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• 句法
• 描述
• 范例
• 规范
• 浏览器兼容性
• 另请参阅
• 相关话题

Promise.race() method returns a promise that fulfills or rejects as soon as one of the promises in an iterable fulfills or rejects, with the value or reason from that promise.

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.

句法

Promise.race(iterable);
					

参数

iterable

An iterable object, such as an Array 。见 iterable .

返回值

A pending Promise that asynchronously yields the value of the first promise in the given iterable to fulfill or reject.

描述

race function returns a Promise that is settled the same way (and takes the same value) as the first promise that settles amongst the promises of the iterable passed as an argument.

If the iterable passed is empty, the promise returned will be forever pending.

If the iterable contains one or more non-promise value and/or an already settled promise, then Promise.race will resolve to the first of these values found in the iterable.

范例

Asynchronicity of Promise.race

This following example demonstrates the asynchronicity of Promise.race :

// 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 }
					

An empty iterable causes the returned promise to be forever pending:

var foreverPendingPromise = Promise.race([]);
console.log(foreverPendingPromise);
setTimeout(function(){
    console.log('the stack is now empty');
    console.log(foreverPendingPromise);
});
// logs, in order:
// Promise { <state>: "pending" }
// the stack is now empty
// Promise { <state>: "pending" }
					

If the iterable contains one or more non-promise value and/or an already settled promise, then Promise.race will resolve to the first of these values found in the array:

var foreverPendingPromise = Promise.race([]);
var alreadyFulfilledProm = Promise.resolve(100);
var arr = [foreverPendingPromise, alreadyFulfilledProm, "non-Promise value"];
var arr2 = [foreverPendingPromise, "non-Promise value", Promise.resolve(100)];
var p = Promise.race(arr);
var p2 = Promise.race(arr2);
console.log(p);
console.log(p2);
setTimeout(function(){
    console.log('the stack is now empty');
    console.log(p);
    console.log(p2);
});
// logs, in order:
// Promise { <state>: "pending" }
// Promise { <state>: "pending" }
// the stack is now empty
// Promise { <state>: "fulfilled", <value>: 100 }
// Promise { <state>: "fulfilled", <value>: "non-Promise value" }
					

Using Promise.race – examples with setTimeout

var p1 = new Promise(function(resolve, reject) {
    setTimeout(() => resolve('one'), 500);
});
var p2 = new Promise(function(resolve, reject) {
    setTimeout(() => resolve('two'), 100);
});
Promise.race([p1, p2])
.then(function(value) {
  console.log(value); // "two"
  // Both fulfill, but p2 is faster
});
var p3 = new Promise(function(resolve, reject) {
    setTimeout(() => resolve('three'), 100);
});
var p4 = new Promise(function(resolve, reject) {
    setTimeout(() => reject(new Error('four')), 500);
});
Promise.race([p3, p4])
.then(function(value) {
  console.log(value); // "three"
  // p3 is faster, so it fulfills
}, function(reason) {
  // Not called
});
var p5 = new Promise(function(resolve, reject) {
    setTimeout(() => resolve('five'), 500);
});
var p6 = new Promise(function(resolve, reject) {
    setTimeout(() => reject(new Error('six')), 100);
});
Promise.race([p5, p6])
.then(function(value) {
  // Not called
}, function(error) {
  console.log(error.message); // "six"
  // p6 is faster, so it rejects
});
					

规范

规范
ECMAScript (ECMA-262) The definition of 'Promise.race' in that specification.

浏览器兼容性

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. 更新 GitHub 上的兼容性数据

	Desktop						Mobile						Server
	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
race()	Chrome 32	Edge 12	Firefox 29	IE No	Opera 19	Safari 8	WebView Android 4.4.3	Chrome Android 32	Firefox Android 29	Opera Android 19	Safari iOS 8	Samsung Internet Android 2.0	nodejs 0.12

图例

完整支持

完整支持

不支持

不支持

另请参阅

• Promise
• Promise.all()

元数据

• 最后修改： Jun 15, 2020

相关话题

1. 标准内置对象
2. Promise
3. 方法
   1. Promise.all()
   2. Promise.allSettled()
   3. Promise.any()
   4. Promise.prototype.catch()
   5. Promise.prototype.finally()
   6. Promise.prototype.then()
   7. Promise.race()
   8. Promise.reject()
   9. Promise.resolve()
4. 继承：
5. Function
6. 特性
   1. Function.arguments
   2. Function.caller
   3. Function.displayName
   4. Function.length
   5. Function.name
7. 方法
   1. Function.prototype.apply()
   2. Function.prototype.bind()
   3. Function.prototype.call()
   4. Function.prototype.toSource()
   5. Function.prototype.toString()
8. Object
9. 特性
   1. Object.prototype.__proto__
   2. Object.prototype.constructor
10. 方法
    1. Object.prototype.__defineGetter__()
    2. Object.prototype.__defineSetter__()
    3. Object.prototype.__lookupGetter__()
    4. Object.prototype.__lookupSetter__()
    5. Object.prototype.hasOwnProperty()
    6. Object.prototype.isPrototypeOf()
    7. Object.prototype.propertyIsEnumerable()
    8. Object.prototype.toLocaleString()
    9. Object.prototype.toSource()
    10. Object.prototype.toString()
    11. Object.prototype.valueOf()
    12. Object.setPrototypeOf()