求助

<script type="text/javascript">
window.onload = function(){
var mainNav = document.getElementById("mainNav");
var nav = mainNav.getElementsByClassName("oNav");
var navs = mainNav.getElementsByTagName("ul");
for(i=0;i<nav.length;i++){
nav[i].onclick = function(i){
document.write(j);
navs[i].style.display = "none";}
}
}
</script>
为什么i 的参数变成了[object MouseEvent]