选中全选框

这里是HTML代码

<table>
        <tr>
            <th>
                <input type="checkbox" name="" id="checkAll"/>全选/全不选
            </th>
            <th>菜名</th>
            <th>商家</th>
            <th>价格</th>
        </tr>
        <tr>
            <td>
                <input type="checkbox" name="check"/>
            </td>
            <td>红烧肉</td>
            <td>隆*脚饭</td>
            <td>￥200</td>
        </tr>
        <tr>
            <td>
                <input type="checkbox" name="check"/>
            </td>
            <td>香酥排骨</td>
            <td>隆*脚饭</td>
            <td>￥998</td>
        </tr>
        <tr>
            <td>
                <input type="checkbox" name="check"/>
            </td>
            <td>北京烤鸭</td>
            <td>隆*脚饭</td>
            <td>￥88</td>
        </tr>
    </table>

这里是css代码

    *{
        margin: 0;
        padding: 0;
    }
    table {
        border-collapse: collapse;
        border-spacing: 0;
        border: 1px solid #c0c0c0;
        width: 500px;
        margin: 100px auto;
        text-align: center;
    }
    th {
        background-color: #09c;
        font: bold 16px "微软雅黑";
        color: #fff;
        height: 24px;
    }
    td {
        border: 1px solid #d0d0d0;
        color: #404060;
        padding: 10px;
    }

这里是js代码

var checkAll = document.getElementById("checkAll");
        var checks = document.getElementsByName("check");
        checkAll.onclick=function(){
            var statu = this.checked;
            for(var i =0;i<checks.length;i++){
              checks[i].checked=statu;
            }
        }
        for(var i=0;i<checks.length;i++){
          checks[i].onclick=function(){
            var flag = true;
            for(var j=0;j<checks.length;j++){
              if(checks[j].checked==false){
                  flag=false;
                  break;
              }
            }
              if(flag){
                checkAll.checked=true;
              }else{
                checkAll.checked=false;
            }
          }
        }