Promise.race
Promise.race(iterable)方法返回一个?promise，并伴随着 promise对象解决的返回值或拒绝的错误原因, 只要 iterable 中有一个 promise 对象"解决(resolve)"或"拒绝???????(reject)"。
语法
Promise.race(iterable);参数
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
下面的这个例子演示了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 }一个空的迭代器会导致返回的promise永远等待：
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" }如果迭代包含一个或多个非承诺值和/已解决/reject的promise，Promise.race则将解析为在数组中找到的第一个值：
var foreverPendingPromise = Promise.race([]);
var alreadyResolvedProm = Promise.resolve(666);
var arr = [foreverPendingPromise, alreadyResolvedProm, "non-Promise value"];
var arr2 = [foreverPendingPromise, "non-Promise value", Promise.resolve(666)];
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>: 666 }
// Promise { <state>: "fulfilled", <value>: "non-Promise value" }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"
// Both resolve, but p2 is faster
});
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 is faster, so it resolves
}, function(reason) {
// Not called
});
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) {
// Not called
}, function(reason) {
console.log(reason); // "six"
// p6 is faster, so it rejects
});规范
Specification | Status | Comment |
|---|---|---|
ECMAScript 2015 (6th Edition, ECMA-262)The definition of 'Promise.race' in that specification. | Standard | Initial definition in an ECMA standard. |
ECMAScript Latest Draft (ECMA-262)The definition of 'Promise.race' in that specification. | Living Standard | ? |
浏览器兼容性
Feature | Chrome | Edge | Firefox | Internet Explorer | Opera | Safari |
|---|---|---|---|---|---|---|
Basic Support | 32.0 | (Yes) | 29.0 | No | 19 | 7.1 |
Feature | Android | Chrome for Android | Edge mobile | Firefox for Android | IE mobile | Opera Android | iOS Safari |
|---|---|---|---|---|---|---|---|
Basic Support | 4.4.4 | 32.0 | (Yes) | 29 | No | (Yes) | 8.0 |
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