Live Demo: prime number in JavaScript

<html>
    <body>
        <form name="f1">
            enter the table <input type="number" name="no">
            <input type="button" value="output" onclick="PrimeNo()" value="output">
        </form>
        <p id="prime">output here</p>
        <script>
            function PrimeNo()
            {
                var i,flag=0;
                var p=" ";
                var a=parseInt(document.f1.no.value);
                for(i=1;i<=a;i++)
                {
                    if(a%i==0)
                    {
                        flag+=1;
                    }
                if(flag==2)
                {p=a+" is prime number"; }
                
                else{  p=a+" is not a prime number";}
                }
                var d=document.getElementById("prime").innerHTML=p;
                   
            }
        </script>
    </body>
</html>