//emit value every 1s

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

​

const example = source.mergeMap(

    //project

    val => Rx.Observable.interval(5000).take(2),

  //resultSelector

  (oVal, iVal, oIndex, iIndex) => [oIndex, oVal, iIndex, iVal],

  //concurrent

  2 

);

/*

        Output:

        [0, 0, 0, 0] <--1st inner observable

        [1, 1, 0, 0] <--2nd inner observable

        [0, 0, 1, 1] <--1st inner observable

        [1, 1, 1, 1] <--2nd inner observable

        [2, 2, 0, 0] <--3rd inner observable

        [3, 3, 0, 0] <--4th inner observable

*/

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