JS模拟select下拉菜单

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>JS模拟select下拉菜单</title>
    <style>
    body{ font-size: 15px;}
    em{ font-style: normal;}
    ul{ padding: 0; margin: 0; list-style: none;}
    #box{ position: relative;}
    .select{ display: inline-block; border: 1px solid #ccc; padding: 5px; width: 10%; cursor: pointer;}
    .arrow{ color: #888; position: relative; left: -25px;}
    .sub{ display: none; border: 1px solid #ccc; border-top: none; border-bottom: none; width: 10.5%;}
    .sub li{ border-bottom: 1px solid #ccc; padding: 5px; cursor: pointer;}
    .sub li:hover ,.sub li.hover{ background: #eee;}
    </style>
</head>
<body>
    <h3>JS模拟selec下拉菜单</h3>
    <div id="box">
        <form action="">
            <span class="select">请选择下拉列表项</span><em class="arrow">▼</em>
            <button type="submit">搜索</button>
        </form>
        <ul class="sub">
            <li>项目1</li>
            <li>项目2</li>
            <li>项目3</li>
            <li>项目4</li>
            <li>项目5</li>
            <li>项目6</li>
            <li>项目7</li>
            <li>项目8</li>
        </ul>    
    </div>
    <script>
    window.onload = function(){

        var oBox = document.getElementById('box');
        var oSel = oBox.getElementsByTagName('span')[0];
        var oSub = oBox.getElementsByTagName('ul')[0];
        var oEm = oBox.getElementsByTagName('em')[0];
        var aLi = oSub.getElementsByTagName('li');

        // 点击显示隐藏下拉列表
        oSel.onclick = function(ev){
            var oEvent = ev || event;

            if(oSub.style.display == 'block'){
                oEm.innerHTML = "▼";
                // alert(oEm.innerHTML);
                oSub.style.display = 'none';
            }else{
                oEm.innerHTML = "▲";
                oSub.style.display = 'block';
            }
            // 阻止默认事件
            oEvent.cancelBubble = true;
        };

        // 点击文档任意空白处，隐藏下拉菜单
        document.onclick = function(){
            oSub.style.display = 'none';
        };

        // 循环遍历所有li，为每个li添加相应事件
        for(var i = 0; i < aLi.length; i++){

            // 鼠标移入，给li添加hover类
            aLi[i].onmouseover = function(){
                this.className = 'hover';
            };

            // 鼠标移出，移出li的class
            aLi[i].onmouseout = function(){
                this.className = '';
            };

            // 点击li，将oSel的内容替换成当前li的值
            aLi[i].onclick = function(){
                oEm.innerHTML = "▼";
                oSel.innerHTML = this.innerHTML;
            };
        }
    };
    </script>
</body>
</html>
运用到js的display显示隐藏、阻止默认事件，添加删除class，innerHTML等相关知识点。
代码只实现了它的功能，美化方面，可根据项目实际需要，自行调整修改。