javascriptsleep

What is the JavaScript version of sleep()?


Is there a better way to engineer a sleep in JavaScript than the following pausecomp function (taken from here)?

function pausecomp(millis)
{
    var date = new Date();
    var curDate = null;
    do { curDate = new Date(); }
    while(curDate-date < millis);
}

This is not a duplicate of Sleep in JavaScript - delay between actions; I want a real sleep in the middle of a function, and not a delay before a piece of code executes.


Solution

  • 2017 — 2023 update

    Since 2009 when this question was asked, JavaScript has evolved significantly. All other answers are now obsolete or overly complicated. Here is the current best practice:

    var sleepSetTimeout_ctrl;
    
    function sleep(ms) {
        clearInterval(sleepSetTimeout_ctrl);
        return new Promise(resolve => sleepSetTimeout_ctrl = setTimeout(resolve, ms));
    }
    

    Or as a one-liner:

    await new Promise(r => setTimeout(r, 2000));
    

    As a function:

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

    or in Typescript:

    const sleep = (ms: number) => new Promise((r) => setTimeout(r, ms));
    

    use it as:

    await sleep(<duration>);
    

    Demo:

    function sleep(ms) {
        return new Promise(resolve => setTimeout(resolve, ms));
    }
    
    async function demo() {
        for (let i = 0; i < 5; i++) {
            console.log(`Waiting ${i} seconds...`);
            await sleep(i * 1000);
        }
        console.log('Done');
    }
    
    demo();

    Note that,

    1. await can only be executed in functions prefixed with the async keyword, or at the top level of your script in an increasing number of environments.
    2. await only pauses the current async function. This means it does not block the execution of the rest of the script, which is what you want in the vast majority of the cases. If you do want a blocking construct, see this answer using Atomics.wait, but note that most browsers will not allow it on the browser's main thread.

    Two new JavaScript features (as of 2017) helped write this "sleep" function:

    Compatibility

    If for some reason you're using Node older than 7 (which reached end of life in 2017), or are targeting old browsers, async/await can still be used via Babel (a tool that will transpile JavaScript + new features into plain old JavaScript), with the transform-async-to-generator plugin.