RxJS: How can I do an "if" with Observables?

Question

Say I have two observables and one I want to listen on changes in one observable, if the other on matches a certain condition. I tried it with zip but it seems I will only be notified, if both observables change, but I want to be notified for every change on the one observable, if the condition of the other one is true.

What I tried:

var firstState = new Rx.BehaviorSubject(undefined);
var secondState = new Rx.BehaviorSubject(undefined);

Rx.Observable.zip(firstState, secondState, function (first, second) {
  return {
    first: first,
    second: second
  }
}).filter(function (value) {
  return value.first !== undefined;
}).subscribe(function (value) {
  // do something with value.second
}); 

I noticed there is an Rx.Observable.if, but I couldn't got it to work.

Answer 1

Use pausable:

secondState
    .pausable(firstState.map(function (s) { return s !== undefined; }))
    .subscribe(function (second) {
        // only occurs when first is truthy
    });