javascript 在 catch 之后执行 then
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Executing then after catch
提问by TGH
I have the following fiddle: http://jsfiddle.net/thelgevold/3uv9nnjm/6/
我有以下小提琴：http: //jsfiddle.net/thelgevold/3uv9nnjm/6/
angular.module('hello',[]).controller('helloController',function($q){
console.clear();
function someService(){
var deferred = $q.defer();
deferred.reject({e:'error'});
return deferred.promise;
}
function callService(){
return someService().then(function(obj){
console.log('first then');
}).
catch(function(e){
console.log('error1');
var deferred = $q.defer();
deferred.reject({e:'error'});
return deferred.promise;
});
}
callService().catch(function(e){
console.log('error2');
}).then(function(e){
console.log('second then');
});
});
It's essentially just a quick $q promise POC. My question is: Why does the last then clause get called when the promise is rejected? The output is as follows:
它本质上只是一个快速的 $q 承诺 POC。我的问题是：当承诺被拒绝时，为什么会调用最后一个 then 子句？输出如下：
error1
错误1
error2
错误2
second then
第二然后
I understand why error1/error2 are printed, but I thought the second then string should not be printed since the promise was rejected. I thought it would omit "second then" for the same reason the "first then" is omitted. Any thoughts?
我明白为什么会打印 error1/error2，但我认为第二个 then 字符串不应该被打印，因为 promise 被拒绝了。我认为它会省略“second then”，原因与省略“first then”的原因相同。有什么想法吗？
回答by JLRishe
Before I get started, don't do this:
在我开始之前，不要这样做：
var deferred = $q.defer();
deferred.reject({e:'error'});
return deferred.promise;
Do this:
做这个：
return $q.reject({e:'error'});
Or preferably, this:
或者最好是这样：
return $q.reject(new Error('error'));
Beware the deferred antipattern.
Now, for the answer to your question.
现在，回答你的问题。
将
.catch().catch()你的电话后，callService()callService()正赶上错误，而不是产生新的错误。它基本上“处理”了错误，并且.then().then()可以自由调用以下处理程序。The synchronous code equivalent of your example would be:
与您的示例等效的同步代码是：
function someService() {
throw { e: 'error' };
}
function callService() {
try {
var obj = someService();
console.log('first then');
} catch (e) {
console.log('error1');
throw { e: 'error' };
}
}
var e;
try {
e = callService();
} catch (e) {
console.log('error2');
}
console.log('second then');I think that if you look at it this way, it makes perfect sense.
我认为，如果你这样看，它是完全有道理的。
The relevant text in the Promises/A+ spec is here. For all intents and purposes, you can view the catchhandler as the same thing as an onRejectedhandler:
Promises/A+ 规范中的相关文本在这里。出于所有意图和目的，您可以将catch处理程序视为与处理程序相同的事物onRejected：
2.2.7. then must return a promise [3.3].
promise2 = promise1.then(onFulfilled, onRejected);2.2.7.1. If either onFulfilled or onRejected returns a value x, run the Promise Resolution Procedure [[Resolve]](promise2, x).
2.2.7. 然后必须返回一个承诺 [3.3]。
promise2 = promise1.then(onFulfilled, onRejected);2.2.7.1. 如果 onFulfilled 或 onRejected 返回值 x，则运行 Promise Resolution Procedure [[Resolve]](promise2, x)。
Basically, your onRejectedhandler is "returning" the value undefined, so the promise produced by catch()resolves with the value undefined.
基本上，您的onRejected处理程序正在“返回” value undefined，因此由catch()value产生的承诺会解析undefined。
