Live Demo: factorial number in javascript

<html>
    <body>
        <script>
            function factorial()
            {
            var fact=1,i;
            var a=prompt("enter a number:");
            for(i=1;i<=a;i++)
            {
                fact=fact*i;
            }
            document.write("factorial of number "+a+" is:",fact);
            }
        </script>
        <form>
            <input type="button" value="factorial" onclick="factorial();">
        </form>
    </body>
</html>