JavaScript Timers


When writing JavaScript code, you might want to delay the execution of a function.

This is the job of setTimeout. You specify a callback function to execute later, and a value expressing how later you want it to run, in milliseconds:

setTimeout(() => {
  // runs after 2 seconds
}, 2000)

setTimeout(() => {
  // runs after 50 milliseconds
}, 50)

This syntax defines a new function. You can call whatever other function you want in there, or you can pass an existing function name, and a set of parameters:

const myFunction = (firstParam, secondParam) => {
  // do something

// runs after 2 seconds
setTimeout(myFunction, 2000, firstParam, secondParam)

setTimeout returns the timer id. This is generally not used, but you can store this id, and clear it if you want to delete this scheduled function execution:

const id = setTimeout(() => {
  // should run after 2 seconds
}, 2000)

// I changed my mind

Zero delay

If you specify the timeout delay to 0, the callback function will be executed as soon as possible, but after the current function execution:

setTimeout(() => {
  console.log( 'after ')
}, 0)

console.log(' before ')

will print before after.

This is especially useful to avoid blocking the CPU on intensive tasks, and let other functions be executed while performing a heavy calculation, by queuing functions in the scheduler.

Some browsers (IE and Edge) implement a setImmediate() method that does this same exact functionality, but it’s not standard and unavailable on other browsers. But it’s a standard function in Node.js.


setInterval is a function similar to setTimeout, with a difference: instead of running the callback function once, it will run it forever, at the specific time interval you specify (in milliseconds):

setInterval(() => {
  // runs every 2 seconds
}, 2000)

The function above runs every 2 seconds unless you tell it to stop, using clearInterval, passing it the interval id that setInterval returned:

const id = setInterval(() => {
  // runs every 2 seconds
}, 2000)


It’s common to call clearInterval inside the setInterval callback function, to let it auto-determine if it should run again or stop. For example this code runs something unless App.somethingIWait has the value arrived:

const interval = setInterval(function() {
  if (App.somethingIWait === 'arrived') {

    // otherwise do things
}, 100)

Recursive setTimeout

setInterval starts a function every n milliseconds, without any consideration about when a function finished its execution.

If a function takes always the same amount of time, it’s all fine:

setInterval working fine

Maybe the function takes different execution times, depending on network conditions for example:

setInterval varying duration

And maybe one long execution overlaps the next one:

setInterval overlapping

To avoid this, you can schedule a recursive setTimeout to be called when the callback function finishes:

const myFunction = () => {
  // do something

  setTimeout(myFunction, 1000)

}, 1000)

to achieve this scenario:

Recursive setTimeout


setTimeout and setInterval are also available in Node.js, through the Timers module.

Node.js also provides setImmediate(), which is equivalent to using setTimeout(() => {}, 0), mostly used to work with the Node.js Event Loop.