# 📚 JavaScript Tutorial - Part 8/10: Functional Programming in JavaScript
#JavaScript #FunctionalProgramming #FP #PureFunctions #HigherOrderFunctions

Welcome to Part 8 of our JavaScript series! Today we'll explore functional programming (FP) concepts that will transform how you write JavaScript, making your code more predictable, reusable, and maintainable.

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## 🔹 Core Principles of Functional Programming
### 1. Pure Functions

// Pure function (same input → same output, no side effects)
function add(a, b) {
  return a + b;
}

// Impure function (side effect + depends on external state)
let taxRate = 0.1;
function calculateTax(amount) {
  return amount * taxRate; // Depends on external variable
}

### 2. Immutability

// Bad (mutates original array)
const addToCart = (cart, item) => {
  cart.push(item); // Mutation!
  return cart;
};

// Good (returns new array)
const addToCartFP = (cart, item) => [...cart, item];

### 3. Function Composition

const compose = (...fns) => x => fns.reduceRight((v, f) => f(v), x);

const toUpperCase = str => str.toUpperCase();
const exclaim = str => `${str}!`;
const shout = compose(exclaim, toUpperCase);

shout('hello'); // "HELLO!"

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## 🔹 First-Class Functions
### 1. Functions as Arguments

const numbers = [1, 2, 3];

// Passing function as argument
numbers.map(num => num * 2); // [2, 4, 6]

### 2. Returning Functions

const createMultiplier = factor => num => num * factor;
const double = createMultiplier(2);
double(5); // 10

### 3. Storing Functions

const mathOps = {
  add: (a, b) => a + b,
  subtract: (a, b) => a - b
};
mathOps.add(3, 5); // 8

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## 🔹 Higher-Order Functions
### 1. Array Methods

const products = [
  { id: 1, name: 'Laptop', price: 999, inStock: true },
  { id: 2, name: 'Mouse', price: 25, inStock: false }
];

// Transform data
const productNames = products.map(p => p.name);

// Filter data
const availableProducts = products.filter(p => p.inStock);

// Reduce to single value
const totalPrice = products.reduce((sum, p) => sum + p.price, 0);

### 2. Custom HOF Example

function withLogging(fn) {
  return (...args) => {
    console.log(`Calling with args: ${args}`);
    const result = fn(...args);
    console.log(`Result: ${result}`);
    return result;
  };
}

const loggedAdd = withLogging(add);
loggedAdd(3, 5);

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## 🔹 Closures
Functions that remember their lexical scope.

### 1. Basic Closure

function createCounter() {
  let count = 0;
  return () => ++count;
}

const counter = createCounter();
counter(); // 1
counter(); // 2

### 2. Practical Use Case

function createApiClient(baseUrl) {
  return {
    get(endpoint) {
      return fetch(`${baseUrl}${endpoint}`).then(res => res.json());
    },
    post(endpoint, data) {
      return fetch(`${baseUrl}${endpoint}`, {
        method: 'POST',
        body: JSON.stringify(data)
      });
    }
  };
}

const jsonPlaceholder = createApiClient('https://jsonplaceholder.typicode.com');
jsonPlaceholder.get('/posts/1').then(console.log);

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## 🔹 Currying
Transforming a multi-argument function into a sequence of single-argument functions.

// Regular function
const add = (a, b, c) => a + b + c;

// Curried version
const curriedAdd = a => b => c => a + b + c;
curriedAdd(1)(2)(3); // 6

// Practical example
const createUser = username => email => password => ({
  username,
  email,
  password
});

const registerUser = createUser('js_dev');
const withEmail = registerUser('[email protected]');
const finalUser = withEmail('secure123');

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## 🔹 Recursion
### 1. Basic Recursion

function factorial(n) {
  return n <= 1 ? 1 : n * factorial(n - 1);
}

### 2. Tail Call Optimization

function factorial(n, acc = 1) {
  return n <= 1 ? acc : factorial(n - 1, n * acc);
}

### 3. Recursive Array Processing

function deepMap(arr, fn) {
  return arr.map(item => 
    Array.isArray(item) ? deepMap(item, fn) : fn(item)
  );
}

deepMap([1, [2, [3]]], x => x * 2); // [2, [4, [6]]]

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