在 Promise 中混淆错误和拒绝 [英] Confuse about error and reject in Promise

问题描述

全部:

我对 JS Promise 还很陌生，当谈到 Promise 链时，有一个令人困惑的地方，比如我有一个像 Promise 链:

I am pretty new to JS Promise, there is one confuse when it comes to Promise chaining, say I have a promise chaining like:

var p = new Promise(function(res, rej){
})
.then(
    function(data){
    }, 
    function(err){
    })
.then(
    function(data){
    }, 
    function(err){
    })
.catch(
    function(err){
    })

让我困惑的是:

1. 函数(err)何时被调用，catch 何时被调用?
2. 如何在then中解决和拒绝?

1. When the function(err) get called and when the catch get called?
2. How to resolve and reject in then?

谢谢

推荐答案

使用 Promise 的公式是:

The formular for using a Promise is:

var p = new Promise(function(resolve, reject) {

  var condition = doSomething();

  if (condition) {
    resolve(data);
  } else {
    reject(err);
  }

});

.catch 没有什么特别的，只是.then (undefined, func) 的糖，但是.catch 更清楚传达它纯粹是一个错误处理程序.

There is nothing special about .catch, it is just sugar for .then (undefined, func), but .catch more clearly communicates that it is purely an error handler.

如果一个 Promise 没有解析，并且其中没有提供拒绝回调，它会向前跳到链中的下一个 .then ，其中包含一个拒绝回调.拒绝回调是 reject(err).

If a Promise does not resolve and no rejection callback is provided in it, it skips forward to the next .then in the chain with a rejection callback in it. The rejection callback is the reject(err).

有关更详细的说明，请参阅:Javascript Promises - 来来回回.

For more detailed explanations see: Javascript Promises - There and Back again.

也就是说:在您的示例中，只有在前面的拒绝回调中有错误时才会调用 .catch .那是 reject(err) 函数本身存在错误 - 这与前面的 Promise 未解析无关.

That is: in your example the .catch only gets called if the preceding rejection callback has an error in it. That is there is an error in the reject(err) function itself - which has nothing to do with the preceding Promise not resolving.

您基本上可以将自己限制在 .then 链末端的 .catch 中的拒绝回调.任何.then 中的任何Error 都将落入.catch.不过有一个微妙之处:.catch 中的任何错误都不会被捕获.

You can essentially limit yourself to a rejection callback in the .catch at you end of the .then chain. Any Error in any .then will then fall through to the .catch. One subtlety though: any error in the .catch is not caught.

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