Promise.all

Promise.all

Promise.all() 方法返回一个Promise, 在可迭代(iterable)参数中所有的 promises 都已经解决了或者当iterable参数不包含 promise 时, 返回解决。或者返回拒绝, 当传递的 promise 包含一个拒绝(reject)时。

语法

Promise.all(iterable

迭代（iterable）一个可迭代的对象（如ArrayorString）。

返回值

  • An already resolved Promise 如果传递空的可迭代对象

  • An asynchronously resolved Promise 如果传递可迭代对象中不包含Promise. 比如, Google Chrome 58 返回一个 already resolved promise .

  • A pending Promise 在其他范例中. 返回的 promise 将会 处于异步等待 resolved/rejected的状态 (as soon as the stack is empty) 当可迭代对象中包含的所有Promise被resolved时, 或者其中任何一个Promise被reject时, 异步等待结束. See the example about "Asynchronicity or synchronicity of Promise.all" below.

描述

这个方法可以用来汇总多个promise的结果。

执行结果：

如果一个空的iterable被传递，那么这个方法返回（同步）一个已经解决的promise。

Promise.all()会异步执行,它的返回值是所有已经执行完毕的Promise,还有尚未执行的Promise.

在所有情况下，返回的promise都是通过一个数组实现的，该数组包含作为参数传递的所有 iterable的值（也是非promise值）。

拒绝:

如果任何Promise被reject，Promise.all会拒绝那些被拒绝的Promise，不论其他promise是否已经解决。

示例

使用Promise.all

Promise.all 等待所有代码的完成（或第一个代码的失败）。

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iterable包含非promise值，则它们将被忽略，但仍然计入返回的promise数组值（如果promise已满足）：

// 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的同步和异步

下面的示例展示了Promise.all的异步(当传递的可迭代对象是空时，是同步的):

// 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] }

如果Promise.all reject时会出现同样的结果：

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，当且只当传递的可迭代对象为空时，Promise.all会变成同步执行

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 快速失败行为

如果任意一个元素是rejected，那么Promise.all就是rejected。例如，如果你传递了五个参数，其中四个是经过一定时间间隔后执行resolve，另外一个立刻执行reject，那么Promise.all将会立刻执行reject。

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"

规范

SpecificationStatusComment
ECMAScript 2015 (6th Edition, ECMA-262)The definition of 'Promise.all' in that specification.StandardInitial definition in an ECMA standard.
ECMAScript Latest Draft (ECMA-262)The definition of 'Promise.all' in that specification.Draft​

浏览器兼容性

FeatureChromeEdgeFirefoxInternet ExplorerOperaSafari
Basic Support32.0(Yes)29.0No197.1

FeatureAndroidChrome for AndroidEdge mobileFirefox for AndroidIE mobileOpera AndroidiOS Safari
Basic Support4.4.432.0(Yes)29No(Yes)8.0