# JavaScript Sleep Function With Promises and Async Await

> Create a JavaScript sleep function using a Promise constructor and setTimeout, then pause execution with async/await in place of a built-in delay

**URL:** https://sentry.io/answers/what-is-the-javascript-version-of-sleep/

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## The Problem

You may be familiar with another language, such as Python, that has a `sleep()` function. The Python `time` module has a built-in `sleep()` method that you can use to delay the execution of a program. The time delay, in seconds, is passed in as an argument:

```python
import time
time.sleep(delayInSeconds)
```

What is the JavaScript version of `sleep()`?

## The Solution

To replicate the Python `sleep()` method, you can make a JavaScript `sleep` function using a [`Promise()` constructor](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Promise/Promise) and [`setTimeout`](https://developer.mozilla.org/en-US/docs/Web/API/setTimeout):

```javascript
function sleep(ms = 0) {
  return new Promise(resolve => setTimeout(resolve, ms));
}
```

You could also write it on one line using an [arrow function](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Functions/Arrow_functions):

```javascript
const sleep = (ms = 0) => new Promise(resolve => setTimeout(resolve, ms));
```

You can then use the `sleep` function to delay the execution of a program. For example, the `delayedGreeting` function as shown in the code below is paused for five seconds:

```javascript
async function delayedGreeting() {
  await sleep(5000);
  console.log("hello");
}

delayedGreeting();
```

The `sleep` function takes in a time in milliseconds as an argument. It's set to 0 as a default here. It returns a [`Promise`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Promise), which is an object that represents the eventual completion or failure of an asynchronous (async) operation.

A promise has three possible states:

- *Pending*: The object's initial state, neither fulfilled nor rejected.

- *Fulfilled*: The operation was completed successfully.

- *Rejected*: The operation failed.

You can attach callbacks to an async function that returns a promise to handle the two possible outcomes:

```javascript
asyncFunctionThatReturnsPromise(options).then(successCallback, failureCallback);
```

If the promise is fulfilled, `successCallback` will be called. If it is rejected, `failureCallback` will be called.

Promises allow async methods to return values like synchronous methods. The async method returns a promise to give the value later. You can use the [`await`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/await) operator to wait for the promise to be resolved.

```javascript
const value = await asyncFunctionThatReturnsPromise();
```

The `Promise()` constructor is used to wrap around callback functions that don't support promises, such as `setTimeout`. It allows us to resolve or reject a promise manually, which means we can use `setTimeout` to delay the execution of a function. The `sleep` function's returned promise is resolved using the [`resolve`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Promise/resolve) method after the `setTimeout` timer expires. This occurs after the successful completion of the `setTimeout` function. The value of the resolved promise is `undefined`; the promise is used to pause execution so it does not need a returned value. The promise does not include a [`reject`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Promise/reject) as `setTimeout` does not really fail.

When using the `sleep` function to pause execution, the [`await`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/await) operator is used to wait for the returned promise in the `sleep` function to be resolved. It can only be used in an async function or at the top level of a module.

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*Source: [sentry.io/answers/what-is-the-javascript-version-of-sleep/](https://sentry.io/answers/what-is-the-javascript-version-of-sleep/)*