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);            }        });    };});