Live Demo: Prime number program in JavaScript using for loop

<html>
<body>
<script>
function PrimeNo()
{  
  var i,flag=0;
  var n=prompt("enter a number");
  for(i=1;i<=n;i++)
  {
    if(n%i==0)
    {
      flag+=1;
    }
  }
    if(flag==2)
      { document.write(n+" is prime number"); }
       
    else{ document.write(n+" is not a prime number"); }              
}
</script>
  <form>
         <input type="button" value="prime number" onclick="PrimeNo();">
  </form>
</body>
</html>