Functions in JS

What is a Function?

• A function in JavaScript is a reusable block of code designed to perform a specific task. Functions are executed when they are called or invoked
• Instead of writing the same code multiple times, we can define it once and reuse it.

Why Use Functions?

✅ Code reusability
✅ Better readability
✅ Easy debugging
✅ Modular programming

Basic Syntax

function functionName(parameters) {
    // code
    return value;
}

Example

function greet(name) {
    return "Hello " + name;
}

console.log(greet("Akalya"));

Output:

Hello Akalya

Types of Functions in JavaScript

1. Function Declaration

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

✔️ Can be called before declaration (hoisting)

2. Function Expression

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

• A function expression is a function stored in a variable
• The variable name can be used to call the function
• Cannot be called before declaration

3. Arrow Function (ES6)

const add = (a, b) => a + b;

• Arrow Functions is a short syntax for function expressions
• You can skip the function keyword
• You can skip the return keyword
• You can skip the curly brackets

4.Anonymous Function(Need to be discussed)

setTimeout(function() {
    console.log("Hello");
}, 1000);

Function without a name

5. Immediately Invoked Function (IIFE)(Need to be discussed)

(function() {
    console.log("Runs immediately");
})();

Executes as soon as it is defined

Parameters vs Arguments

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

add(2, 3);             // arguments

Function Parameters

• Parameters allow you to send values to a function
• Parameters are listed in parentheses in the function definition

Function Arguments

• Function parameters and arguments are distinct concepts
• Parameters are the names listed in the function definition
• Arguments are the values received by the function

Return Statement

• Used to send value back from function
• Stops execution after returning

function square(n) {
    return n * n;
}

Default Parameters

function greet(name = "Guest") {
    return "Hello " + name;
}

Function Scope

• Variables inside function → local scope
• Variables outside → global scope

let x = 10;

function test() {
    let y = 20;
    console.log(x); // accessible
}

Callback Functions(Need to be discussed)

Function passed as argument to another function

function greet(name, callback) {
    console.log("Hi " + name);
    callback();
}

function sayBye() {
    console.log("Bye!");
}

greet("Akalya", sayBye);

Higher Order Functions(Need to be discussed)

Functions that:

Take another function as argument OR
Return a function

Example: map(), filter()