关于前端的复选框

<!DOCTYPE html>
<html>
<head>
  <meta charset="UTF-8">
  <title></title>
</head>
<script type="text/javascript">
  function check(){
   document.getElementById("ck1").checked=true;
   document.getElementById("ck2").checked=true;
   document.getElementById("ck3").checked=true;
   document.getElementById("ck4").checked=true;
  }
  function uncheck(){
   document.getElementById("ck1").checked=false;
   document.getElementById("ck2").checked=false;
   document.getElementById("ck3").checked=false;
   document.getElementById("ck4").checked=false;
  }

  function panduan(){
   if(document.getElementById("ck").checked){
    check();
   }else{
    uncheck();
   }
  }
</script>
<body>
  <input type="checkbox" id="ck1"/>篮球<br />
  <input type="checkbox" id="ck2"/>足球<br />
  <input type="checkbox" id="ck3"/>排球<br />
  <input type="checkbox" id="ck4"/>乒乓球<br />
  <input type="checkbox" id="ck" onclick="panduan()"/>全选<br />
</body>
</html>