JavaScript Functions

A function is a block of code designed to perform a particular task. Functions are reusable.

Function Declaration

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

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

Function Expression

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

Arrow Functions (ES6+)

// Regular arrow function
const greet = (name) => {
  return `Hello, ${name}!`;
};

// Concise (implicit return)
const greet = name => `Hello, ${name}!`;

// No parameters
const sayHi = () => "Hi!";

// Multiple params
const add = (a, b) => a + b;

Default Parameters

function greet(name = "World", greeting = "Hello") {
  return `${greeting}, ${name}!`;
}

greet();              // "Hello, World!"
greet("Alice");       // "Hello, Alice!"
greet("Bob", "Hey");  // "Hey, Bob!"

Rest Parameters

function sum(...numbers) {
  return numbers.reduce((total, n) => total + n, 0);
}

sum(1, 2, 3, 4, 5); // 15

Higher-Order Functions

Functions that take other functions as arguments or return functions:

// Map, Filter, Reduce
const numbers = [1, 2, 3, 4, 5];

const doubled = numbers.map(n => n * 2);
// [2, 4, 6, 8, 10]

const evens = numbers.filter(n => n % 2 === 0);
// [2, 4]

const total = numbers.reduce((sum, n) => sum + n, 0);
// 15

// Sort with callback
const words = ["banana", "apple", "cherry"];
words.sort((a, b) => a.localeCompare(b));
// ["apple", "banana", "cherry"]

Closures

function makeCounter(initial = 0) {
  let count = initial;
  
  return {
    increment: () => ++count,
    decrement: () => --count,
    value: () => count,
  };
}

const counter = makeCounter(10);
counter.increment(); // 11
counter.increment(); // 12
counter.decrement(); // 11
counter.value();     // 11

Async Functions

// Async/Await
async function fetchUser(id) {
  try {
    const response = await fetch(`/api/users/${id}`);
    const user = await response.json();
    return user;
  } catch (error) {
    console.error("Failed to fetch user:", error);
    throw error;
  }
}

// Usage
const user = await fetchUser(42);