Promise.race()

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

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.

相关链接

• Promise
• Promise.all()