Unable to cancel a chained promise returned by $interval

Question

If I create a promise using $interval, it is cancelable See: http://jsbin.com/jeweke/2/

timer = $interval(intervalFunc, intervalDelay, 10);
timer.then(
  function(res) {console.log('ok', res);},
  function(err) {console.log('err', err);}
);

However, if I chain the promise, the returned promise is not cancelable. See: http://jsbin.com/jeweke/1/

timer = $interval(intervalFunc, intervalDelay, 10)
.then(
  function(res) {console.log('ok', res);},
  function(err) {console.log('err', err);}
);

What gives? Is this just how it's supposed to work?

Note - the example here is loosely adapted from http://jsfiddle.net/ExpertSystem/fZc3W/

Answer 1

That is because the result of promise chain from the $interval does not have the property which contains the interval id ($$intervalId). First case you are saving timer promise which has the $intervalId, in the second case you are saving the promise returned from the chain which is a raw q promise without the $intervalId property (which is a custom property added on the promise to store the respective setInterval's id when you call $interval(...). When you cancel the timer it needs the $intervalId to cancelInterval and reject the respective timer promise.

This is what interval.cancel does

 interval.cancel = function(promise) {
      if (promise && promise.$$intervalId in intervals) {
        intervals[promise.$$intervalId].reject('canceled');
        clearInterval(promise.$$intervalId);
        delete intervals[promise.$$intervalId];
        return true;
      }
      return false;
    };

Note the line:-

 if (promise && promise.$$intervalId in intervals) {

intervals are nothing but a map of intervalId and its respective promise (example:- {1:promiseOfInterval1, 2:promiseOfInterval2}), so without intervalId no cancellation happens. So in short the promise returned by the $interval is q promise plus $intervalId property and when you chain it thorough it is just the $q implementation which returns back a new deferred object's promise.