Recursive Function

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.

Found a mistake? Have a question or improvement idea? Let me know.

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