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articles/snippets/javascript-sleep.md

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+---
+title: How can I implement a sleep function in JavaScript?
+shortTitle: Sleep function
+type: question
+tags: [javascript,date,promise]
+author: chalarangelo
+cover: sleepy-cat
+excerpt: Learn all the different ways you can implement a `sleep()` function in JavaScript.
+dateModified: 2021-11-06T20:51:47+03:00
+---
+
+JavaScript does not come with a `sleep()` function out of the box. That's probably a good idea considering the environments where it runs and the trouble it could cause if used incorrectly. The closest equivalent is the `setTimeout()` function, but there are other, less common ways to implement a function that will pause execution for a specified amount of time.
+
+### setTimeout
+
+JavaScript's `setTimeout()` sets a timer which executes some code once the timer expires. Only the code inside the `setTimeout()` callback will execute after the timer expires. This can lead to nesting issues, as well as code executing out of order if you are not careful.
+
+```js
+const printNums = () => {
+  console.log(1);
+  setTimeout(() => console.log(2), 500);
+  console.log(3);
+};
+
+printNums(); // Logs: 1, 3, 2 (2 logs after 500ms)
+```
+
+### Synchronous version
+
+While strongly discouraged, `Date.prototype.getTime()` can be used inside a `while` loop to pause execution for a set amount of time. You can easily define a synchronous `sleep()` function like this:
+
+```js
+const sleepSync = (ms) => {
+  const end = new Date().getTime() + ms;
+  while (new Date().getTime() < end) { /* do nothing */ }
+}
+
+const printNums = () => {
+  console.log(1);
+  sleepSync(500);
+  console.log(2);
+  console.log(3);
+};
+
+printNums(); // Logs: 1, 2, 3 (2 and 3 log after 500ms)
+```
+
+### Asynchronous version
+
+A less intrusive way to go about implementing a `sleep()` function is to utilize the `async` and `await` keywords added in JavaScript ES6, a `Promise` and `setTimeout()`. Note that the resulting function must be executed in an `async` function and has to be called with `await`:
+
+```js
+const sleep = (ms) =>
+  new Promise(resolve => setTimeout(resolve, ms));
+
+const printNums = async() => {
+  console.log(1);
+  await sleep(500);
+  console.log(2);
+  console.log(3);
+};
+
+printNums(); // Logs: 1, 2, 3 (2 and 3 log after 500ms)
+```