custom event and dragdrop solution in javascript

//custom event

function EventTarget() {
        this.handlers = {};
    }
    EventTarget.prototype = {
        constructor: EventTarget,
        addHandler: function (type, handler) {
            if (typeof this.handlers[type] == "undefined") {
                this.handlers[type] = [];
            }
            this.handlers[type].push(handler);
        },

        fire: function (event) {
            if (!event.target) {
                event.target = this;
            }
            if (this.handlers[event.type] instanceof Array) {
                var handlers = this.handlers[event.type];
                for (var i = 0, len = handlers.length; i < len; i++) {
                    handlers[i](event);
                }
            }
        },

        removeHandler: function (type, handler) {
            if (this.handlers[type] instanceof Array) {
                var handlers = this.handlers[type];
                for (var i = 0, len = handlers.length; i < len; i++) {
                    if (handlers[i] === handler) {
                        break;
                    }
                }

                handlers.splice(i, 1);
            }
        }
    };

//DragDrop solution

var DragDrop = function () {

        var dragdrop = new EventTarget();
        var dragging = null;
        var diffX = 0;
        var diffY = 0;

        function handleEvent(event) {

            //get event and object
            event = EventUtil.getEvent(event);
            var target = EventUtil.getTarget(event);

            //confirm event type
            switch (event.type) {
                case "mousedown":
                    if (target.className.indexOf("draggable") > -1) {
                        dragging = target;
                        diffX = event.clientX - target.offsetLeft;
                        diffY = event.clientY - target.offsetTop;
                        dragdrop.fire({ type: "dragstart", target: dragging, x: event.clientX, y: event.clientY });
                    }
                    break;
                case "mousemove":
                    if (dragging !== null) {

                        //get event
                        event = EventUtil.getEvent(event);

                        //get position
                        dragging.style.left = (event.clientX - diffX) + "px";
                        dragging.style.top = (event.clientY - diffY) + "px";

                        //raise customized event
                        dragdrop.fire({ type: "dragstart", target: dragging, x: event.clientX, y: event.clientY });
                    }
                    break;

                case "mouseup":
                    dragdrop.fire({ type: "dragstart", target: dragging, x: event.clientX, y: event.clientY });
                    dragging = null;
                    break;
            }
        };

        //public interface
        dragdrop.enable = function () {
            EventUtil.addHandler(document, "mousedown", handleEvent);
            EventUtil.addHandler(document, "mousemove", handleEvent);
            EventUtil.addHandler(document, "mouseup", handleEvent);
        };

        dragdrop.disable = function () {
            EventUtil.removeHandler(document, "mousedown", handleEvent);
            EventUtil.removeHandler(document, "mousemove", handleEvent);
            EventUtil.removeHandler(document, "mouseup", handleEvent);
        };

        return dragdrop;
    } ();