A recursive function is simply a function, that would call itself.
function factorial (n) {
if (n <= 1) {
return 1;
}
return n * factorial(n - 1);
}The above function shows a basic example of how to perform a recursive function to return a factorial.
Another example, would be to retrieve the sum of even numbers in an array.
function countEvenNumbers (arr) {
// Sentinel value. Recursion stops on empty array.
if (arr.length < 1) {
return 0;
}
// The shift() method removes the first element from an array
// and returns that element. This method changes the length of the array.
var value = arr.shift();
// `value % 2 === 0` tests if the number is even or odd
// If it's even we add one to the result of counting the remainder of
// the array. If it's odd, we add zero to it.
return ((value % 2 === 0) ? 1 : 0) + countEvens(arr);
}It is important that such functions make some sort of sentinel value check to avoid infinite loops. In the first example above, when n is less than or equal to 1, the recursion stops, allowing the result of each call to be returned back up the call stack.