RxJS: How to not subscribe to initial value and/or undefined?

mySubject.pipe( skipWhile( v => !v ) );

As mentioned by Will you should be able to just skip the first value by using the skip operator:

var mySubject = new Rx.BehaviorSubject(undefined);

mySubject.pipe(skip(1)).subscribe(function(value) {
  // do something with the value
});

Use new Rx.ReplaySubject(1) instead of BehaviorSubject.