JS编写全选，复选按钮

<!DOCTYPE html>
<html lang="en">
<head>
   <meta charset="UTF-8">
   <title>复选</title>
</head>
<body>
   <input type="button" name="" value="全选" id="one">
   <input type="button" name="" value="反选" id="two">
   <input type="checkbox" name="" id="three">
   <ul>
       <li>
           <input type="checkbox" name="li">
       </li>
       <li>
           <input type="checkbox" name="li">
       </li>
       <li>
           <input type="checkbox" name="li">
       </li>
       <li>
           <input type="checkbox" name="li">
       </li>
       <li>
           <input type="checkbox" name="li">
       </li>
       <li>
           <input type="checkbox" name="li">
       </li>
   </ul>
   <script type="text/javascript">
   var btn1 = document.getElementById('one')
   var btn2 = document.getElementById('two')
   var btn3 = document.getElementById('three')
   var li = document.getElementsByName('li')
   btn1.onclick = function(){
       for(var i = 0;i < li.length;i++){
           if(li[i].checked ==0 ){
               li[i].checked = 1;
           }
       }
       isAll();
   }

   btn2.onclick = function(){
       for(var f = 0;f < li.length;f++){
           if (li[f].checked == 1) {
               li[f].checked = 0;
           }else{
               li[f].checked = 1;
           }
       }
       isAll();
   }

　　// 每个checkbox状态修改时判断是否全选
   for(var i = 0;i < li.length;i++){
       li[i].onchange = function(){
           isAll();
       }
   }

　　// 判断当前是否全选
   function isAll(){
       var num = 0;
       for(var i = 0;i < li.length;i++){
           if(li[i].checked == 1){
               num++
           }
       }
       if(num == 6){
           btn3.checked = 1;
       }else{
           btn3.checked = 0;
       }
   }
   </script>

</body>
</html>