// utility functions

const takeUntilFunc = (endRange, currentNumber) => {

  return endRange > currentNumber

    ? val => val <= endRange

    : val => val >= endRange;

};

​

const positiveOrNegative = (endRange, currentNumber) => {

  return endRange > currentNumber ? 1 : -1;

};

​

const updateHTML = id => val => document.getElementById(id).innerHTML = val;

// display

const input = document.getElementById('range');

const updateButton = document.getElementById('update');

​

const subscription = (function(currentNumber) {

  return Rx.Observable

      .fromEvent(updateButton, 'click')

      .map(_ => parseInt(input.value))

      .switchMap(endRange => {

        return Rx.Observable.timer(0, 20)

            .mapTo(positiveOrNegative(endRange, currentNumber))

            .startWith(currentNumber)

            .scan((acc, curr) => acc + curr)

            // .delayWhen(//easing here)

            .takeWhile(takeUntilFunc(endRange, currentNumber))

      })

      .do(v => currentNumber = v)

      .startWith(currentNumber)

      .subscribe(updateHTML('display'));

}(0))

​

​