为什么会出现Uncaught TypeError: Cannot read property 'getElementsByClassName' of undefined

Question

.= ()
{
    Lis = .()Lis.onmouseover = () {
            .= ..().()..().=}
        Lis.onmouseout = () {
            .= }
    }

Answer 1

111

Answer 2

window.onload = function (){    var Lis = document.getElementById("list1");        Lis.onmouseover = function () {            this.className = "lihover";            this.document.getElementById("picture1").setAttribute("src","image/uparrow.png");            this.document.getElementsByClassName("submenue").display="block";        }        Lis.onmouseout = function () {            this.className = "";        }    }JS代码是这个