为什么这段代码报错？

Question

<script>

var x =document.getElementById("leftul");

var y=x.getElementsByTagName("ul");

var z=x.getElementsByTagName("a");

for (var i =0;i<y.length;i++){

z[i].addEventListener("mouseover",show);

z[i].addEventListener("mouseout",hide);

function show(){

y[i].style.display="block"; 

}

function hide(){

y[i].style.display="none";

}

}

</script>

//调试报错：Cannot read property 'style' of undefined

Answer 1

你这段代码要放在你的html代码后面，或者给这段代码加载入事件