Tracks how long you've been typing by creating a stream of state change-events and then sampling from that stream every second.

typing-clock.js

/*** Helper Functions ***/

const showTyping = () => $('.typing').text('User is typing...');
const showIdle = () => $('.typing').text('');
const updateTimer = (x) => $('.timer').text(x);
const handleTypingStateChange = state =>
  state === 1 ? showTyping() : showIdle();

/*** Program Logic ***/

const inputEvents$ = Rx.Observable.fromEvent($('#input'), 'input').share();

// streams to indicate when user is typing or has become idle
const typing$ = inputEvents$.mapTo(1);
const isIdle$ = inputEvents$.debounceTime(1000).mapTo(0);

// stream to emit "typing state" change-events
const typingState$ = Rx.Observable.merge(typing$, isIdle$)
  .distinctUntilChanged()
  .share();

// every second, sample from typingState$
// if user is typing, add 1, otherwise 0
const timer$ = Rx.Observable
  .interval(1000)
  .withLatestFrom(typingState$, (tick, typingState) => typingState)
  .scan((a, b) => a + b, 0)

// subscribe to streams
timer$.subscribe(updateTimer);
typingState$.subscribe(handleTypingStateChange);