Functions

// Declaration — can be called before it's defined (hoisted)
function greet(name) {
  return "Hello, " + name + "!";
}

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

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

// Default parameters
function greet(name = "stranger") {
  return `Hello, ${name}!`;
}
greet();          // Hello, stranger!
greet("Nelson");  // Hello, Nelson!

// Function expression — stored in a variable
const double = function(n) {
  return n * 2;
};

// Arrow function — concise syntax (most common in modern JS)
const double = (n) => n * 2;          // single expression, implicit return
const square = n => n * n;            // one param, no parentheses needed
const add    = (a, b) => a + b;
const greet  = (name) => {            // block body — explicit return needed
  const message = `Hello, ${name}!`;
  return message;
};

// Arrow functions capture the surrounding 'this'
// (important when working with classes and React)

// Rest — collects extra arguments into an array
function sum(...numbers) {
  return numbers.reduce((total, n) => total + n, 0);
}
sum(1, 2, 3, 4, 5);  // 15

// Spread — expands an array/object into individual values
const nums = [1, 2, 3];
console.log(Math.max(...nums));  // 3

const first  = [1, 2, 3];
const second = [4, 5, 6];
const all    = [...first, ...second];  // [1, 2, 3, 4, 5, 6]