CODEWITHSUNDEEP
javascriptangular-promise
Devesh Agrawal
Devesh Agrawal

Reputation: 9212

Difference between $q.when and $q.defer in angularJS

I am seeing angularJS files where in some places reviewer has commented these 3 lines:

var deferred = $q.defer();
deferred.resolve(BrandList);
return deferred.promise;

and replaced with this one:

return $q.when(BrandList);

I would like to understand the difference between two. Do both serve same purpose? Which should be used when?

Upvotes: 7

Views: 4773

Answers (2)

Daniel Weiner
Daniel Weiner

Reputation: 1904

$q.defer() is appropriate when you want to decorate a function to use promises instead of callbacks. Usually, you will specify a deferred.resolve() and a deferred.reject(), and maybe even a deferred.notify().

$q.when() is appropriate when you want to immediately create a new promise and resolve it to a value. EDIT it also normalizes variables into promises, which is useful if the variable may or may not be a promise. See Jim's answer.

$q.when() seems to be appropriate for your case.

Upvotes: 1

JimTheDev
JimTheDev

Reputation: 3469

Ben Nadal does a great job of explaining this here.

In his words:

The $q.when() method doesn't just create a promise that is immediately resolved; rather, it normalizes a value that may or may not be a "thenable" object. If the given value is a promise, $q.when() will properly chain off of it. If the given value is not a promise, $q.when() will create promise resolved with the given value.

So essentially it takes either promises or non-promises and does some magic to make sure they are either resolved or promise wrapped so as not to break the promise chain.

Upvotes: 4

Related Questions