Dropdown with RxJs

dropdown.js

let Observable = Rx.Observable;

let div:Element = document.querySelector('.to-drag');

let mousedown = Observable.fromEvent(div, 'mousedown');
let mouseup = Observable.fromEvent(document, 'mouseup');
let mousemove = Observable.fromEvent(document, 'mousemove');

mousedown.forEach((e) => {
  return mousemove
    .takeUntil(mouseup)
    .map((e:MouseEvent) => {
      return {
        x: e.pageX,
        y: e.pageY
      };
    })
    .forEach((pos:Object) => {    
      let element:Element = e.target;
      let rect:ClientRect = element.getBoundingClientRect();

      element.style.position = 'absolute';    
      element.style.left = (pos.x - (rect.width / 2)) + 'px';    
      element.style.top = (pos.y - (rect.height / 2)) + 'px';
    });
});