Function Declarations

A function is a reusable block of code that performs a specific task. In JavaScript, the most traditional way to define a function is with a function declaration:

// Function declaration
function greet(name) {
    console.log(`Hello, ${name}!`);
}

// Calling the function
greet("Alice");   // "Hello, Alice!"
greet("Bob");     // "Hello, Bob!"

// A function that performs a calculation
function add(a, b) {
    return a + b;
}

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

Function declarations are hoisted, which means you can call them before they appear in your code:

// This works — declarations are hoisted
sayHello(); // "Hello!"

function sayHello() {
    console.log("Hello!");
}

Function Expressions

A function expression assigns a function to a variable. The function itself can be anonymous (no name) or named:

// Anonymous function expression
const greet = function(name) {
    console.log(`Hello, ${name}!`);
};

greet("Alice"); // "Hello, Alice!"

// Named function expression (useful for debugging/recursion)
const factorial = function fact(n) {
    if (n <= 1) return 1;
    return n * fact(n - 1);
};

console.log(factorial(5)); // 120

Unlike declarations, function expressions are not hoisted. You must define them before you use them:

// This will NOT work — expressions are not hoisted
// sayHi(); // TypeError: sayHi is not a function

const sayHi = function() {
    console.log("Hi!");
};

sayHi(); // "Hi!" — works after the definition

Arrow Functions

Arrow functions (introduced in ES6) provide a shorter syntax for writing functions. They are especially popular for callbacks and short operations:

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

// Arrow function equivalent
const addArrow = (a, b) => {
    return a + b;
};

// Concise body — if there's just one expression, you can omit {} and return
const addShort = (a, b) => a + b;

// Single parameter — parentheses are optional
const double = n => n * 2;

// No parameters — empty parentheses required
const getRandom = () => Math.random();

// Examples in action
console.log(addShort(3, 5)); // 8
console.log(double(4));      // 8
console.log(getRandom());    // 0.7234... (random)

// Arrow functions and this
const counter = {
    count: 0,

    // Regular function — 'this' refers to counter
    increment: function() {
        this.count++;
        console.log(this.count);
    },

    // Arrow function — 'this' is inherited (NOT counter!)
    // incrementArrow: () => {
    //     this.count++;  // 'this' is the outer scope, not counter
    // }
};

counter.increment(); // 1
counter.increment(); // 2

// Where arrow functions shine: callbacks
const numbers = [1, 2, 3, 4, 5];
const doubled = numbers.map(n => n * 2);
console.log(doubled); // [2, 4, 6, 8, 10]

Parameters and Default Values

Functions can accept parameters (inputs). JavaScript is flexible — you can call a function with fewer or more arguments than defined:

function greet(name, greeting) {
    console.log(`${greeting}, ${name}!`);
}

greet("Alice", "Hello");     // "Hello, Alice!"
greet("Bob");                // "undefined, Bob!" — missing args become undefined

// Default parameters (ES6)
function greetDefault(name, greeting = "Hello") {
    console.log(`${greeting}, ${name}!`);
}

greetDefault("Alice", "Hi"); // "Hi, Alice!"
greetDefault("Bob");         // "Hello, Bob!" — uses default

// Default values can be expressions
function createUser(name, role = "viewer", joinDate = new Date()) {
    return { name, role, joinDate };
}

console.log(createUser("Alice"));
// { name: "Alice", role: "viewer", joinDate: [current date] }
console.log(createUser("Bob", "admin"));
// { name: "Bob", role: "admin", joinDate: [current date] }

Rest Parameters

// Collect remaining arguments into an array
function sum(...numbers) {
    let total = 0;
    for (const num of numbers) {
        total += num;
    }
    return total;
}

console.log(sum(1, 2, 3));       // 6
console.log(sum(10, 20, 30, 40)); // 100

// Rest must be the last parameter
function logAll(first, ...rest) {
    console.log(`First: ${first}`);
    console.log(`Rest: ${rest}`);
}

logAll("a", "b", "c", "d");
// "First: a"
// "Rest: b,c,d"

Return Values

Functions send data back using return. A function without a return statement (or with an empty return) returns undefined.

// Returning a value
function multiply(a, b) {
    return a * b;
}

const product = multiply(4, 5);
console.log(product); // 20

// Early return (guard clause pattern)
function divide(a, b) {
    if (b === 0) {
        return "Cannot divide by zero";
    }
    return a / b;
}

console.log(divide(10, 2)); // 5
console.log(divide(10, 0)); // "Cannot divide by zero"

// Returning objects (wrap in parentheses with arrow functions)
const makeUser = (name, age) => ({ name, age });
console.log(makeUser("Alice", 25)); // { name: "Alice", age: 25 }

// Functions without return give undefined
function doSomething() {
    console.log("Working...");
    // no return statement
}

const result = doSomething(); // prints "Working..."
console.log(result);          // undefined

Scope

Scope determines where variables are accessible. JavaScript has three types of scope:

Global Scope

// Variables declared outside any function or block are global
const appName = "MyApp";
let userCount = 0;

function showApp() {
    // Global variables are accessible everywhere
    console.log(appName);  // "MyApp"
    userCount++;
}

showApp();
console.log(userCount); // 1

Function (Local) Scope

function calculateTax(amount) {
    const taxRate = 0.2;  // local to this function
    const tax = amount * taxRate;
    return tax;
}

console.log(calculateTax(100)); // 20
// console.log(taxRate);  // ReferenceError: taxRate is not defined
// console.log(tax);      // ReferenceError: tax is not defined

Block Scope

// let and const are block-scoped (limited to { })
if (true) {
    const blockConst = "I'm block-scoped";
    let blockLet = "Me too";
    console.log(blockConst); // works
    console.log(blockLet);   // works
}
// console.log(blockConst); // ReferenceError
// console.log(blockLet);   // ReferenceError

// for loop scope
for (let i = 0; i < 3; i++) {
    // i only exists inside this loop
}
// console.log(i); // ReferenceError

// var hoisting demonstration
console.log(x); // undefined (not an error — var is hoisted)
var x = 5;
console.log(x); // 5

// What JavaScript actually sees:
// var x;           // declaration hoisted to top
// console.log(x);  // undefined
// x = 5;           // assignment stays here
// console.log(x);  // 5

// let/const do NOT allow this
// console.log(y); // ReferenceError: Cannot access 'y' before initialization
// let y = 5;

Closures

A closure is a function that remembers the variables from the scope where it was created, even after that scope has finished executing. This is one of JavaScript's most powerful features.

// Basic closure
function createGreeter(greeting) {
    // The inner function "closes over" the greeting variable
    return function(name) {
        console.log(`${greeting}, ${name}!`);
    };
}

const sayHello = createGreeter("Hello");
const sayHi = createGreeter("Hi");

sayHello("Alice"); // "Hello, Alice!"
sayHi("Bob");      // "Hi, Bob!"
// greeting is "remembered" even though createGreeter has returned

Practical Closure: Counter

function createCounter(start = 0) {
    let count = start;

    return {
        increment: () => ++count,
        decrement: () => --count,
        getCount: () => count,
        reset: () => { count = start; }
    };
}

const counter = createCounter(10);
console.log(counter.getCount());  // 10
counter.increment();
counter.increment();
console.log(counter.getCount());  // 12
counter.decrement();
console.log(counter.getCount());  // 11
counter.reset();
console.log(counter.getCount());  // 10

// count is private — there's no way to access it directly
// console.log(counter.count); // undefined

Closure Gotcha: Loops

// Common mistake with var in loops
for (var i = 0; i < 3; i++) {
    setTimeout(function() {
        console.log(i); // prints 3, 3, 3 (not 0, 1, 2)
    }, 100);
}
// var is function-scoped, so all callbacks share the same i

// Fix: use let (block-scoped — each iteration gets its own i)
for (let i = 0; i < 3; i++) {
    setTimeout(function() {
        console.log(i); // prints 0, 1, 2
    }, 100);
}

Callback Functions

A callback is a function passed as an argument to another function. Callbacks are fundamental to JavaScript — they're used for event handling, array methods, timers, and asynchronous operations.

// Passing a function as an argument
function doMath(a, b, operation) {
    return operation(a, b);
}

const sum = doMath(5, 3, (a, b) => a + b);
const product = doMath(5, 3, (a, b) => a * b);

console.log(sum);     // 8
console.log(product); // 15

Callbacks with Array Methods

const numbers = [1, 2, 3, 4, 5];

// forEach — run a function on each element
numbers.forEach(num => console.log(num * 2));
// 2, 4, 6, 8, 10

// map — transform each element (returns new array)
const doubled = numbers.map(num => num * 2);
console.log(doubled); // [2, 4, 6, 8, 10]

// filter — keep elements that pass a test
const evens = numbers.filter(num => num % 2 === 0);
console.log(evens); // [2, 4]

// find — get the first element that passes a test
const firstBig = numbers.find(num => num > 3);
console.log(firstBig); // 4

Callbacks with Timers

// setTimeout — run once after a delay (milliseconds)
setTimeout(() => {
    console.log("This runs after 2 seconds");
}, 2000);

// setInterval — run repeatedly at an interval
let count = 0;
const intervalId = setInterval(() => {
    count++;
    console.log(`Tick ${count}`);
    if (count >= 5) {
        clearInterval(intervalId); // stop after 5 ticks
    }
}, 1000);

Callbacks with Event Listeners

// In a browser environment:
// document.getElementById("myButton").addEventListener("click", function(event) {
//     console.log("Button clicked!");
//     console.log(event.target); // the element that was clicked
// });

// Arrow function version:
// document.getElementById("myButton").addEventListener("click", (event) => {
//     console.log("Button clicked!");
// });

Summary

• Function declarations are hoisted — can be called before they appear in code
• Function expressions assign functions to variables — not hoisted
• Arrow functions provide concise syntax and inherit this from their scope
• Default parameters let you set fallback values: (name = "World")
• Rest parameters collect extra arguments: (...args)
• Functions return undefined unless you explicitly return a value
• Scope: global, function (local), and block (let/const inside { })
• Closures remember variables from their creation scope — essential for data privacy and factory patterns
• Callbacks are functions passed to other functions — used everywhere in JavaScript