🔰Promises in parallel

JS ⟩ async ⟩ Promise ⟩ in parallel

⭐️ 重點

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)

🔴 主題

• await promises (in parallel or in series)

💈範例

// 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'
});

📘 手冊

• Promise.all() - rejected immediately if any rejetcted, fulfilled if all fulfilled.

• Promise.allSettled() - a Promise that will always be fulfilled (with an array of outcome objects) when all settled.

• Promise.race() - settled immediately if any is settled.

👥 相關

• await promises