Live Demo: Fibonacci Series in JavaScript using for loop

<!DOCTYPE html>
<html>
<body>
    <script>
        function fibonacci()
        {
          var a=0,b=1,fib=0,n,i;
          var n=prompt("Enter the length of series:");
          document.write(a+"</br> "+b+"</br>");
          for(i=2;i<n;i++)
          {
            fib=a+b;
            document.write(fib+"</br>");
            a=b;
            b=fib;
          }
        }
    </script>
    <form>
        <input type="button" value="fibonacci" onclick="fibonacci();">
	</form>
</body>
</html>