setTimeout or setInterval?

They essentially try to do the same thing, but the setInterval approach will be more accurate than the setTimeout approach, since setTimeout waits 1000ms, runs the function and then sets another timeout. So the wait period is actually a bit more than 1000ms (or a lot more if your function takes a long time to execute).

Although one might think that setInterval will execute exactly every 1000ms, it is important to note that setInterval will also delay, since JavaScript isn't a multi-threaded language, which means that - if there are other parts of the script running - the interval will have to wait for that to finish.

In this Fiddle, you can clearly see that the timeout will fall behind, while the interval is almost all the time at almost 1 call/second (which the script is trying to do). If you change the speed variable at the top to something small like 20 (meaning it will try to run 50 times per second), the interval will never quite reach an average of 50 iterations per second.

The delay is almost always negligible, but if you're programming something really precise, you should go for a self-adjusting timer (which essentially is a timeout-based timer that constantly adjusts itself for the delay it's created)

Is there any difference?

Yes. A Timeout executes a certain amount of time after setTimeout() is called; an Interval executes a certain amount of time after the previous interval fired.

You will notice the difference if your doStuff() function takes a while to execute. For example, if we represent a call to setTimeout/setInterval with ., a firing of the timeout/interval with * and JavaScript code execution with [-----], the timelines look like:

Timeout:

.    *  .    *  .    *  .    *  .
     [--]    [--]    [--]    [--]

Interval:

.    *    *    *    *    *    *
     [--] [--] [--] [--] [--] [--]

The next complication is if an interval fires whilst JavaScript is already busy doing something (such as handling a previous interval). In this case, the interval is remembered, and happens as soon as the previous handler finishes and returns control to the browser. So for example for a doStuff() process that is sometimes short ([-]) and sometimes long ([-----]):

.    *    *    •    *    •    *    *
     [-]  [-----][-][-----][-][-]  [-]

• represents an interval firing that couldn't execute its code straight away, and was made pending instead.

So intervals try to ‘catch up’ to get back on schedule. But, they don't queue one on top of each other: there can only ever be one execution pending per interval. (If they all queued up, the browser would be left with an ever-expanding list of outstanding executions!)

.    *    •    •    x    •    •    x
     [------][------][------][------]

x represents an interval firing that couldn't execute or be made pending, so instead was discarded.

If your doStuff() function habitually takes longer to execute than the interval that is set for it, the browser will eat 100% CPU trying to service it, and may become less responsive.

Which do you use and why?

Chained-Timeout gives a guaranteed slot of free time to the browser; Interval tries to ensure the function it is running executes as close as possible to its scheduled times, at the expense of browser UI availability.

I would consider an interval for one-off animations I wanted to be as smooth as possible, whilst chained timeouts are more polite for ongoing animations that would take place all the time whilst the page is loaded. For less demanding uses (such as a trivial updater firing every 30 seconds or something), you can safely use either.

In terms of browser compatibility, setTimeout predates setInterval, but all browsers you will meet today support both. The last straggler for many years was IE Mobile in WinMo <6.5, but hopefully that too is now behind us.

The setInterval makes it easier to cancel future execution of your code. If you use setTimeout, you must keep track of the timer id in case you wish to cancel it later on.

var timerId = null;
function myTimeoutFunction()
{
    doStuff();
    timerId = setTimeout(myTimeoutFunction, 1000);
}

myTimeoutFunction();

// later on...
clearTimeout(timerId);

versus

function myTimeoutFunction()
{
    doStuff();
}

myTimeoutFunction();
var timerId = setInterval(myTimeoutFunction, 1000);

// later on...
clearInterval(timerId);

I find the setTimeout method easier to use if you want to cancel the timeout:

function myTimeoutFunction() {
   doStuff();
   if (stillrunning) {
      setTimeout(myTimeoutFunction, 1000);
   }
}

myTimeoutFunction();

Also, if something would go wrong in the function it will just stop repeating at the first time error, instead of repeating the error every second.

I use setTimeout.

Apparently the difference is setTimeout calls the method once, setInterval calls it repeatdly.

Here is a good article explaining the difference: Tutorial: JavaScript timers with setTimeout and setInterval

Both setInterval and setTimeout return a timer id that you can use to cancel the execution, that is, before the timeouts are triggered. To cancel you call either clearInterval or clearTimeout like this:

var timeoutId = setTimeout(someFunction, 1000);
clearTimeout(timeoutId);
var intervalId = setInterval(someFunction, 1000),
clearInterval(intervalId);

Also, the timeouts are automatically cancelled when you leave the page or close the browser window.

If you set the interval in setInterval too short, it may fire before the previous call to the function has been completed. I ran into this problem with a recent browser (Firefox 78). It resulted in the garbage collection not being able to free memory fast enough and built up a huge memory leak. Using setTimeout(function, 500); gave the garbage collection enough time to clean up and keep the memory stable over time.

Serg Hospodarets mentioned the problem in his answer and I fully agree with his remarks, but he didn't include the memory leak/garbage collection-problem. I experienced some freezing, too, but the memory usage ran up to 4 GB in no time for some minuscule task, which was the real bummer for me. Thus, I think this answer is still beneficial to others in my situation. I would have put it in a comment, but lack the reputation to do so. I hope you don't mind.

The reason why Option A and Option B seem like they work the same is mostly because the places of the setInterval and the setTimeout functions.

function myTimeoutFunction()
{
    doStuff();
    setTimeout(myTimeoutFunction, 1000);
}

myTimeoutFunction();

This one is a recursive function, and if doStuff is very complex, setTimeout has to keep track of all calls of the setTimout plus the current doStuff, which makes it become slower and s l o w e r.

function myTimeoutFunction()
{
    doStuff();
}

myTimeoutFunction();
setInterval(myTimeoutFunction, 1000);

On the other hand, the setInterval only has to keep track of the last setInterval and the current doStuff, making it staying at a constant speed.

From the above, you should probably be able to conclude that the better one is setInterval.

The important point to consider is the performance. The only way to run a function periodically using setTimeout is to call it recursively with the target function, and when you check it, it appears that it works asynchronously put when you see the call stack you will find it keep growing by the time. In fact, it is sensible. Since Javascript does not support multi-threading, it is impossible to finish calling the parent function before finishing the child function, therefor, the stack will keep growing as long as there is recursive calling. Whilst, with setInterval we don't need to call the target function recursively since it has a logic that runs it periodically as a loop. So, this keeps the call stack clean. You can watch the call stack using developer's tools in your browser and you will notice the difference.

The difference will be clear when using small interval for a long period of time.