You can edit / change the code and click run button <script> function factorial(aNumber) { // If the number is not an integer, round it down. aNumber = Math.floor(aNumber); // The number must be equal to or bigger than zero if (aNumber < 0) { return "undefined"; } if ((aNumber == 0) || (aNumber == 1)) { // If the number is 0 or 1, its factorial is 1. return 1; } else { // Make a recursive call return (aNumber * factorial(aNumber - 1)); } } number = prompt("Enter a number:", "5") document.write(factorial(number)); </script> <p>It calculates the factorial of the given number