获取relatedTarget属性

在做mouseenter与mouseleave的兼容时，我们需要用到事件对象的relatedTarget属性

function getRelatedTarget(e) {

    var t = e.relatedTarget

    if (t) {

        return t

    }

    return e.fromElement === e.target ?

        e.toElement :

        e.fromElement;

}

String("mouseenter,mouseleave").replace(/\w+/g, function (type) {

    eventHooks[type] = function (dom) {

        var eventType = type === "mouseenter" ? "mouseover" : "mouseout";

        addEvent(dom, eventType, function (e) {

            var t = getRelatedTarget(e)

            if (!t || (t !== dom && !dom.contains(t))) {

                dispatchEvent(e, type, true);

            }

        });

    };

});