switch.js

  var SwitchObservable = (function(__super__) {
    inherits(SwitchObservable, __super__);
    function SwitchObservable(source) {
      this.source = source;
      __super__.call(this);
    }

    SwitchObservable.prototype.subscribeCore = function (o) {
      var inner = new SerialDisposable(), s = this.source.subscribe(new SwitchObserver(o, inner));
      return new BinaryDisposable(s, inner);
    };

    inherits(SwitchObserver, AbstractObserver);
    function SwitchObserver(o, inner) {
      this.o = o;
      this.inner = inner;
      this.stopped = false;
      this.latest = 0;
      this.hasLatest = false;
      AbstractObserver.call(this);
    }

    SwitchObserver.prototype.next = function (innerSource) {
      var d = new SingleAssignmentDisposable(), id = ++this.latest;
      this.hasLatest = true;
      this.inner.setDisposable(d);
      isPromise(innerSource) && (innerSource = observableFromPromise(innerSource));
      d.setDisposable(innerSource.subscribe(new InnerObserver(this, id)));
    };

    SwitchObserver.prototype.error = function (e) {
      this.o.onError(e);
    };

    SwitchObserver.prototype.completed = function () {
      this.stopped = true;
      !this.hasLatest && this.o.onCompleted();
    };

    inherits(InnerObserver, AbstractObserver);
    function InnerObserver(parent, id) {
      this.parent = parent;
      this.id = id;
      AbstractObserver.call(this);
    }
    InnerObserver.prototype.next = function (x) {
      this.parent.latest === this.id && this.parent.o.onNext(x);
    };

    InnerObserver.prototype.error = function (e) {
      this.parent.latest === this.id && this.parent.o.onError(e);
    };

    InnerObserver.prototype.completed = function () {
      if (this.parent.latest === this.id) {
        this.parent.hasLatest = false;
        this.parent.stopped && this.parent.o.onCompleted();
      }
    };

    return SwitchObservable;
  }(ObservableBase));

  /**
  * Transforms an observable sequence of observable sequences into an observable sequence producing values only from the most recent observable sequence.
  * @returns {Observable} The observable sequence that at any point in time produces the elements of the most recent inner observable sequence that has been received.
  */
  observableProto['switch'] = observableProto.switchLatest = function () {
    return new SwitchObservable(this);
  };