Lesson 1

Hello, fellow explorer! Today, 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`

.

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:

JavaScript`1function recursiveFunction(x) { 2 if(x <= 0){ // Base case 3 console.log("Base case reached"); 4 } else { 5 console.log(x); 6 recursiveFunction(x - 1); // Recursive call 7 } 8} 9recursiveFunction(5); 10/*Output: 115 124 133 142 151 16Base case reached 17*/`

In this function, `x`

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

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.

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):

JavaScript`1function factorial(n) { 2 if (n === 1) { // base case 3 return 1; 4 } else { 5 return n * factorial(n-1); // recursive case 6 } 7} 8console.log(factorial(3)); // Will print 6 (3 * 2 * 1)`

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`

.

Try visualizing 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.

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 utilization of recursion, the solution becomes simpler:

JavaScript`1function sumOfDigits(num) { 2 if(num < 10) { // Base case 3 return num; 4 } 5 else { 6 return num % 10 + sumOfDigits(Math.floor(num / 10)); // Recursive case 7 } 8} 9console.log(sumOfDigits(12345)); // Will print 15 (1+2+3+4+5)`

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.

Let's wrap up here. We dove into the sea of recursion, familiarized ourselves with vital concepts like base and recursive cases, and implemented a simple JavaScript recursive function. We'll deepen our understanding of recursion with continuous practice and be well-prepared for succeeding lessons involving complex sorting and searching algorithms. Remember — knowledge is the illumination of practice. Keep experimenting, and happy coding!