javascript - Is there a difference between passing a function to `setInterval` once and repeatedly passing it to `setTimeout`? -

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So I know that there are differences between setTimeout and setInterval, but consider these two code examples:

function myFunction(){
   setTimeout('myFunction();', 100);
   doSomething();
}
setTimeout('myFunction();', 100);

and

function myFunction(){
   doSomething();
}
setInterval('myFunction();', 100);

Note that in the first example I call setTimeout at the beginning of the function and then I doSomething. Therefore there is no extra delay from doSomething(). Does that mean that those two examples do exactly the same? Or is there even more subtle difference?

So I know that there are differences between setTimeout and setInterval, but consider these two code examples:

function myFunction(){
   setTimeout('myFunction();', 100);
   doSomething();
}
setTimeout('myFunction();', 100);

and

function myFunction(){
   doSomething();
}
setInterval('myFunction();', 100);

Note that in the first example I call setTimeout at the beginning of the function and then I doSomething. Therefore there is no extra delay from doSomething(). Does that mean that those two examples do exactly the same? Or is there even more subtle difference?

• javascript
• settimeout
• setinterval

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2 Answers 2

Sorted by: Reset to default 11

They're functionally about the same, but there are differences. One difference is in how browsers handle it if doSomething takes longer than the interval. With setInterval, at least some browsers will just skip the next interval if doSomething is still running. So if you use 100ms as you have, and doSomething takes 110 ms to run, the next run won't happen until 90ms later (of course, all of these times are approximate).

Another difference is that with the setTimeout, you'll get a new handle every time, whereas with setInterval you get one handle.

Another difference with your examples as given is that in the setTimeout example, you're firing up a JavaScript parser/piler every time, whereas with setInterval you're only firing up the parser/piler once. But this difference shouldn't matter, because you shouldn't be doing that at all — see below.

But subtleties aside, what you have there is functionally the same.

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Side note: It's not best practice to pass strings into either setTimeout or setInterval. Instead, pass in a function reference:

// setTimeout
function myFunction(){
   setTimeout(myFunction, 100);
   doSomething();
}
setTimeout(myFunction, 100);

// setInterval
function myFunction(){
   doSomething();
}
setInterval(myFunction, 100);

Passing in a string fires up a JavaScript parser and does the same thing as eval. It should be avoided whenever possible (and it's almost always possible).

T.J. Crowder explained the main differences, one other more subtle could appear (I change the time scale as it's easier to explain) :

Lets plot the difference with a very big timeout time : 1 Day. You call both methods at 00:00 on Day 1 and let it run for 1 year...

1 Year latter your method called by setInterval will execute at 00:00 + some milliseconds (because you may not be the only one asking for the processors to do things at this exact moment and the OS timers have granularity anyway).

But your setTimeout method will occur latter, maybe around 00:01 because each day it would have been called a little after the requested time and requested to be called the next day at the same time...

PS: It could also be called before the requested time in some cases but more often than not it run after :-D

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