Promise.all() 方法回傳一個 Promise 物件,當引數 iterable 中所有的 promises 都被實現(resolved),或引數 iterable 不含任何 promise 時,被實現。或以第一個被拒絕的 promise 的原因被拒絕。

語法

Promise.all(iterable);
iterable
一個 iterable 物件像是 Array 或 String

回傳值

  • 一個已被實現(already resolved)的 Promise,若傳入的 iterable 為空。
  • 一個非同步地被實現(asynchronously resolved)的 Promise 若傳入的 iterable 不含 promise。注意,Google Chrome 58 對此情形回傳一個已被解決的 promise。
  • 一個擱置(pending)的 Promise,對所有剩餘情形。此 promise 接著被非同步地被 resolved/rejected(只要堆疊為空)當 iterable 中所有的 promises 都被實現,或其中一個被拒絕。參見下方關於"Promise.all 的非同步與同步性質"的例子。

描述

此方法在聚集(aggregating)多個 promises 的結果時很有幫助。

實現(Fulfillment):
若傳入空的 iterable,此方法(同步地)回傳一個已被解決的 promise。若所有傳入的 promises 都被實現,或都不是 promise,Promise.all 回傳的 promise 被非同步地實現。無論是哪個情形,回傳一個以 iterable 其內所有值(包含非 promise 值)作為引數的陣列被實現。
 

拒絕(Rejection):
若任一個傳入的 promise 被拒絕,Promise.all 非同步地以其值被拒絕,無論其他 promises 是否被解決。

範例

使用 Promise.all

Promise.all 等到全部實現(或一個拒絕)。

var p1 = Promise.resolve(3);
var p2 = 1337;
var p3 = new Promise((resolve, reject) => {
  setTimeout(resolve, 100, 'foo');
}); 

Promise.all([p1, p2, p3]).then(values => { 
  console.log(values); // [3, 1337, "foo"] 
});

若 iterable 含非 promise 值,它們將被忽略,但依然會被記入回傳 promise 陣列值(若被實現):

// this will be counted as if the iterable passed is empty, so it gets fulfilled
var p = Promise.all([1,2,3]);
// this will be counted as if the iterable passed contains only the resolved promise with value "444", so it gets fulfilled
var p2 = Promise.all([1,2,3, Promise.resolve(444)]);
// this will be counted as if the iterable passed contains only the rejected promise with value "555", so it gets rejected
var p3 = Promise.all([1,2,3, Promise.reject(555)]);

// using setTimeout we can execute code after the stack is empty
setTimeout(function(){
    console.log(p);
    console.log(p2);
    console.log(p3);
});

// logs
// Promise { <state>: "fulfilled", <value>: Array[3] }
// Promise { <state>: "fulfilled", <value>: Array[4] }
// Promise { <state>: "rejected", <reason>: 555 }

Promise.all 的非同步與同步性質

以下例子驗證了 Promise.all 的非同步性質(asynchronicity)(或同步性質(synchronicity),若傳入的 iterable 是空的):

// we are passing as argument an array of promises that are already resolved,
// to trigger Promise.all as soon as possible
var resolvedPromisesArray = [Promise.resolve(33), Promise.resolve(44)];

var p = Promise.all(resolvedPromisesArray);
// immediately logging the value of p
console.log(p);

// using setTimeout we can execute code after the stack is empty
setTimeout(function(){
    console.log('the stack is now empty');
    console.log(p);
});

// logs, in order:
// Promise { <state>: "pending" } 
// the stack is now empty
// Promise { <state>: "fulfilled", <value>: Array[2] }

Promise.all 被拒絕時發生一樣的事情:

var mixedPromisesArray = [Promise.resolve(33), Promise.reject(44)];
var p = Promise.all(mixedPromisesArray);
console.log(p);
setTimeout(function(){
    console.log('the stack is now empty');
    console.log(p);
});

// logs
// Promise { <state>: "pending" } 
// the stack is now empty
// Promise { <state>: "rejected", <reason>: 44 }

注意!Promise.all 同步地被解決若且唯若傳入的 iterable 為空:

var p = Promise.all([]); // will be immediately resolved
var p2 = Promise.all([1337, "hi"]); // non-promise values will be ignored, but the evaluation will be done asynchronously
console.log(p);
console.log(p2)
setTimeout(function(){
    console.log('the stack is now empty');
    console.log(p2);
});

// logs
// Promise { <state>: "fulfilled", <value>: Array[0] }
// Promise { <state>: "pending" }
// the stack is now empty
// Promise { <state>: "fulfilled", <value>: Array[2] }

Promise.all 的失敗優先(fail-fast)行為

當任一個陣列成員被拒絕則 Promise.all 被拒絕。例如,若傳入四個將在一段時間後被解決的 promises,而其中一個立刻被拒絕,則 Promise.all 將立刻被拒絕。

var p1 = new Promise((resolve, reject) => { 
  setTimeout(resolve, 1000, 'one'); 
}); 
var p2 = new Promise((resolve, reject) => { 
  setTimeout(resolve, 2000, 'two'); 
});
var p3 = new Promise((resolve, reject) => {
  setTimeout(resolve, 3000, 'three');
});
var p4 = new Promise((resolve, reject) => {
  setTimeout(resolve, 4000, 'four');
});
var p5 = new Promise((resolve, reject) => {
  reject('reject');
});

Promise.all([p1, p2, p3, p4, p5]).then(values => { 
  console.log(values);
}, reason => {
  console.log(reason)
});

//From console:
//"reject"

//You can also use .catch
Promise.all([p1, p2, p3, p4, p5]).then(values => { 
  console.log(values);
}).catch(reason => { 
  console.log(reason)
});

//From console: 
//"reject"

規範

Specification Status Comment
ECMAScript 2015 (6th Edition, ECMA-262)
The definition of 'Promise.all' in that specification.
Standard Initial definition in an ECMA standard.
ECMAScript Latest Draft (ECMA-262)
The definition of 'Promise.all' in that specification.
Draft  

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