了解promise.race（）用法 [英] Understanding promise.race() usage

问题描述

据我所知， 承诺 ：

• promise.all （）

promise.race（）

好的，我知道 promise.all（）是什么。它并行运行promises，并且 .then 为两个值提供成功解析的值。这是一个例子：

Ok, I know what promise.all() does. It runs promises in parallel, and .then gives you the values if both resolved successfully. Here is an example:

Promise.all([
  $.ajax({ url: 'test1.php' }),
  $.ajax({ url: 'test2.php' })
])
.then(([res1, res2]) => {
  // Both requests resolved
})
.catch(error => {
  // Something went wrong
});

但我不明白 promise.race（）应该做的确切吗？换句话说，不使用它有什么区别？假设：

But I don't understand what does promise.race() is supposed to do exactly? In other word, what's the difference with not using it? Assume this:

$.ajax({
    url: 'test1.php',
    async: true,
    success: function (data) {
        // This request resolved
    }
});

$.ajax({
    url: 'test2.php',
    async: true,
    success: function (data) {
        // This request resolved
    }
});

看？我没有使用 promise.race（），它的行为类似于 promise.race（）。无论如何，是否有任何简单而干净的例子告诉我何时应该使用 promise.race（）？

See? I haven't used promise.race() and it behaves like promise.race(). Anyway, is there any simple and clean example to show me when exactly should I use promise.race() ?

推荐答案

如您所见， race（）将返回首先解决或拒绝的promise实例：

As you see, the race() will return the promise instance which is firstly resolved or rejected:

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
});

对于要使用的场景，您可能希望限制请求的成本时间：

For a scenes to be used， maybe you want to limit the cost time of a request ：

var p = Promise.race([
    fetch('/resource-that-may-take-a-while'),
    new Promise(function (resolve, reject) {
         setTimeout(() => reject(new Error('request timeout')), 5000)
    })
])
p.then(response => console.log(response))
p.catch(error => console.log(error))

使用 race（）你只需要获得返回的承诺，你不必关心 race（[]）首先返回，

With the race() you just need to get the returned promise, you needn't care about which one of the promises in the race([]) firstly returned,

然而，如果没有种族，就像你的例子一样，你需要关心哪一个将首先返回，并在 success 回调中调用回调。

However, without the race, just like your example, you need to care about which one will firstly returned, and called the callback in the both success callback.

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