JavaScript Scheduling: setTimeout and setInterval

We may decide to execute a function not right now, but at a certain time later. That’s called “scheduling a call”.

There are two methods for it:

• setTimeout allows us to run a function once after the interval of time.
• setInterval allows us to run a function repeatedly, starting after the interval of time, then repeating continuously at that interval.

These methods are not a part of JavaScript specification. But most environments have the internal scheduler and provide these methods. In particular, they are supported in all browsers and Node.js.

1. setTimeout

The syntax:

let timerId = setTimeout(func|code, [delay], [arg1], [arg2], ...)


Parameters:func|codeFunction or a string of code to execute. Usually, that’s a function. For historical reasons, a string of code can be passed, but that’s not recommended.delayThe delay before run, in milliseconds (1000 ms = 1 second), by default 0.arg1arg2…Arguments for the function (not supported in IE9-)

For instance, this code calls sayHi() after one second:

function sayHi() {
}

setTimeout(sayHi, 1000);


With arguments:

function sayHi(phrase, who) {
alert( phrase + ', ' + who );
}

setTimeout(sayHi, 1000, "Hello", "John"); // Hello, John


If the first argument is a string, then JavaScript creates a function from it.

So, this will also work:

setTimeout("alert('Hello')", 1000);


But using strings is not recommended, use arrow functions instead of them, like this:

setTimeout(() => alert('Hello'), 1000);


Pass a function, but don’t run it

Novice developers sometimes make a mistake by adding brackets () after the function:

// wrong!
setTimeout(sayHi(), 1000);


That doesn’t work, because setTimeout expects a reference to a function. And here sayHi() runs the function, and the result of its execution is passed to setTimeout. In our case the result of sayHi() is undefined (the function returns nothing), so nothing is scheduled.

2. Canceling with clearTimeout

A call to setTimeout returns a “timer identifier” timerId that we can use to cancel the execution.

The syntax to cancel:

let timerId = setTimeout(...);
clearTimeout(timerId);


In the code below, we schedule the function and then cancel it (changed our mind). As a result, nothing happens:

let timerId = setTimeout(() => alert("never happens"), 1000);
alert(timerId); // timer identifier

clearTimeout(timerId);
alert(timerId); // same identifier (doesn't become null after canceling)


As we can see from alert output, in a browser the timer identifier is a number. In other environments, this can be something else. For instance, Node.js returns a timer object with additional methods.

Again, there is no universal specification for these methods, so that’s fine.

For browsers, timers are described in the timers section of HTML5 standard.

3. setInterval

The setInterval method has the same syntax as setTimeout:

let timerId = setInterval(func|code, [delay], [arg1], [arg2], ...)


All arguments have the same meaning. But unlike setTimeout it runs the function not only once, but regularly after the given interval of time.

To stop further calls, we should call clearInterval(timerId).

The following example will show the message every 2 seconds. After 5 seconds, the output is stopped:

// repeat with the interval of 2 seconds
let timerId = setInterval(() => alert('tick'), 2000);

// after 5 seconds stop
setTimeout(() => { clearInterval(timerId); alert('stop'); }, 5000);


Time goes on while alert is shown

In most browsers, including Chrome and Firefox the internal timer continues “ticking” while showing alert/confirm/prompt.

So if you run the code above and don’t dismiss the alert window for some time, then the next alert will be shown immediately as you do it. The actual interval between alerts will be shorter than 2 seconds.

4. Nested setTimeout

There are two ways of running something regularly.

One is setInterval. The other one is a nested setTimeout, like this:

/** instead of:
let timerId = setInterval(() => alert('tick'), 2000);
*/

let timerId = setTimeout(function tick() {
timerId = setTimeout(tick, 2000); // (*)
}, 2000);


The setTimeout above schedules the next call right at the end of the current one (*).

The nested setTimeout is a more flexible method than setInterval. This way the next call may be scheduled differently, depending on the results of the current one.

For instance, we need to write a service that sends a request to the server every 5 seconds asking for data, but in case the server is overloaded, it should increase the interval to 10, 20, 40 seconds…

Here’s the pseudocode:

let delay = 5000;

let timerId = setTimeout(function request() {
...send request...

if (request failed due to server overload) {
// increase the interval to the next run
delay *= 2;
}

timerId = setTimeout(request, delay);

}, delay);


And if the functions that we’re scheduling are CPU-hungry, then we can measure the time taken by the execution and plan the next call sooner or later.

Nested setTimeout allows to set the delay between the executions more precisely than setInterval.

Let’s compare two code fragments. The first one uses setInterval:

let i = 1;
setInterval(function() {
func(i++);
}, 100);


The second one uses nested setTimeout:

let i = 1;
setTimeout(function run() {
func(i++);
setTimeout(run, 100);
}, 100);


For setInterval the internal scheduler will run func(i++) every 100ms:

Did you notice?

The real delay between func calls for setInterval is less than in the code!

That’s normal, because the time taken by func‘s execution “consumes” a part of the interval.

It is possible that func‘s execution turns out to be longer than we expected and takes more than 100ms.

In this case the engine waits for func to complete, then checks the scheduler and if the time is up, runs it again immediately.

In the edge case, if the function always executes longer than delay ms, then the calls will happen without a pause at all.

And here is the picture for the nested setTimeout:

The nested setTimeout guarantees the fixed delay (here 100ms).

That’s because a new call is planned at the end of the previous one.Garbage collection and setInterval/setTimeout callback

When a function is passed in setInterval/setTimeout, an internal reference is created to it and saved in the scheduler. It prevents the function from being garbage collected, even if there are no other references to it.

// the function stays in memory until the scheduler calls it
setTimeout(function() {...}, 100);


For setInterval the function stays in memory until clearInterval is called.

There’s a side-effect. A function references the outer lexical environment, so, while it lives, outer variables live too. They may take much more memory than the function itself. So when we don’t need the scheduled function anymore, it’s better to cancel it, even if it’s very small.

5. Zero delay setTimeout

There’s a special use case: setTimeout(func, 0), or just setTimeout(func).

This schedules the execution of func as soon as possible. But the scheduler will invoke it only after the currently executing script is complete.

So the function is scheduled to run “right after” the current script.

For instance, this outputs “Hello”, then immediately “World”:

setTimeout(() => alert("World"));



The first line “puts the call into calendar after 0ms”. But the scheduler will only “check the calendar” after the current script is complete, so "Hello" is first, and "World" – after it.

There are also advanced browser-related use cases of zero-delay timeout, that we’ll discuss in the chapter Event loop: microtasks and macrotasks.

Zero delay is in fact not zero (in a browser)

In the browser, there’s a limitation of how often nested timers can run. The HTML5 standard says: “after five nested timers, the interval is forced to be at least 4 milliseconds.”.

Let’s demonstrate what it means with the example below. The setTimeout call in it re-schedules itself with zero delay. Each call remembers the real time from the previous one in the times array. What do the real delays look like? Let’s see:

let start = Date.now();
let times = [];

setTimeout(function run() {
times.push(Date.now() - start); // remember delay from the previous call

if (start + 100 < Date.now()) alert(times); // show the delays after 100ms
else setTimeout(run); // else re-schedule
});

// an example of the output:
// 1,1,1,1,9,15,20,24,30,35,40,45,50,55,59,64,70,75,80,85,90,95,100


First timers run immediately (just as written in the spec), and then we see 9, 15, 20, 24.... The 4+ ms obligatory delay between invocations comes into play.

The similar thing happens if we use setInterval instead of setTimeoutsetInterval(f) runs f few times with zero-delay, and afterwards with 4+ ms delay.

That limitation comes from ancient times and many scripts rely on it, so it exists for historical reasons.

For server-side JavaScript, that limitation does not exist, and there exist other ways to schedule an immediate asynchronous job, like setImmediate for Node.js. So this note is browser-specific.

6. Summary

• Methods setTimeout(func, delay, ...args) and setInterval(func, delay, ...args) allow us to run the func once/regularly after delay milliseconds.
• To cancel the execution, we should call clearTimeout/clearInterval with the value returned by setTimeout/setInterval.
• Nested setTimeout calls are a more flexible alternative to setInterval, allowing us to set the time between executions more precisely.
• Zero delay scheduling with setTimeout(func, 0) (the same as setTimeout(func)) is used to schedule the call “as soon as possible, but after the current script is complete”.
• The browser limits the minimal delay for five or more nested calls of setTimeout or for setInterval (after 5th call) to 4ms. That’s for historical reasons.

Please note that all scheduling methods do not guarantee the exact delay.

For example, the in-browser timer may slow down for a lot of reasons:

• The CPU is overloaded.
• The browser tab is in the background mode.
• The laptop is on battery.

All that may increase the minimal timer resolution (the minimal delay) to 300ms or even 1000ms depending on the browser and OS-level performance settings.