RxJs: Combine two Observables - Take all from first but not all from second

Question

Lets say I have two observables in rxjs.

I want to take all elements coming from the first and combine them with the second but only take the element from the second if its not already in the first.

Elements are distinct by id.

Answer 1

I found the answer, also thanks to the link.

var a = Observable.of([1, 2, 3])
var b = Observable.of([3, 4])
a.concat(b).distinct().subscribe(..)

which will output [1, 2, 3, 4] but the 3 was taken from the first observable.