//emit value every 1s

const source = Rx.Observable.interval(1000);

//is number even?

const isEven = val => val % 2 === 0;

//only allow values that are even

const evenSource = source.filter(isEven);

//keep a running total of the number of even numbers out

const evenNumberCount = evenSource

    .scan((acc, _) => acc + 1, 0);

//do not emit until 5 even numbers have been emitted

const fiveEvenNumbers = evenNumberCount.filter(val => val > 5);

const example = evenSource

    //also give me the current even number count for display

  .withLatestFrom(evenNumberCount)

    .map(([val, count]) => `Even number (${count}) : ${val}`)

  //when five even numbers have been emitted, complete source observable

  .takeUntil(fiveEvenNumbers);

/*

    Even number (1) : 0,

    Even number (2) : 2

    Even number (3) : 4

    Even number (4) : 6

    Even number (5) : 8

*/

const subscribe = example.subscribe(val => console.log(val));