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。
The source for this interactive demo is stored in a GitHub repository. If you'd like to contribute to the interactive demo project, please clone https://github.com/mdn/interactive-examples and send us a pull request.
语法
Promise.all(iterable);
参数
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)的
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 promise
,Promise.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
To contribute to this compatibility data, please write a pull request against this repository: https://github.com/mdn/browser-compat-data.
Desktop | Mobile | Server | |||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|---|
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 |
Legend
- Full support
- Full support
- No support
- No support