🔰Promises in parallel

JS ⟩ async ⟩ Promise ⟩ in parallel

Promise.all()

will reject immediately upon any of the input promises rejecting.

Promise.allSettled() (⭐️ ES2020)

will wait for all input promises to complete, regardless of whether or not one rejects.
will always return the final result of every promise and function from the input iterable. (order is preserved)

// rejected if any of the input Promises are rejected,
// otherwise, fulfilled with an array of values of each Promise.
Promise.all(
    // array of URLs -> array of Promises
    urls.map(url => fetch(url).then(response => response.text())
).then(
    texts => handle(texts)    // do something with array of strings
)

💾 replit：Promise.all()

// custom Promise
const promiseFoo = new Promise((resolve, reject) => {
    setTimeout(resolve, 2000, 'foo');
    //                        ╰───╯ <---- parameter to `resolve`
});

// ⭐ Promise.all()
// - rejected if any is rejected
// - fulfilled (with array of values) if all are fulfilled
Promise.all([
    Promise.resolve(3),
    42,                     // non-Promise value is OK
    promiseFoo
]).then(values => {
    console.log(values);    // [ 3, 42, 'foo' ]
});

// ⭐ Promise.allSettled()
// - always fulfilled with array of outcomes (objects)
Promise.allSettled([
    Promise.resolve(1),
    Promise.reject('bad boy'),
    3,
]).then(results => {
    results.forEach(result => console.log(result));
    // { status: 'fulfilled',  value: 1         }    // ⭐ with `value`
    // { status: 'rejected' , reason: 'bad boy' }    // ⭐ with `reason`
    // { status: 'fulfilled',  value: 3         }
});

💾 replit：Promise.race()

// ⭐ Promise.race()
// - settled immediately if any is settled.
Promise.race([
    new Promise(resolve => setTimeout(resolve, 500, 'one')),
    new Promise(resolve => setTimeout(resolve, 200, 'two')),    // resolve faster
]).then(value => {
    console.log(value);    // 'two'
});

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Last updated 3 years ago