Promise.all()

Promise.all(iterable) 方法返回一个 Promise 实例,此实例在 iterable 参数内所有的 promise 都“完成(resolved)”或参数中不包含 promise 时回调完成(resolve);如果参数中  promise 有一个失败(rejected),此实例回调失败(reject),失败原因的是第一个失败 promise 的结果。

语法

Promise.all(iterable);

参数

iterable
一个 可迭代对象,如  Array 或  String

返回值

  • 如果传入的参数是一个空的可迭代对象,则返回一个已完成(already resolved)状态的 Promise
  • 如果传入的参数不包含任何 promise,则返回一个异步完成(asynchronously resolved) Promise
  • 其它情况下返回一个处理中(pending)Promise。这个返回的 promise 之后会在所有的 promise 都完成或有一个 promise 失败时异步地变为完成或失败。 见下方关于“Promise.all 的异步或同步”示例。返回值将会按照参数内的 promise 顺序排列,而不是由调用 promise 的完成顺序决定。

说明

此方法在集合多个 promise 的返回结果时很有用。

完成(Fulfillment):
如果传入的可迭代对象为空,Promise.all 会同步地返回一个已完成(resolved)状态的promise
如果所有传入的 promise 都变为完成状态,或者传入的可迭代对象内没有 promisePromise.all 返回的 promise 异步地变为完成。
在任何情况下,Promise.all 返回的 promise 的完成状态的结果都是一个数组,它包含所有的传入迭代参数对象的值(也包括非 promise 值)。

失败/拒绝(Rejection):
如果传入的 promise 中有一个失败(rejected),Promise.all 异步地将失败的那个结果给失败状态的回调函数,而不管其它 promise 是否完成。

示例

Promise.all 的使用

Promise.all 等待所有都完成(或第一个失败)。

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"] 
});

如果参数中包含非 promise 值,这些值将被忽略,但仍然会被放在返回数组中(如果 promise 完成的话):

// 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 的异步和同步

下面的例子中演示了 Promise.all 的异步性(如果传入的可迭代对象是空的,就是同步):

// 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] }

如果 Promise.all 失败,也是一样的:

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 }

但是,Promise.all 当且仅当传入的可迭代对象为空时为同步:

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 的快速返回失败行为

Promise.all 在任意一个传入的 promise 失败时返回失败。例如,如果你传入的 promise中,有四个 promise 在一定的时间之后调用成功函数,有一个立即调用失败函数,那么 Promise.all 将立即变为失败。

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"

技术规范

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

浏览器兼容性

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Chrome Edge Firefox Internet Explorer Opera Safari Android webview Chrome for Android Firefox for Android Opera for Android Safari on iOS Samsung Internet Node.js
all() Chrome Full support 32 Edge Full support 12 Firefox Full support 29 IE No support No Opera Full support 19 Safari Full support 8 WebView Android Full support 4.4.3 Chrome Android Full support 32 Firefox Android Full support 29 Opera Android Full support Yes Safari iOS Full support 8 Samsung Internet Android Full support 2.0 nodejs Full support 0.12

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参见