var source = Rx.Observable.from('hello') .zip(Rx.Observable.interval(600), (x, y) => x); var example = source.scan((origin, next) => origin + next, ''); example.subscribe({ next: (value) => { console.log(value); }, error: (err) => { console.log('Error: ' + err); }, complete: () => { console.log('complete'); } }); // h // he // hel // hell // hello // complete