Simple Javascript draggable element

Draggable.js

/**
    Pass in a dom ID string or a dom node and to make it draggable.
*/
var Draggable = function(elem) {
    var that = this;
    var el = (typeof elem === 'string') ? document.getElementById(elem) : elem;
    var offX, offY;
    this.mouseDown = function(e) {
        offX = e.clientX - parseInt(el.offsetLeft);
        offY = e.clientY - parseInt(el.offsetTop);
        window.addEventListener('mousemove', that.mouseMove);
        window.addEventListener('mouseup', function() {
            window.removeEventListener('mousemove', that.mouseMove);
        });

    };
    this.mouseMove = function(m) {
        el.style.position = 'absolute';
        el.style.top = (m.clientY - offY);
        el.style.left = (m.clientX - offX);
    };
    el.addEventListener('mousedown', that.mouseDown);
};