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🔁 JavaScript – Functional Programming, Higher-Order Functions, and Currying

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🧲 Introduction – Why Learn Functional Programming in JavaScript?

Have you ever struggled with reusable, testable, and clean JavaScript code? Functional Programming (FP) can change the way you approach problems. It emphasizes immutability, pure functions, and function composition, making your code modular and predictable.

By the end of this guide, you’ll understand:

✅ The core principles of functional programming in JavaScript
✅ How to write and use Higher-Order Functions (HOFs)
✅ The power of currying and function composition
✅ Real-world examples and best practices

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🧠 1. What is Functional Programming in JavaScript?

Functional Programming is a declarative paradigm that treats computation as the evaluation of pure functions.

🔑 Key Principles:

Principle	Description
Pure Functions	No side-effects, same input = same output
Immutability	Data is never modified directly
First-Class Functions	Functions are treated as values
Higher-Order Functions	Functions that operate on other functions
Function Composition	Combine small functions to build complex ones

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📦 2. Pure Functions and Immutability

✅ Pure Function Example

function add(a, b) {
  return a + b;
}

✅ Explanation:

• Always returns the same output for the same input.
• Has no side effects (doesn’t modify external state).

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❌ Impure Function Example

let count = 0;
function increment() {
  count += 1;
}

⚠️ Explanation:

• Modifies a global variable.
• Not predictable or testable in isolation.

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🔁 3. Higher-Order Functions (HOFs)

A Higher-Order Function is a function that:

• Accepts another function as a parameter, OR
• Returns a function

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🔧 Example: map, filter, reduce

const numbers = [1, 2, 3, 4];

const doubled = numbers.map(n => n * 2);        // [2, 4, 6, 8]
const evens = numbers.filter(n => n % 2 === 0); // [2, 4]
const sum = numbers.reduce((acc, n) => acc + n, 0); // 10

✅ Explanation:

• .map() transforms each item.
• .filter() returns a subset based on a condition.
• .reduce() accumulates a result from the array.

📘 These methods take callback functions, making them higher-order.

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🛠️ Custom Higher-Order Function

function withLogging(fn) {
  return function(...args) {
    console.log(`Calling ${fn.name} with`, args);
    return fn(...args);
  };
}

const add = (a, b) => a + b;
const loggedAdd = withLogging(add);

loggedAdd(3, 5); // Logs: Calling add with [3, 5] → 8

✅ Explanation:

• withLogging() enhances any function with logging behavior.
• Demonstrates function wrapping, a key FP strategy.

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🧮 4. Currying in JavaScript

Currying is the process of transforming a function with multiple arguments into a sequence of functions, each taking one argument.

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📌 Simple Currying Example

function multiply(a) {
  return function(b) {
    return a * b;
  };
}

const double = multiply(2);
console.log(double(5)); // 10

✅ Explanation:

• multiply(2) returns a new function.
• That function takes b and multiplies it by 2.

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⚡ ES6 Arrow Syntax for Currying

const multiply = a => b => a * b;

const triple = multiply(3);
console.log(triple(4)); // 12

💡 Use When: You want partial application of arguments or to build customizable functions.

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🔀 5. Function Composition

Function Composition means combining multiple functions to produce a new function.

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🔧 Compose Example

const toUpper = str => str.toUpperCase();
const exclaim = str => str + '!';
const compose = (f, g) => x => f(g(x));

const shout = compose(exclaim, toUpper);
console.log(shout('hello')); // HELLO!

✅ Explanation:

• compose(f, g) returns a new function that executes g, then f.
• This creates pipeline-like transformations.

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📘 Real-World Functional Programming Use Cases

Use Case	FP Strategy
Form validation	Compose functions to sanitize input
Redux reducers	Use pure functions to manage state
Event handling	Use HOFs to wrap listeners
Middleware systems	Chain and compose operations

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🧠 Best Practices

✅ Favor pure functions and avoid side-effects
✅ Prefer map, filter, reduce over manual loops
✅ Use currying for reusability and configuration
✅ Keep functions small, testable, and composable
✅ Avoid mutating original arrays/objects (use spread/cloning)

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📌 Summary

In this guide, you’ve learned:

• ✅ The principles of functional programming
• ✅ How to write and use higher-order functions
• ✅ How currying works and why it matters
• ✅ How to compose functions for clean pipelines

Mastering these tools helps you build cleaner, safer, and more testable JavaScript.

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❓FAQs – Functional Programming in JavaScript

❓ What is the difference between a pure and impure function?
➡️ A pure function always returns the same result for the same input and has no side effects. An impure function may depend on or modify external state.

❓ Why use higher-order functions?
➡️ They make code modular, reusable, and expressive, especially in callbacks, data processing, and event handling.

❓ What is the benefit of currying?
➡️ Currying allows for partial function application, improving code reuse and customization.

❓ Is functional programming better than object-oriented programming?
➡️ Each has strengths. FP is ideal for stateless logic and data transformation, while OOP excels at modeling complex entities with state and behavior.

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