`Promise.race(iterable)` 方法返回一个 promise，一旦迭代器中的某个promise解决或拒绝，返回的 promise就会解决或拒绝。

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.race(iterable);

### 参数

iterable
  可迭代对象，类似`Array`。详见 iterable。

### 返回值

一个待定的 `Promise` 只要给定的迭代中的一个promise解决或拒绝，就采用第一个promise的值作为它的值，从而异步地解析或拒绝（一旦堆栈为空）。

## 描述

`race` 函数返回一个 `Promise`，它将与第一个传递的 promise 相同的完成方式被完成。它可以是完成（ resolves），也可以是失败（rejects），这要取决于第一个完成的方式是两个中的哪个。

如果传的迭代是空的，则返回的 promise 将永远等待。

如果迭代包含一个或多个非承诺值和/或已解决/拒绝的承诺，则`Promise.race` 将解析为迭代中找到的第一个值。

## 示例

### `Promise.race`的异步性

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

    var p = Promise.race(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>: 33 }

### `使用 Promise.race` – `setTimeout 的示例`

    var p1 = new Promise(function(resolve, reject) {
        setTimeout(resolve, 500, "one");
    });
    var p2 = new Promise(function(resolve, reject) {
        setTimeout(resolve, 100, "two");
    });

    Promise.race([p1, p2]).then(function(value) {
      console.log(value); // "two"
      // 两个都完成，但 p2 更快
    });

    var p3 = new Promise(function(resolve, reject) {
        setTimeout(resolve, 100, "three");
    });
    var p4 = new Promise(function(resolve, reject) {
        setTimeout(reject, 500, "four");
    });

    Promise.race([p3, p4]).then(function(value) {
      console.log(value); // "three"
      // p3 更快，所以它完成了
    }, function(reason) {
      // 未被调用
    });

    var p5 = new Promise(function(resolve, reject) {
        setTimeout(resolve, 500, "five");
    });
    var p6 = new Promise(function(resolve, reject) {
        setTimeout(reject, 100, "six");
    });

    Promise.race([p5, p6]).then(function(value) {
      // 未被调用
    }, function(reason) {
      console.log(reason); // "six"
      // p6 更快，所以它失败了
    });

## 规范

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

## 浏览器兼容

The compatibility table in this page is generated from structured data. If you'd like to contribute to the data, please check out https://github.com/mdn/browser-compat-data and send us a pull request.

Update compatibility data on GitHub

| Desktop | Mobile | Server
| Chrome | Edge | Firefox | Internet Explorer | Opera | Safari | Android webview | Chrome for Android | Edge Mobile | Firefox for Android | Opera for Android | Safari on iOS | Samsung Internet | Node.js
`race` | 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 | Edge Mobile Full support Yes | Firefox Android Full support 29 | Opera Android Full support Yes | Safari iOS Full support 8 | Samsung Internet Android Full support Yes | nodejs Full support 0.12

### Legend

Full support
  Full support
No support
  No support

## 相关链接

- `Promise`
- `Promise.all()`