[рони]

novicer,

<script>
var input = 5//prompt("Введите число: ");
input = parseInt(input);

function factorial(x) {
   if (x <= 1)
      return 1;
    var result = 1;
    for(var i = x; i;) {
        result *= i--
    }
   return result;
}
document.write("Факториал " + input + "! = " + factorial(input));

  </script>