JS 實現checkbox多選按鈕 全選

<script>
//name爲多選按鈕的name
function allCheck(name) {
      var checkbox = document.getElementsByName(name); 
      
      var count = 0;//count用於輔助實現反選擇
      for (var i = 0; i < checkbox.length; i++) {        
          if (checkbox[i].checked != true)  
       	  {
        	  checkbox[i].checked=true;
        	  count++;
       	  }
      }
      if(count == 0)
   	  {
    	  for (var i = 0; i < checkbox.length; i++) {        
           	  checkbox[i].checked=false;
          }
   	  }
  }  
  </script>