately before any callback has been executed. Each callback will then run "in the background" - they will be placed in the event loop, and each one will yield to the browser to do it's thing before the next callback is executed.

This seems like a good idea in event processing, because you don't want event processing to freeze the browser even if there are a lot of them or some take a long time to finish.

About the documentation - as stated, it is incorrect. Javascript is single-threaded. But if you were to call publish() a few times in a row, it is true that those calls would all finish before any callbacks are executed, because of the setTimeout. Maybe this is what the documentation means?

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