Rx.Observable.latestOn

example.js

var model = new Rx.Subject();
var click = new Rx.Subject();

model.latestOn(click, function (model, e) { return [model, e]; })
  .subscribe(function (x) {
    console.log(x);
  });

latestOn.js

Rx.Observable.prototype.latestOn = function (trigger, selector) {

  var source = this;

  return Rx.Observable.createWithDisposable(function (o) {

    var hasValue = false,
        value;

    var sourceSubscription = source.subscribe(function (x) {
      latestValue = x;
      hasValue = true;
    });

    var triggerSubscription = trigger.subscribe(
      function (y) {
        if (hasValue) { o.onNext(selector(latestValue, y)); }
      }
    );

    return new Rx.CompositeDisposable(sourceSubscription, triggerSubscription);
  });
};