const ENTER_KEY_CODE = 13
const MAX_INTERVAL_BETWEEN_EVENTS_IN_MS = 50

const keyCode$ = Rx.Observable.fromEvent(document, "keypress")
  .pluck('keyCode')

const keyCodesBuffer$ = keyCode$
  .buffer(keyCode$.debounce(MAX_INTERVAL_BETWEEN_EVENTS_IN_MS))
  .filter(isFromScan)

function isFromScan(keyCodes) {
  return keyCodes.length > 1 && keyCodes[keyCodes.length - 1] === ENTER_KEY_CODE 
}

function fillInputWith(keyCodes) {
  // …
}

keyCodesBuffer$.subscribe(fillInputWith)