Update recursion concept documentation

concepts/recursion/about.md

@@ -1 +1,93 @@
-# About
+# Understanding Recursion in JavaScript
+
+Recursion is a powerful concept in programming that involves a function calling itself.
+It can be a bit tricky to grasp at first, but once you understand the fundamentals, it becomes a valuable tool in solving complex problems.
+In this tutorial, we'll explore recursion in JavaScript with easy-to-understand examples.
+
+## What is Recursion?
+
+Recursion occurs when a function calls itself, either directly or indirectly.
+It's similar to a loop, but it involves breaking a problem down into smaller, more manageable sub-problems.
+
+### Example 1: Countdown
+
+Let's start with a simple example: a countdown function.
+
+```javascript
+function countdown(num) {
+  // Base case
+  if (num <= 0) {
+    console.log("Blastoff!");
+    return;
+  }
+
+  // Recursive case
+  console.log(num);
+  countdown(num - 1);
+}
+
+// Call the function
+countdown(5);
+```
+
+In this example:
+
+- **Base case**: When `num` becomes less than or equal to 0, the function prints "Blastoff!" and stops calling itself.
+- **Recursive case**: The function prints the current `num` and calls itself with `num - 1`.
+
+### Example 2: Factorial
+
+Now, let's look at a classic example of recursion: calculating the factorial of a number.
+
+```javascript
+function factorial(n) {
+  // Base case
+  if (n === 0 || n === 1) {
+    return 1;
+  }
+
+  // Recursive case
+  return n * factorial(n - 1);
+}
+
+// Test the function
+console.log(factorial(5)); // Output: 120
+```
+
+In this example:
+
+- **Base case**: When `n` is 0 or 1, the function returns 1.
+- **Recursive case**: The function multiplies `n` by the factorial of `n - 1`.
+
+## Key Concepts
+
+### Base Case
+
+Every recursive function should have at least one base case, a condition where the function stops calling itself.
+Without a base case, the recursion would continue indefinitely, leading to a stack overflow.
+
+### Recursive Case
+
+The recursive case defines how the function calls itself with a smaller or simpler version of the problem.
+
+## Pros and Cons of Recursion
+
+**Pros:**
+
+- Elegant solution for certain problems.
+- Mimics the mathematical induction concept.
+
+**Cons:**
+
+- Can be less efficient than iterative solutions.
+- May lead to stack overflow for deep recursion.
+
+## Conclusion
+
+Recursion is a valuable technique that simplifies complex problems by breaking them into smaller, more manageable sub-problems.
+Understanding base cases and recursive cases is crucial for implementing effective recursive solutions in JavaScript.
+
+**Learn More:**
+
+- [MDN: Recursion in JavaScript](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/Functions#recursion)
+- [Eloquent JavaScript: Chapter 3 - Functions](https://eloquentjavascript.net/03_functions.html)

concepts/recursion/introduction.md

@@ -1,3 +1,93 @@
-# Introduction
+# Understanding Recursion in JavaScript
 
-TODO: add introduction for recursion concept
+Recursion is a powerful concept in programming that involves a function calling itself.
+It can be a bit tricky to grasp at first, but once you understand the fundamentals, it becomes a valuable tool in solving complex problems.
+In this tutorial, we'll explore recursion in JavaScript with easy-to-understand examples.
+
+## What is Recursion?
+
+Recursion occurs when a function calls itself, either directly or indirectly.
+It's similar to a loop, but it involves breaking a problem down into smaller, more manageable sub-problems.
+
+### Example 1: Countdown
+
+Let's start with a simple example: a countdown function.
+
+```javascript
+function countdown(num) {
+  // Base case
+  if (num <= 0) {
+    console.log("Blastoff!");
+    return;
+  }
+
+  // Recursive case
+  console.log(num);
+  countdown(num - 1);
+}
+
+// Call the function
+countdown(5);
+```
+
+In this example:
+
+- **Base case**: When `num` becomes less than or equal to 0, the function prints "Blastoff!" and stops calling itself.
+- **Recursive case**: The function prints the current `num` and calls itself with `num - 1`.
+
+### Example 2: Factorial
+
+Now, let's look at a classic example of recursion: calculating the factorial of a number.
+
+```javascript
+function factorial(n) {
+  // Base case
+  if (n === 0 || n === 1) {
+    return 1;
+  }
+
+  // Recursive case
+  return n * factorial(n - 1);
+}
+
+// Test the function
+console.log(factorial(5)); // Output: 120
+```
+
+In this example:
+
+- **Base case**: When `n` is 0 or 1, the function returns 1.
+- **Recursive case**: The function multiplies `n` by the factorial of `n - 1`.
+
+## Key Concepts
+
+### Base Case
+
+Every recursive function should have at least one base case, a condition where the function stops calling itself.
+Without a base case, the recursion would continue indefinitely, leading to a stack overflow.
+
+### Recursive Case
+
+The recursive case defines how the function calls itself with a smaller or simpler version of the problem.
+
+## Pros and Cons of Recursion
+
+**Pros:**
+
+- Elegant solution for certain problems.
+- Mimics the mathematical induction concept.
+
+**Cons:**
+
+- Can be less efficient than iterative solutions.
+- May lead to stack overflow for deep recursion.
+
+## Conclusion
+
+Recursion is a valuable technique that simplifies complex problems by breaking them into smaller, more manageable sub-problems.
+Understanding base cases and recursive cases is crucial for implementing effective recursive solutions in JavaScript.
+
+**Learn More:**
+
+- [MDN: Recursion in JavaScript](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/Functions#recursion)
+- [Eloquent JavaScript: Chapter 3 - Functions](https://eloquentjavascript.net/03_functions.html)

exercises/concept/pizza-order/.docs/introduction.md

@@ -1,73 +1,93 @@
-# Introduction
+# Understanding Recursion in JavaScript
 
-The ability for something to be defined in terms of itself is called recursion.
-Recursive functions are functions that call themselves.
+Recursion is a powerful concept in programming that involves a function calling itself.
+It can be a bit tricky to grasp at first, but once you understand the fundamentals, it becomes a valuable tool in solving complex problems.
+In this tutorial, we'll explore recursion in JavaScript with easy-to-understand examples.
 
-Suppose that you have a function called `recurse`.
-This function is recursive if it calls itself inside its body, like this:
+## What is Recursion?
 
-```js
-function recurse() {
-  // ...
-  recurse();
-  // ...
-}
-```
+Recursion occurs when a function calls itself, either directly or indirectly.
+It's similar to a loop, but it involves breaking a problem down into smaller, more manageable sub-problems.
 
-A recursive function usually has a condition to stop calling itself and return a value, known as a _base case_.
-If a base case is missing, in most cases, because it will call itself indefinitely, it would be able to run forever.
-In reality, in most of those situations, you'll end up with a "StackSize error": an error raised by the runtime because the _stack_ of function calls has grown beyond a predefined limit because each recursive call adds to this _stack_ until it returns (and it doesn't).
-The message of this error is `Maximum call stack size exceeded`.
-
-```js
-function recurse() {
-  if (baseCondition) {
-    // stop calling itself
-    //...
-  } else {
-    recurse();
-  }
-}
-```
+### Example 1: Countdown
 
-Recursive functions often can be used instead of `for` loops for more succinct code.
-For example, take a countdown.
-Here's the more intuitive `for` loop approach:
+Let's start with a simple example: a countdown function.
 
-```js
-function countDown(fromNumber) {
-  for (let i = fromNumber; i > 0; i--) {
-    console.log(i);
+```javascript
+function countdown(num) {
+  // Base case
+  if (num <= 0) {
+    console.log("Blastoff!");
+    return;
   }
+
+  // Recursive case
+  console.log(num);
+  countdown(num - 1);
 }
 
-countDown(3); // 3, 2, 1 in separate lines
+// Call the function
+countdown(5);
 ```
 
-We could solve this using recursion too:
+In this example:
+
+- **Base case**: When `num` becomes less than or equal to 0, the function prints "Blastoff!" and stops calling itself.
+- **Recursive case**: The function prints the current `num` and calls itself with `num - 1`.
 
-```js
-function countDown(fromNumber) {
-  console.log(fromNumber);
-  if (fromNumber > 1) {
-    countDown(fromNumber - 1);
+### Example 2: Factorial
+
+Now, let's look at a classic example of recursion: calculating the factorial of a number.
+
+```javascript
+function factorial(n) {
+  // Base case
+  if (n === 0 || n === 1) {
+    return 1;
   }
+
+  // Recursive case
+  return n * factorial(n - 1);
 }
 
-countDown(3); // same result
+// Test the function
+console.log(factorial(5)); // Output: 120
 ```
 
-Here, our base case is when `fromNumber` is 1, in which case we don't call `countDown` again.
+In this example:
 
-Apart from just displaying numbers, recursive functions can be used for more complicated procedures, such as keeping a sum or total.
+- **Base case**: When `n` is 0 or 1, the function returns 1.
+- **Recursive case**: The function multiplies `n` by the factorial of `n - 1`.
 
-```js
-function sum(n) {
-  if (n <= 1) {
-    return n;
-  }
-  return n + sum(n - 1);
-}
+## Key Concepts
 
-sum(3); // 6
-```
+### Base Case
+
+Every recursive function should have at least one base case, a condition where the function stops calling itself.
+Without a base case, the recursion would continue indefinitely, leading to a stack overflow.
+
+### Recursive Case
+
+The recursive case defines how the function calls itself with a smaller or simpler version of the problem.
+
+## Pros and Cons of Recursion
+
+**Pros:**
+
+- Elegant solution for certain problems.
+- Mimics the mathematical induction concept.
+
+**Cons:**
+
+- Can be less efficient than iterative solutions.
+- May lead to stack overflow for deep recursion.
+
+## Conclusion
+
+Recursion is a valuable technique that simplifies complex problems by breaking them into smaller, more manageable sub-problems.
+Understanding base cases and recursive cases is crucial for implementing effective recursive solutions in JavaScript.
+
+**Learn More:**
+
+- [MDN: Recursion in JavaScript](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/Functions#recursion)
+- [Eloquent JavaScript: Chapter 3 - Functions](https://eloquentjavascript.net/03_functions.html)