$ q.reject和AngularJS链接承诺中的处理错误

Question

I'm having trouble understanding a basic concept of error handling with chaining promises. In order to learn the rules, I have written a simple example, guessing what the result will be. But unfortunatly it doesn't behave as I though it will. I have read multiple articles about the subject but perhaps can't I get details because of my poor english language.

Anyway, here is my code :

    var promiseStart = $q.when("start");
    var promise1 = promiseStart.then(function() {
            return Serviceforpromise1.get();
    });
    var promise2 = promise1.then(function(data1)
    {
            return Serviceforpromise2.get(data1);
    },function(error)
    {
            return $q.reject();
    });
    var promiseend = promise2.then(function(data2)
    {
            return data2;
    },function(error)
    {
            return error;
    });
    return promiseend;

Well I know that it can be way better coded but it's just for the purpose. Here is the code of Serviceforpromise1 function :

    function Serviceforpromise1()
    {
            ...
            return $http.get(*whatever*).then(function (data){
                return data;
            },function(error)
            {
                return $q.reject();
            });
    }

Consider only the case of Serviceforpromise1's failure. A $q.rejec t is sent back to main chain so I'm waiting the error callback of " promise1 .then( " to be called and it worked as expected. I decided for the example to transfert the error to the " promise2 .then " so in this error callback I added the line return $q.reject() ; But it never reached the second error callback (the " promise2 .then " one) and I don't understand why (like Serviceforpromise1, I returned a rejected promise !)

I will be happy to deeply understand what is happening here. Thanks for your help.

Answer 1

Your understanding is correct, and the problem appears to lie somewhere in the way you are trying to observe this behavior (in something you haven't shown us).

If you return a rejected promise from either a success or error handler in then() , then the promise returned by then() will resolve to a rejected promise. Observe:

 angular.module('app', []) .controller('C', [ '$q', function ($q) { var promiseStart = $q.when("start"); var promise1 = promiseStart.then(function (value) { console.log('Got a value:', value); return $q.reject('Error!'); }); var promise2 = promise1.then(function (data1) { return "Got some stuff"; }, function (error) { console.log("Caught an error:", error); return $q.reject('New error'); }); var promiseend = promise2.then(function (data2) { return data2; }, function (error) { console.log('Caught an error:', error); // <-- this is logged to the console return error; }); return promiseend; }]); 

 <script src="https://ajax.googleapis.com/ajax/libs/angularjs/1.2.10/angular.min.js"></script> <div ng-app='app' ng-controller='C'></div> 

One thing to note here is that in that last handler, you are returning the error variable, and not throwing an exception or returning a rejected promise. So in this case, promiseend will successfully resolve with the value of that error variable.