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.


setInterval()

setInterval() is a time interval based code execution method that has the native ability to repeatedly run a specified script when the interval is reached. It should not be nested into its callback function by the script author to make it loop, since it loops by default. It will keep firing at the interval unless you call clearInterval().

If you want to loop code for animations or on a clock tick, then use setInterval().

function doStuff() {
    alert("run your code here when time interval is reached");
}
var myTimer = setInterval(doStuff, 5000);

setTimeout()

setTimeout() is a time based code execution method that will execute a script only one time when the interval is reached. It will not repeat again unless you gear it to loop the script by nesting the setTimeout() object inside of the function it calls to run. If geared to loop, it will keep firing at the interval unless you call clearTimeout().

function doStuff() {
    alert("run your code here when time interval is reached");
}
var myTimer = setTimeout(doStuff, 5000);

If you want something to happen one time after a specified period of time, then use setTimeout(). That is because it only executes one time when the specified interval is reached.


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);

Tags:

Javascript

Setinterval

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