checkbox的全选与取消全选

function selectall() {
    for(var i=0;i<document.getElementsByName("checkbox2").length;i++) {
        document.getElementsByName("checkbox2")[i].checked=document.getElementById("selectflag").checked;
    }
}

1. <table>  
2.         <tr>  
3.             <td>  
4.                 <input type="checkbox" name="selectflag"  id="selectflag" onclick="selectall();"">选中所有显示记录   
5.             </td>  
6.         </tr>  
7.         <tr>  
8.             <td>  
9.                 <input type="checkbox" name="list" value="1" >  
10.             </td>       
11.         </tr>  
12.         <tr>  
13.             <td>  
14.                 <input type="checkbox" name="checkbox2  id="checkbox2“>  
15.             </td>       
16.         </tr>  
17.         <tr>  
18.             <td>  
19.                 <input type="checkbox"  name="checkbox2  id="checkbox2“>  
20.             </td>       
21.         </tr>  
22.     </table>