DOM ------ 下拉菜单

  //html代码
  <ul class='nav'>
        <li>
            <a href="#">微博</a>
            <ul>
                <li>
                    <a href="">私信</a>
                </li>
                <li>
                    <a href="">评论</a>
                </li>
                <li>
                    <a href="">@我</a>
                </li>
            </ul>
        </li> 
    </ul>
    //js代码
    var nav = doument.querySelector('.nav')
    var lis = nav.children
    for(var i=0; i<lis.length; i++){
        lis[i].onmouseover = function(){
            this.children[1].style.display  = 'block'
        }
        lis[i].onmouseout = function(){
            this.children[1].style.display  = 'none'
        }
    }