Factorial function

by Lucille Kenney

JavaScript

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:", " ")
document.write(factorial(number));