The issue is that AngularJS deferred.reject function is not functioning properly, while the $q

Question

The issue is that AngularJS deferred.reject function is not functioning properly, while the $q

I'm struggling to understand the difference between Angular JS deferred and $q. I recently came across this SO Question that delves into the variance between $q.defer() and $q. According to the post:

$q.reject serves as a quick way to create a deferred object and immediately reject it.

So logically, $q.reject() should equate to

var deferred = $q.defer(); deferred.reject()

. If this is not the case, please elucidate the actual discrepancy between the two.

However, in my scenario, $q.reject() is functioning properly, while deferred.reject() is not. Additionally, it seems necessary to return a rejected promise using $q.reject() and not deferred.reject(). I've noticed instances where deferred.reject() lacks a return statement.

Here is the specific code snippet:

 var deferred = $q.defer();
 myService.getData()
 .then(function(response){
   deferred.notify('Just a notification');
   deferred.reject('rejected');
 })
 .then(function(response) {
   console.log('done');      
 }, function(response) {
   console.log('rejected');
 })

Despite the above code not achieving the desired outcome, substituting deferred.reject with $q.reject() successfully rejects the promise and directs the control flow to the error function of the subsequent then block.

Any assistance on this matter would be greatly appreciated. Thank you in advance.

javascript angularjs angular-promise

Answer 1

Answer №1

When utilizing deferred.reject, it's important to note that you must return a new rejected promise. Both $q.reject() and deferred.reject() can be used, but the key is to ensure that a promise is returned in both scenarios.

It is essential to understand that

$q.reject() represents a rejected promise object
deferred.reject() is not a promise, but a deferred object containing a rejected promise in one of its properties (specifically, $promise).

Therefore, any object or value can be returned to create a new promise object that will be passed to the subsequent then block in the chain. However, if you return deferred.reject(), it will be treated as an object (since it is not a promise, but contains a promise within), resulting in the successful resolution of the next promise.

The process will function correctly with deferred as well, provided that the corresponding promise is returned:

var deferred = $q.defer();
myService.getData()
    .then(function(response) {
        deferred.notify('Just a notification');
        deferred.reject('rejected');
        return deferred.promise;
        //return $q.reject();
    })
    .then(function(response) {
        console.log('done');
    }, function(response) {
        console.log('rejected');
    });

To address your query: $q.reject() is a promise object denoting a "rejected" status. deferred.reject(), on the other hand, is not a promise, but contains a rejected promise object within as deferred.$promise. Which should you use? Opt for $q.reject(), as using a dummy deferred object is unnecessary in this context and is deemed a flawed practice, often referred to as the deferred anti-pattern.

Answer 2

When utilizing deferred.reject, it's important to note that you must return a new rejected promise. Both $q.reject() and deferred.reject() can be used, but the key is to ensure that a promise is returned in both scenarios.

It is essential to understand that

$q.reject() represents a rejected promise object
deferred.reject() is not a promise, but a deferred object containing a rejected promise in one of its properties (specifically, $promise).

Therefore, any object or value can be returned to create a new promise object that will be passed to the subsequent then block in the chain. However, if you return deferred.reject(), it will be treated as an object (since it is not a promise, but contains a promise within), resulting in the successful resolution of the next promise.

The process will function correctly with deferred as well, provided that the corresponding promise is returned:

var deferred = $q.defer();
myService.getData()
    .then(function(response) {
        deferred.notify('Just a notification');
        deferred.reject('rejected');
        return deferred.promise;
        //return $q.reject();
    })
    .then(function(response) {
        console.log('done');
    }, function(response) {
        console.log('rejected');
    });

To address your query: $q.reject() is a promise object denoting a "rejected" status. deferred.reject(), on the other hand, is not a promise, but contains a rejected promise object within as deferred.$promise. Which should you use? Opt for $q.reject(), as using a dummy deferred object is unnecessary in this context and is deemed a flawed practice, often referred to as the deferred anti-pattern.

Answer 3

Answer №2

Always remember to ensure that your function returns a promise for asynchronous tasks.

function fetchData() {
    var deferredData = $q.defer();
    myService.getData()
        .then(function (response) {
           deferredData.resolve('Notification successfully received');
        }).catch(function (err) {
           deferredData.reject(err); 
        };

    return deferredData.promise;
}

fetchData().then(function (response) {
    console.log('Task completed');
}, function (response) {
    console.log('Task rejected');
});

Answer 4

Always remember to ensure that your function returns a promise for asynchronous tasks.

function fetchData() {
    var deferredData = $q.defer();
    myService.getData()
        .then(function (response) {
           deferredData.resolve('Notification successfully received');
        }).catch(function (err) {
           deferredData.reject(err); 
        };

    return deferredData.promise;
}

fetchData().then(function (response) {
    console.log('Task completed');
}, function (response) {
    console.log('Task rejected');
});

Answer 5

Answer №3

Utilizing Q library for asynchronous operations (https://github.com/kriskowal/q)

var def = Q.defer();
def.promise
.then(
    function(success){
        return success;
    },
    function(error){
        var d = Q.defer();
        d.reject(error);
        return d.promise;
    }
)
.then(
    function(success){
        console.log('success', success);
    },
    function(error){
        console.log('error', error);
    }
);
def.reject('custom error');

Answer 6

Utilizing Q library for asynchronous operations (https://github.com/kriskowal/q)

var def = Q.defer();
def.promise
.then(
    function(success){
        return success;
    },
    function(error){
        var d = Q.defer();
        d.reject(error);
        return d.promise;
    }
)
.then(
    function(success){
        console.log('success', success);
    },
    function(error){
        console.log('error', error);
    }
);
def.reject('custom error');

The issue is that AngularJS deferred.reject function is not functioning properly, while the $q

Answer №1

Answer №2

Answer №3

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