var foo = Rx.Observable.interval(400)

.zip(Rx.Observable.of('h', 'e', 'l', 'l', 'o'), (__, x) => x);

var bar = Rx.Observable.interval(300)

.zip(Rx.Observable.of(0,1,1,0,0,1,0,0,1), (__ ,x) =>  x);

/*

----h----e----l----l----o|     (foo)

--0--1--1--0--0--1--0--0--1|   (bar)

  withLatestFrom((c,n) => n === 1 ? c.toUpperCase() : c.toLowerCase())

----h----E----l----L----o|

*/

var combined = foo.withLatestFrom(bar, (c,n) => n === 1 ? c.toUpperCase() : c.toLowerCase());

combined.subscribe(

  function (x) { console.log('next ' + x); },

  function (err) { console.log('error ' + err); },

  function () { console.log('done'); },

);

  /*

 "next h"

"next E"

"next l"

"next l"

"next O"

"done"

  */