RxJS: Spread an array over time then reduce when all have been received.

rxjs-array-over-time-reduce.js

var Rx = require('rxjs');

// values
var letters = 'J,A,M,I,E'.split(',');
var interval = 2000;

// streams
var letters$ = Rx.Observable.from(letters);
var timer$ = Rx.Observable.timer(0, interval);
var lettersOverTime$ = Rx.Observable.zip(letters$, timer$, (item, i) => item);
var eventualWord$ = lettersOverTime$.scan((word, letter) => word + letter, '').last();

// log the next letter every 2 secs
lettersOverTime$.subscribe(console.log);

// log the full word when complete
eventualWord$.subscribe(console.log);

/* output:
 * 
 * lettersOverTime$ : "J"---"A"---"M"---"I"---"E"---|
 * eventualWord$    : -------------------------------"JAMIE"|
 */