JavaScript - sleep function

In this short article, we're going to have a look at how in JavaScript create sleep(ms) function.

Quick solution:

1. create new function called sleep,
2. create inside and return Promise object,
3. resolve Promise with setTimeout after indicated time,
4. call the sleep function with await before inside other async function.

Code:

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

// await sleep(1000); // <------ calling inside async function

Node: async/await operators were introduced in ES8 (ES2017). Use Babel or TypeScript to provide this functionality in older browsers.

Explaination

There is no available sleep function in JavaScript, but is is possible to emulate sleeping with acync functions. The main idea is to postpone resolve function call with setTimeout wrapped in Promise object. Later that create logic can be placed inside own sleep function and called with await keyword inside async method only - it is main disadvantage/andvantage of the approach. Advantage becasue our logic will not freeze web browser like infinity loop does.

More complicated runnable example code below:

// ONLINE-RUNNER:browser;

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

// Usage example:

const action = async () => {
    console.log('[START]');
    for (let i = 0; i < 5; ++i) {
        await sleep(1000); // sleeps 1s each iteration
        console.log(i);
    }
    console.log('[STOP]');
};
action();