麻烦请问，这样写会导致内存泄露吗？多谢回复！ for examle:

var Pro = {
    doc : document,
    getLeftArrowEvent : function(){
        //引入this，即整个对象为方便以后可能的引用
        var is = this,
            doc = is.doc,
            leftArrow = doc.getElementById('leftArrow');
        leftArrow.onmouseover = function(){
            var th = is,
                doc = th.doc,
                leftArrow = doc.getElementById('leftArrow');
            leftArrow.style.cursor = 'pointer';
        };
        leftArrow.onmouseout = function(){
            var th = is,
                doc = th.doc,
                leftArrow = doc.getElementById('leftArrow');
            leftArrow.style.cursor = '';
        };
        leftArrow = null;
    }
};