js实现下拉菜单

html代码如下：

<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>	

css代码如下：

*{
	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;

}

js代码如下：

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";
		}

	}

效果图：