自定义下拉框样式，利用prototype制作

<script type="text/javascript" src="js/jquery-1.7.2.min.js" ></script>
    <script type="text/javascript">
        //
        function createDropdown(ele){
            this.dropdown=ele;
            this.val='';
            this.index=-1;//默认值时-1
            this.palceholder=this.dropdown.find('.palceholder');
            this.options=this.dropdown.find('ul.dropdown_menu > li');
            this.clickEvent();
        }
        createDropdown.prototype={
            clickEvent:function(){
                var obj=this;
                obj.dropdown.on('click',function(){
                    $(this).toggleClass('active');
                });
                //下拉框点击事件
                obj.options.on('click',function(){
                    var opt=$(this);
                    obj.val=opt.attr("value");
                    obj.index=opt.index();
                    obj.text=opt.find('a').text();
                    obj.palceholder.text(obj.text);
                });
            }
        }
        var drop1=new createDropdown($('#testDropdown'));
    </script>

<div class="dropdownBox" style="background: red;margin: 50 auto 0px; 120px;">
            <div class="dropdown" id='testDropdown'>
                <a>
                    <span class="palceholder">1</span><!--存值-->
                    <i></i>                             <!--图标-->
                </a>
                <ul class="dropdown_menu">
                    <li value="1"><a>1</a></li>
                    <li value="2"><a>2</a></li>
                    <li value="3"><a>3</a></li>
                    <li value="4"><a>4</a></li>
                    <li value="5"><a>5</a></li>
                </ul>
            </div>
        </div>

#testDropdown .dropdown_menu{display: none;}
            #testDropdown.active .dropdown_menu{display: block;}
            .dropdown_menu li:hover{cursor: pointer;background: pink;color: #fff;}
            .palceholder{display: inline-block;width: 120px;height: 30px;border: 1px solid #bbb;}