JavaScript 复选框全选取消全选反选

<!DOCTYPE html>
<html>
<head> 
<meta charset="UTF-8"> 
<title>小鸟启蒙(facesoho.com)</title>
</head>
<body>
看书：<input type="checkbox" name="checkbox" value=1><br>
写字：<input type="checkbox" name="checkbox"value=2><br>
打飞机：<input type="checkbox" name="checkbox"value=3><br>
玩游戏：<input type="checkbox" name="checkbox"value=4><br>
<button onclick="opcheckboxed('checkbox', 'checkall')">全选</button>
<button onclick="opcheckboxed('checkbox', 'uncheckall')">取消全选</button>
<button onclick="opcheckboxed('checkbox', 'reversecheck')">反选</button>
<script>
function opcheckboxed(objName, type){
  var objNameList=document.getElementsByName(objName);
  if(null!=objNameList){
    for(var i=0;i<objNameList.length;i++){  
      if(objNameList[i].checked==true)
      {
        if(type != 'checkall') {  // 非全选
          objNameList[i].checked=false;
        }
        
      } else {
        if(type != 'uncheckall') {  // 非取消全选
          objNameList[i].checked=true;
        }
      }
    }  
  }
}
</script>
</body>
</html>