JavaScript Functions

Reusable blocks of code that perform specific tasks

🔧 What are Functions?

Functions are reusable blocks of code that perform specific tasks. Think of them as mini-programs that you can call whenever you need them. 


// Creating a simple function
function sayHello() {
    console.log("Hello, World!");
}

// Calling the function
sayHello(); // Output: Hello, World!

Function Features

🔄

Reusable

Write once, use many times 

function greet() {
    console.log("Hi!");
}
greet(); // Use it
greet(); // Use again
📥

Parameters

Accept input values 

function greet(name) {
    console.log("Hi " + name);
}
greet("Alice");
📤

Return Values

Send results back 

function add(a, b) {
    return a + b;
}
let result = add(5, 3);
🏠

Local Scope

Variables stay inside functions 

function test() {
    let x = 10; // Only inside
}
// x not available here

🔹 Basic Function Syntax

Here's how to create and use functions: 

// Function declaration
function functionName() {
    // Code to execute
    console.log("This is a function!");
}

// Call the function
functionName();

// Function with parameters
function greetUser(userName) {
    console.log("Welcome, " + userName + "!");
}

// Call with arguments
greetUser("Sarah");
greetUser("Mike");

Console Output:

This is a function!
Welcome, Sarah!
Welcome, Mike!

🔹 Functions with Return Values

Functions can calculate and return results: 

// Function that returns a value
function calculateArea(width, height) {
    let area = width * height;
    return area;
}

// Use the returned value
let roomArea = calculateArea(10, 12);
console.log("Room area:", roomArea + " square feet");

// Function that returns a greeting
function createGreeting(name, time) {
    return "Good " + time + ", " + name + "!";
}

let morning = createGreeting("Emma", "morning");
let evening = createGreeting("John", "evening");

console.log(morning);
console.log(evening);

Console Output:

Room area: 120 square feet
Good morning, Emma!
Good evening, John!

🔹 Function Expressions

Another way to create functions: 

// Function expression
const multiply = function(a, b) {
    return a * b;
};

// Arrow function (modern way)
const divide = (a, b) => {
    return a / b;
};

// Short arrow function
const square = x => x * x;

// Using the functions
console.log("Multiply:", multiply(4, 5));
console.log("Divide:", divide(20, 4));
console.log("Square:", square(6));

Console Output:

Multiply: 20
Divide: 5
Square: 36

🔹 Default Parameters

Set default values for parameters: 

// Function with default parameters
function orderCoffee(size = "medium", type = "regular") {
    return "One " + size + " " + type + " coffee, please!";
}

// Call with different arguments
console.log(orderCoffee());                    // Uses defaults
console.log(orderCoffee("large"));            // Uses default type
console.log(orderCoffee("small", "decaf"));   // Uses both arguments

// Function with default calculation
function calculateTip(bill, tipPercent = 15) {
    return bill * (tipPercent / 100);
}

console.log("Tip for $50:", "$" + calculateTip(50));
console.log("Tip for $30 at 20%:", "$" + calculateTip(30, 20));

Console Output:

One medium regular coffee, please!
One large regular coffee, please!
One small decaf coffee, please!
Tip for $50: $7.5
Tip for $30 at 20%: $6

🔹 Practical Function Examples

Real-world function examples: 

// Check if a number is even
function isEven(number) {
    return number % 2 === 0;
}

// Convert temperature
function celsiusToFahrenheit(celsius) {
    return (celsius * 9/5) + 32;
}

// Validate email (simple check)
function isValidEmail(email) {
    return email.includes("@") && email.includes(".");
}

// Generate random number
function randomBetween(min, max) {
    return Math.floor(Math.random() * (max - min + 1)) + min;
}

// Test the functions
console.log("Is 8 even?", isEven(8));
console.log("25°C in Fahrenheit:", celsiusToFahrenheit(25));
console.log("Is '[email protected]' valid?", isValidEmail("[email protected]"));
console.log("Random number 1-10:", randomBetween(1, 10));

Console Output:

Is 8 even? true
25°C in Fahrenheit: 77
Is '[email protected]' valid? true
Random number 1-10: 7

🧠 Test Your Knowledge

What keyword is used to send a value back from a function?