Rx.changes.js

Rx.Observable.prototype.changes = function(deleteObs, keySelector) {
  var source = this;

  return new Rx.AnonymousObservable(function(observer) {
    var store = new Map;

    return new Rx.CompositeDisposable(
      deleteObs.subscribe(function(key) {
        var last = store.get(key);
        store.delete(key);

        last && observer.onNext([undefined, last]);
      }),
      source.subscribe(
        function(ele) {
          var key = keySelector(ele);

          var last = store.get(key);
          store.set(key, ele);

          observer.onNext([ele, last]);
        },
        observer.onError.bind(observer),
        observer.onCompleted.bind(observer)
      ),
      Rx.Disposable.create(store.clear.bind(store))
    );
  }, this);
};