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Exploring the magic of Recursion in JavaScript

We are going to dive into the intriguing concept of "Recursion" — a concept reminiscent of the seemingly endless reflections in a room of mirrors. We will dissect recursion, understand its intricacies, and learn to use it in JavaScript.

Understanding Recursion

Think about a stack of pancakes. To get to the bottom, you must lift each pancake from the top, one at a time. This repeated action is a basic example of recursion. In programming, recursion involves a function calling itself repeatedly until a specific condition, known as the base case, is satisfied. This is like walking down a staircase, step by step until you reach the bottom.

Here's a simple JavaScript function to illustrate recursion:

In this function, x decrements by one in each recursive call until it is no longer greater than 0, at which point recursion stops.

Defining the Base Case

Think of the base case as a stop sign guiding our function, indicating when recursion should cease. In our pancake stack example, the base case is when there are no more pancakes to lift. Similarly, x <= 0 is our base case in our function. This base case is vital in avoiding the chaos of an infinite recursion.

Defining the Recursive Case

The recursive case makes recursion tick — it refers to the process that gradually reduces the problem's size. Each recursive function call brings us closer to the base case. Let's take finding a factorial as an example to illustrate this.

To find a factorial, we multiply a number by the factorial of the number minus one and repeat this process until we reach one (our base case):

In this example, factorial(3) returns 3 * factorial(2), which in turn returns 2 * factorial(1). As factorial(1) equals 1, the entire chain of recursive calls results in the computation of 3 * 2 * 1.

Tips for Thinking Recursively

Try visualising problems as a nesting doll to wrap your thoughts around recursion. Each time you open the doll, a smaller one reveals itself until you reach the smallest doll - analogous to the base case, and the un-nesting process resembles the recursive case.

Remembering that large problems often break down into smaller, manageable sub-problems is crucial. Solving these smaller problems and combining their solutions can easily tackle the bigger problem.


Another Example of Recursive Function

Let's create a function that calculates the sum of the digits of a number. A loop could suffice for this job, but with clever utilisation of recursion, the solution becomes simpler:

In this function, similarly, with the factorial calculation, we pass Math.floor(num / 10) to the next recursion level, effectively dropping the last digit in each recursive call.
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