JavaScript Functions and Scope

Function Declarations

// Function declaration (hoisted)
function greet(name) {
    return "Hello, " + name + "!";
}
console.log(greet("Alice")); // Hello, Alice!

// Function expression (not hoisted)
const add = function(a, b) {
    return a + b;
};

// Arrow function (ES6)
const multiply = (a, b) => a * b;

// Arrow function with body
const divide = (a, b) => {
    if (b === 0) return null;
    return a / b;
};

Parameters and Arguments

// Default parameters
function power(base, exp = 2) {
    return base ** exp;
}
power(3);    // 9
power(3, 3); // 27

// Rest parameters
function sum(...numbers) {
    return numbers.reduce((acc, n) => acc + n, 0);
}
sum(1, 2, 3, 4); // 10

// Destructuring parameters
function display({ name, age }) {
    console.log(`${name} is ${age}`);
}
display({ name: "Bob", age: 30 }); // Bob is 30

Scope

// Global scope
let globalVar = "I'm global";

function outer() {
    // Function scope
    let outerVar = "I'm in outer";

    function inner() {
        // Block scope
        let innerVar = "I'm in inner";
        console.log(globalVar);  // accessible
        console.log(outerVar);   // accessible (closure)
        console.log(innerVar);   // accessible
    }

    inner();
    // console.log(innerVar); // ERROR: not accessible
}

Closures

function makeCounter() {
    let count = 0;          // private variable
    return function() {
        count++;
        return count;
    };
}

const counter = makeCounter();
console.log(counter()); // 1
console.log(counter()); // 2
console.log(counter()); // 3

Immediately Invoked Function Expression (IIFE)

(function() {
    const secret = "hidden";
    console.log("IIFE runs immediately");
})();
// secret is not accessible outside

Higher-Order Functions

// Function that takes a function as argument
function doTwice(fn) {
    fn();
    fn();
}
doTwice(() => console.log("Hello"));

// Function that returns a function
function multiplier(factor) {
    return n => n * factor;
}
const double = multiplier(2);
console.log(double(5)); // 10

this Keyword

Key Takeaway: Functions are first-class citizens in JavaScript. Understand closures for data encapsulation, arrow functions for concise syntax, and this binding to write correct object-oriented code.