[RxJS] Combination operator: withLatestFrom

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" 
  */