JavaScript参考手册
Promise
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
iterable可迭代对象,例如: 一个Array
。详情可见 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
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 |
Promise相关
JavaScript 是一种高级编程语言,通过解释执行,是一门动态类型,面向对象(基于原型)的解释型语言。它已经由ECMA(欧洲电脑制造商协会)通过 ECMAScript 实现语言的标准化。它被世界上的绝大多数网站所使用,也被世界主流浏览器( Chrome、IE、FireFox、Safari、Opera )支持。JavaScript 是一门基于原型、函数先行的语言,是一门多范式的语言,它支持面向对象编程,命令式编程,以及函数式编程。它提供语法来操控文本、数组、日期以及正则表达式等,不支持 I/O,比如网络