Make a JavaScript Function Sleep

Oct 14, 2019

Unlike many other languages, JavaScript doesn't have a built-in function to pause execution for a given period of time. There's a good reason for this: in languages like C, sleep(2) blocks the current thread for 2 seconds. Since JavaScript is single-threaded, C-style sleep(2) would block the entire JavaScript runtime for 2 seconds, and that isn't very useful.

However, you can pause an async function call without blocking the rest of the JavaScript runtime. You just need to await on a promise that resolves after a given period of time.

function sleep(ms) {
  return new Promise(resolve => setTimeout(resolve, ms));

async function run() {
  // Pause execution of this async function for 2 seconds
  await sleep(2000);

  console.log('Waited for 2 seconds');

The new Promise() call uses JavaScript's promise constructor to create a new promise that resolves after ms milliseconds. And when you await on a promise, you pause execution of that function until the promise resolves, without impacting other functions.

For example, you can call two functions in parallel that call sleep(), and neither function will block the other.

const start =;

async function pauseMe() {
  await sleep(2000);
  console.log('MS since start:', - start);

// Will print something like "MS since start: 2006"
// Will print something like "MS since start: 2010"

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