Recursion (JavaScript)

Recursion is an important programming technique, in which a function calls itself.

One example is the calculation of factorials. The factorial of a number n is calculated by multiplying 1 * 2 * 3 *... n. The following example shows how to calculate factorials iteratively, that is, by using a while loop in which the result is calculated.

function factorial(num)
{
    // If the number is less than 0, reject it.
    if (num < 0) {
        return -1;
    }
    // If the number is 0, its factorial is 1.
    else if (num == 0) {
        return 1;
    }
    var tmp = num;
    while (num-- > 2) {
        tmp *= num;
    }
    return tmp;
}

var result = factorial(8);
document.write(result);

// Output: 40320

You can make the example recursive very simply. Instead of using a while loop to calculate the value, you can simply call factorial again, passing in the next lowest value. The recursion stops when the value is 1.

function factorial(num)
{
    // If the number is less than 0, reject it.
    if (num < 0) {
        return -1;
    }
    // If the number is 0, its factorial is 1.
    else if (num == 0) {
        return 1;
    }
    // Otherwise, call this recursive procedure again.
    else {
        return (num * factorial(num - 1));
    }
}

var result = factorial(8);
document.write(result);

// Output: 40320
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