JavaScript, async and await keywords

Magus

42.5K views

01 Welcome

Previous: Limits and traps Next: Time to pratice

Call multiple functions with `await` in parallel

If you try this code :

Sequential multiple awaits

const start = Date.now();
function timeLog(text) {
    console.log(`${Date.now() - start}ms - ${text}`);
}
function job() {
    return new Promise(function(resolve, reject) {
        timeLog('Job start');
        setTimeout(function() {
            timeLog('Job done');
            resolve('Hello world');
        }, 500);
    });
}
async function main() {
    let message1 = await job(),
        message2 = await job(),
        message3 = await job();
    timeLog(message1);
    timeLog(message2);
    timeLog(message3);
}
main();

You can see that the job function is called sequentially. But in such code, you probably want to make the 3 calls in parallel. You have to use your old friend Promise.all

Parallel multiple awaits

const start = Date.now();
function timeLog(text) {
    console.log(`${Date.now() - start}ms - ${text}`);
}
function job() {
    return new Promise(function(resolve, reject) {
        timeLog('Job start');
        setTimeout(function() {
            timeLog('Job done');
            resolve('Hello world');
        }, 500);
    });
}
async function main() {
    let messages = await Promise.all([job(), job(), job()]);
    messages.forEach(function(message) {
        timeLog(message);
    });
}
main();