Functions

Functions are reusable blocks of code that perform specific tasks.

Function Declaration

function greet(name) {
  return `Hello, ${name}!`;
}

console.log(greet("Alice")); // "Hello, Alice!"

Function Expression

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

console.log(add(5, 3)); // 8

Arrow Functions (ES6)

// Basic arrow function
const multiply = (a, b) => {
  return a * b;
};

// Shorthand for single expression
const square = x => x * x;

// No parameters
const sayHello = () => "Hello!";

// Single parameter (parentheses optional)
const double = x => x * 2;

Parameters and Arguments

// Default parameters
function greetWithDefault(name = "World") {
  return `Hello, ${name}!`;
}

// Rest parameters
function sum(...numbers) {
  return numbers.reduce((total, num) => total + num, 0);
}

console.log(sum(1, 2, 3, 4)); // 10

// Destructuring parameters
function createUser({name, age, email}) {
  return {
    id: Date.now(),
    name,
    age,
    email
  };
}

Higher-Order Functions

// Function that returns a function
function createMultiplier(factor) {
  return function(number) {
    return number * factor;
  };
}

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

// Function that takes a function as parameter
function processArray(arr, callback) {
  return arr.map(callback);
}

const numbers = [1, 2, 3, 4];
const doubled = processArray(numbers, x => x * 2);

Scope and Closures

function outerFunction(x) {
  // Outer scope
  
  function innerFunction(y) {
    // Inner scope - has access to outer scope
    return x + y;
  }
  
  return innerFunction;
}

const addFive = outerFunction(5);
console.log(addFive(3)); // 8

Functions are the building blocks of JavaScript applications!