[Angular2] Map keyboards events to Function

The idea is when we tape the arrow keys on the keyboard, we want the ball move accodingly.

    const leftArrow$ = Observable.fromEvent(document, 'keydown')
      .filter(event => event.key === 'ArrowLeft')
      .mapTo(position => this.increment(position, 'x', 10));
    const rightArrow$ = Observable.fromEvent(document, 'keydown')
      .filter(event => event.key === 'ArrowRight')
      .mapTo(position => this.decrement(position, 'x', 10));
    const upArrow$ = Observable.fromEvent(document, 'keydown')
      .filter(event => event.key === 'ArrowUp')
      .mapTo(position => this.increment(position, 'y', 10));
    const downArrow$ = Observable.fromEvent(document, 'keydown')
      .filter(event => event.key === 'ArrowDown')
      .mapTo(position => this.decrement(position, 'y', 10));

  increment(obj, prop, value) {
    return Object.assign({}, obj, {[prop]: obj[prop] + value});
  }

  decrement(obj, prop, value) {
    return Object.assign({}, obj, {[prop]: obj[prop] - value});
  }

    Observable.merge(leftArrow$, rightArrow$, downArrow$, upArrow$)
      .startWith({
        x: 200,
        y: 200
      })
      .scan((acc, curr) => curr(acc))
      .subscribe(
        p => this.position = p
      )

Here, 'curr' is the function return from 'mapTo', the 'acc' will remember the position return from the function. 

The initial value of 'acc', is from startWith().