javascript - Difference between $q.when and $q.defer in angularJS - Stack Overflow

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I am seeing angularJS files where in some places reviewer has mented 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?

I am seeing angularJS files where in some places reviewer has mented 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?

• javascript
• angular-promise

Share Improve this question Follow edited Jul 6, 2015 at 17:14 Bergi 666k161161 gold badges1k1k silver badges1.5k1.5k bronze badges asked Jul 6, 2015 at 17:06 Devesh AgrawalDevesh Agrawal 9,2221818 gold badges8686 silver badges134134 bronze badges 3

• Yes, they do the same. But only the second conveys the intent (and it's shorter as well). – Bergi Commented Jul 6, 2015 at 17:16
• So can we use either of two at any point of time in our code?? – Devesh Agrawal Commented Jul 6, 2015 at 17:20
• Well, $q.when is a single function, and could be passed around. A call to it makes a single expression that can be used about everywhere, while the first snippet with $q.defer() contains multiple statements. – Bergi Commented Jul 6, 2015 at 17:22

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2 Answers 2

Sorted by: Reset to default 4

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.

$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.

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