javascript - Create an AngularJS promise with no return value - Stack Overflow

I am using $q to wrap a promise around a legacy callback. However, the existing callback doesn't have a value to return. It takes a success function with no parameters.

angular.module('MyModule').service('MyService', function() {
    function initialize() {
        var deferred = $q.defer();
        LegacyFactory.initialize(
            // 'void' Success Callback
            function () {
                deferred.resolve (/* WHAT DO I PUT HERE? */);
            },
            // Error Callback
            function (errorCode) {
                deferred.reject(errorCode);
            });
        return deferred.promise;
    }
});

I can't find a void version of resolve in the AngularJS docs. I can return a dummy value, but then clients might access that dummy value, which would cause confusion.

How do I create an AngularJS promise with no return value?

NOTE: The question AngularJS promise returning empty object is pletely different. That question does return a value in the resolve function.

I am using $q to wrap a promise around a legacy callback. However, the existing callback doesn't have a value to return. It takes a success function with no parameters.

angular.module('MyModule').service('MyService', function() {
    function initialize() {
        var deferred = $q.defer();
        LegacyFactory.initialize(
            // 'void' Success Callback
            function () {
                deferred.resolve (/* WHAT DO I PUT HERE? */);
            },
            // Error Callback
            function (errorCode) {
                deferred.reject(errorCode);
            });
        return deferred.promise;
    }
});

I can't find a void version of resolve in the AngularJS docs. I can return a dummy value, but then clients might access that dummy value, which would cause confusion.

How do I create an AngularJS promise with no return value?

NOTE: The question AngularJS promise returning empty object is pletely different. That question does return a value in the resolve function.

• javascript
• angularjs
• angular-promise

Share Improve this question Follow edited May 23, 2017 at 12:24 CommunityBot 111 silver badge asked Sep 27, 2014 at 0:49 metacubedmetacubed 7,30177 gold badges4040 silver badges6666 bronze badges 6

• Just resolve with nothing.. deferred.resolve()? – PSL Commented Sep 27, 2014 at 0:52
• @PSL Is that possible? The docs don't mention it. – metacubed Commented Sep 27, 2014 at 0:54
• 1 Yes you can resolve with no value. Just give it a try and see what happens.. :) plnkr.co/edit/BpYqlg?p=preview – PSL Commented Sep 27, 2014 at 0:55
• I use deferred.resolve() all the time. – Reactgular Commented Sep 27, 2014 at 2:02
• 2 seriously people. Don't downvote this question. It's a perfectly valid question for someone who's new to JavaScript but ing from a typed language. – Reactgular Commented Sep 27, 2014 at 14:20

 |  Show 1 more ment

1 Answer 1

Sorted by: Reset to default 13

There is no void data type in JavaScript. Instead JavaScript uses the primitive type undefined that is used to represent a variable that has not been assigned a value.

A method or statement that has no return value will return undefined. A function that doesn't use the return statement will return undefined. An argument that isn't passed to a function will be undefined.

I hope you're starting to see the consistent behavior here of undefined.

function foo(x) { console.log(x); }
foo(); // will print undefined
function zoom() {}
console.log(zoom()); // will print undefined

So when you use deferred.resolve() you are passing undefined as the value for the data argument.

To more specifically answer your question.

"How do I create an AngularJS promise with no return value?"

To write JavaScript code that gives the intent of no return value for the promise. You would write this.

deferred.resolve(undefined);

That makes it clear that there is no intended data.

Later in your callback you don't have to define the data argument, but if you want you can.

 foo().then(function(data){
     if(typeof data === 'undefined') {
          // there is no data
     } else {
          // there is data
     });

if you always expect no data, then just do this.

foo().then(function(){
   // handle success
});