65 lines
2.4 KiB
Markdown
65 lines
2.4 KiB
Markdown
---
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title: How can I implement a sleep function in JavaScript?
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type: question
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tags: javascript,date,promise
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authors: chalarangelo
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cover: blog_images/sleepy-cat.jpg
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excerpt: Learn all the different ways you can implement a `sleep()` function in JavaScript.
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---
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JavaScript does not come with a `sleep()` function out of the box and that is probably a good idea considering the environments where it runs and the trouble such a function could cause if used incorrectly. The closest equivalent of such a function is `setTimeout`, however there are other, less common ways to implement a function that will freeze the current thread for a specified amount of time.
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### setTimeout
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JavaScript's `setTimeout` sets a timer which executes a function or specified piece of code once the timer expires. Only the code inside the `setTimeout` callback will execute after the timer expires, which can lead to nesting issues, as well as code executing out of order if you are not careful.
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```js
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const printNums = () => {
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console.log(1);
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setTimeout(() => console.log(2), 500);
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console.log(3);
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};
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printNums(); // Logs: 1, 3, 2 (2 logs after 500ms)
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```
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### Synchronous version
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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:
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```js
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const sleepSync = (ms) => {
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const end = new Date().getTime() + ms;
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while (new Date().getTime() < end) { /* do nothing */ }
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}
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const printNums = () => {
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console.log(1);
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sleepSync(500);
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console.log(2);
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console.log(3);
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};
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printNums(); // Logs: 1, 2, 3 (2 and 3 log after 500ms)
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```
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### Asynchronous version
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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`:
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```js
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const sleep = (ms) =>
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new Promise(resolve => setTimeout(resolve, ms));
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const printNums = async() => {
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console.log(1);
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await sleep(500);
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console.log(2);
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console.log(3);
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};
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printNums(); // Logs: 1, 2, 3 (2 and 3 log after 500ms)
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```
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**Image credit:** [Kate Stone Matheson](https://unsplash.com/@kstonematheson?utm_source=unsplash&utm_medium=referral&utm_content=creditCopyText) on [Unsplash](https://unsplash.com?utm_source=unsplash&utm_medium=referral&utm_content=creditCopyText)
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