A recursive function is a function that calls itself. Recursive functions are useful when a problem can be solved by an algorithm that repeats the same operation multiple times using the results of the preceding repetition. Factorial calculation, used in the following example, is one case where recursion is useful. The Towers of Hanoi game is also solved using a recursive algorithm.

A recursive function, like looping code, must have an end condition that always stops the function. Otherwise, the function will continue until a system error occurs or you stop the ColdFusion server.

The following example calculates the factorial of a number, that is, the product of all the integers from 1 through the number; for example, 4 factorial is 4 X 3 X 2 X 1 = 24.

function Factorial(factor) {
    If (factor LTE 1) 
        return 1;
    else 
        return factor * Factorial(factor -1);
}

If the function is called with a number greater than 1, it calls itself using an argument one less than it received. It multiplies that result by the original argument, and returns the result. Therefore, the function keeps calling itself until the factor is reduced to 1. The final recursive call returns 1, and the preceding call returns 2 * 1, and so on until all the initial call returns the end result.