The `setTimeout()` Function in JavaScript

Jul 3, 2024

JavaScript has a built-in setTimeout() function that registers a function to run after a given amount of time has elapsed.

// First param to `setTimeout()` is a callback function
// Second param is milliseconds to wait
setTimeout(function() {
  console.log('I ran after waiting for 1 second');
}, 1000);

The setTimeout() function registers the given callback function with JavaScript's event loop, so setTimeout() won't block the currently executing function. For example, the following code will print "In setTimeout" after "After setTimeout", even though callback is scheduled to run after 0 milliseconds.

console.log('Before setTimeout');
setTimeout(function callback() {
  console.log('In setTimeout');
}, 0);
console.log('After setTimeout');

Cancelling the Timeout

There's also a global clearTimeout() function. setTimeout() returns a timer ID. Pass the timer ID to clearTimeout() to cancel the timeout.

const timeout = setTimeout(function() {
  console.log('This will not print');
}, 100);

clearTimeout(timeout); // Cancel the timeout

With Promises

setTimeout() doesn't return a promise. If you want to wait a certain amount of time before executing some code, like waiting 1 second, you should use the following pattern.

function delay(time) {
  // Wrap `setTimeout()` in a promise
  return new Promise(resolve => setTimeout(resolve, time));

delay(1000).then(() => console.log('ran after 1 second'));

Did you find this tutorial useful? Say thanks by starring our repo on GitHub!

More Fundamentals Tutorials