JS如何控制checkbox的全选反选

JS代码：

 1 <script language="javascript" type="text/javascript">
 2 
 3         //转载请保留出处 http://www.dwww.cn 
 4         function unselectall() {
 5             if (document.getElementById("chkAll").checked) {
 6                 document.getElementById("chkAll").checked = document.getElementById("chkAll").checked & 0;
 7             }
 8         }
 9         function CheckAll(form) {
10             var r = document.getElementById("Example1");
11             if (r != null) {
12                 var r2 = r.getElementsByTagName('input');
13                 for (var i = 0; i < r2.length; i++) {
14                     var e = r2[i];
15                     if (e.Name != 'chkAll' && e.disabled == false)
16                         e.checked = document.getElementById("chkAll").checked;
17                 }
18             }
19         }
20     </script>

HTML代码：

 1 <div id="Example1">
 2         <input name='id' type='checkbox' οnclick='unselectall()' id='id' value='1' />
 3         A
 4         <input name='id' type='checkbox' οnclick='unselectall()' id='id' value='2' />
 5         B
 6         <input name='id' type='checkbox' οnclick='unselectall()' id='id' value='3' />
 7         C
 8         <input name='id' type='checkbox' οnclick='unselectall()' id='id' value='4' />
 9         D
10         <input name='chkAll' type='checkbox' id='chkAll' οnclick='CheckAll(this.form)' value='checkbox' />
11         全选
12     </div>