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?

Answer 1

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).