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