Editing Technology/Software Development/General knowledge/Programming Fundamentals/Functional Programming (section)

== Functional Programming ==
'''Definition'''

Functional Programming (FP) is a programming paradigm that treats computation as the evaluation of mathematical functions. It focuses on writing pure functions that produce output based solely on their input, without modifying external state or producing side effects.

'''Key Concepts'''

Let's delve into some fundamental concepts of functional programming:

1. '''Pure Functions:'''
   - Pure functions are functions that, given the same input, always produce the same output and do not modify external state.
   - They have no side effects, such as modifying global variables or performing I/O operations.
   - Pure functions are deterministic and easier to reason about and test.

2. '''Immutable Data:'''
   - Functional programming discourages mutable data, where values can be changed after they are created.
   - Instead, immutable data is preferred, where values remain constant throughout their lifetime.
   - Immutable data promotes safer concurrency, easier debugging, and simpler program flow.

3. '''Higher-Order Functions:'''
   - Higher-order functions are functions that can take other functions as arguments or return functions as results.
   - They enable composition and abstraction, allowing developers to create powerful and flexible code.

4. '''Recursion:'''
   - Recursion is a technique where a function calls itself to solve a problem by breaking it down into smaller sub-problems.
   - Functional programming encourages the use of recursion instead of iterative loops for iteration.

'''Code Example'''

Here's an example in JavaScript that demonstrates the usage of functional programming concepts:

<pre>
// Pure function example
function square(number) {
    return number * number;
}

// Higher-order function example
function doTwice(func, value) {
    return func(func(value));
}

// Immutable data example
const numbers = [1, 2, 3, 4, 5];
const doubledNumbers = numbers.map((number) => number * 2);

// Recursion example
function factorial(n) {
    if (n === 0) {
        return 1;
    } else {
        return n * factorial(n - 1);
    }
}

console.log(square(5)); // Output: 25
console.log(doTwice(square, 3)); // Output: 81
console.log(doubledNumbers); // Output: [2, 4, 6, 8, 10]
console.log(factorial(5)); // Output: 120
</pre>

In this example, we define a pure function `square` that calculates the square of a number. We also define a higher-order function `doTwice` that takes a function and a value, and applies the function twice to the value. The `map` function is used to create a new array `doubledNumbers` by doubling each number in the original `numbers` array. Lastly, we demonstrate recursion by implementing a factorial function that calculates the factorial of a given number.