setInterval and long running functions

Question

How does setInterval handle callback functions that take longer than the desired interval?

I've read that the callback may receive the number of milliseconds late as its first argument, but I was unable to find why it would be late (jitter, or long running functions).

And the wonderful follow up, does it behave differently for the common browsers?

Good question. I would suspect the answer is browser specific. Should we run some tests to find out? — user213154
– user213154, Commented Dec 29, 2009 at 19:29
I wouldn't rely on receiving any arguments to the callback. Instead, use a simple Date comparison. That seems the most reliable and cross-browser approach to me. — Roman Nurik
– Roman Nurik, Commented Dec 29, 2009 at 19:35

Community · Accepted Answer · 2020-06-20 09:12:55Z

25

Let me quote an excellent article about timers by John Resig:

setTimeout(function(){
  /* Some long block of code... */
  setTimeout(arguments.callee, 10);
}, 10);

setInterval(function(){
  /* Some long block of code... */
}, 10);

These two pieces of code may appear to be functionally equivalent, at first glance, but they are not. Notably the setTimeout code will always have at least a 10ms delay after the previous callback execution (it may end up being more, but never less) whereas the setInterval will attempt to execute a callback every 10ms regardless of when the last callback was executed.

Intervals may execute back-to-back with no delay if they take long enough to execute (longer than the specified delay).

edited Jun 20, 2020 at 9:12

CommunityBot

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answered Dec 29, 2009 at 19:44

Christian C. Salvadó

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1 Comment

Allain Lalonde Over a year ago

So, in a nutshell, because JavaScript threading is all single threaded, the callbacks cannot be running in parallel. Perfect.

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