JS judgment single mouse double-click event

<input type= "button"  onclick= "fn()"  ondblclick= "fun()"  value= "点我" >

<script language="javascript">

  the  flag;

  function aa(){

    flag = false;

    setTimeout(fn, 500);

    function fn(){

      if(flag != false) return;

      alert ( "Click");

    }

  

  function fun(){

    flag = true;

    alert ( "double-clicking");

  }

</script>