RxJS Detect When Subscription has Closed

Yes, based on the RxJS subscribe() documentation there are 3 arguments that can be passed, last one being the OnCompleted callback.

var observer = Rx.Observer.create(
    function (x) {
        console.log('Next: %s', x);
    },
    function (err) {
        console.log('Error: %s', err);
    },
    function () {
        console.log('Completed');
    });