js实现下拉菜单

<header>
<img src="../img/logoo.png" alt="图标">
<ul id="menu">
<li id="menuli" class="active"><a href="#">课程学习</a>	   <ul class="sub_menu">
<li class="item"><a href="#">Javascript</a></li>
<li class="item"><a href="#">Dom</a></li>
<li class="item"><a href="#">Css</a></li>
<li class="item"><a href="#">HTML</a></li>
</ul>
</li>
<li id="menuli2"><a href="#">课程笔记</a>
<ul class="sub_menu">
<li class="item" id="menuli3"><a href="#">Javascript</a>
<ul class="third_menu">
<li class="item"><a href="#">Object</a></li>
<li class="item"><a href="#">Function</a></li>
<li class="item"><a href="#">String</a></li>
<li class="item"><a href="#">Prototype</a></li>
</ul>
</li>
<li class="item"><a href="#">Dom</a></li>
<li class="item"><a href="#">Css</a></li>
<li class="item"><a href="#">HTML</a></li>
</ul>
</li>
<li><a href="#">课程练习</a></li>
</ul>
</header>

*{
margin:0;
padding:0;
}

body{
background: url("../img/wallhaven2.png");
}
header{
width: 100%;
background: #F5F5F5;
border:1px solid #E5E5E5;
height:50px;

}
img{
display: block;
float:left;
padding: 1px;
}

#menu>li{
float:left;
padding:0 30px;
}
#menuli>a{
border-bottom:2px solid red;

}
ul{
list-style-type: none;
margin-left:150px;
text-align: center;

}
.sub_menu{
margin-left: 0;
position: relative;
border:1px solid #e5e5e5;
border-top:none;
border-radius:0 0 10px 10px;
display: none;
}
.item{
border:1px dotted #E5E5E5;
}
.third_menu{
position: absolute;
left:130px;
top:0px;
margin-left: 0;
border:1px solid #E5E5E5;
border-top:none;
border-left:none;
display: none;
}
.third_menu>li>a{
display: block;
text-decoration: none;
padding:5px;
color:#000;
line-height: 20px;
}
ul>li>a{
display: block;
text-decoration: none;
padding:5px 25px;
color:#000;
line-height: 40px;

}
#menuli>a{
background: url("../img/arrow_drop_down.png") no-repeat 90px center;
}
#menuli>a:hover{
background: url("../img/arrow_drop_down.png") #ddd no-repeat 92px center;
color:#000;

}
ul>li>a:hover{
color:#eee;
background: #ddd;

}

window.onload = function(){
var menuliobj = document.getElementById('menuli');
var menuli2obj = document.getElementById('menuli2');
var menuli3obj = document.getElementById('menuli3');
var third_menuobj = document.getElementsByClassName('third_menu');
var sub_menuobj = document.getElementsByClassName('sub_menu');
menuliobj.addEventListener("mouseover",
menudown1,false);
function menudown1(){
sub_menuobj[0].style.display="block";

}
menuliobj.addEventListener("mouseout",
menuup1,false);
function menuup1(){
sub_menuobj[0].style.display="none";

}
menuli2obj.addEventListener("mouseover",
menudown,false);
function menudown(){
sub_menuobj[1].style.display="block";
}
menuli2obj.addEventListener("mouseout",
menuup,false);
function menuup(){
sub_menuobj[1].style.display="none";
}

menuli3obj.addEventListener("mouseover",menudown2,false);
function menudown2(){

third_menuobj[0].style.display = "block";
}
menuli3obj.addEventListener("mouseout",menuup2,false);
function menuup2(){

third_menuobj[0].style.display = "none";
}

}

window.onload = function(){
var menuliobj = document.getElementById('menuli');
var menuli2obj = document.getElementById('menuli2');
var menuli3obj = document.getElementById('menuli3');
var third_menuobj = document.getElementsByClassName('third_menu');
var sub_menuobj = document.getElementsByClassName('sub_menu');
//菜单显示函数
function show(obj){
obj.style.display = 'block';
}
//菜单隐藏函数
function hide(obj){
obj.style.display = 'none';
}
menuliobj.addEventListener("mouseover",function(){show(sub_menuobj[0]);},false);
menuliobj.addEventListener("mouseout"，function(){hide(sub_menuobj[0]);},false);
menuli2obj.addEventListener("mouseover",function(){show(sub_menuobj[1]);},false)
menuli2obj.addEventListener("mouseout",function(){hide(sub_menuobj[1]);},false)
menuli3obj.addEventListener("mouseover",function(){show(third_menuobj[0]);},false);
menuli3obj.addEventListener("mouseout",function(){hide(third_menuobj[0]);},false);