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