const meta$ = new Rx.Subject();

​

// Create some streams that we can observe.

const stream1$ = Rx.Observable.interval(300);

const stream2$ = Rx.Observable.timer(1000)

  .switchMap(() => Rx.Observable.interval(500).startWith(0))

  .take(10);

const stream3$ = Rx.Observable.timer(1500)

  .switchMap(() => Rx.Observable.interval(500).startWith(0))

  .take(2);

​

​

stream1$.subscribe(

  next => meta$.next({ stream1: true }),

  () => meta$.next({ stream1: false }),

  () => meta$.next({ stream1: false })

);

stream2$.subscribe(

  next => meta$.next({ stream2: true }),

  () => meta$.next({ stream2: false }),

  () => meta$.next({ stream2: false })

);

stream3$.subscribe(

  next => meta$.next({ stream3: true }),

  () => meta$.next({ stream3: false }),

  () => meta$.next({ stream3: false })

);

​

meta$

  .scan((state, next) => {

    return Object.assign({}, state, next);

  })

  .map(obj => {

    return Object.keys(obj)

      .map(key => +obj[key])

      .reduce((acc, x) => acc + x, 0)

  })

  .take(50) // don't create endless loop

  .timestamp()

  .subscribe(x => console.log(x));