# Recursion (JavaScript)

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

## An Example of Recursion

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
``````