Functions are the beating heart of JavaScript. They are reusable blocks of code built to perform a specific task. We can define them once and run them as many times as we need.

A Simple Analogy: Think of a coffee machine.

• Inputs (Parameters): You give it water and coffee grounds.
• The Process (Function Body): You press a button, and the machine does its work.
• The Output (Return Value): It gives you a cup of coffee.

A function is exactly like this reusable machine.

🔬 Anatomy of a Function

Before looking at the different types, let's break down the parts of a classic function:

//      (2)Function Name   (3)Parameter
//           |                |
function greet(name) {
  // (4)Function Body: The work to be done
  const message = `Hello, ${name}!`;
  return message; // (5)The return value
}
// (1)Keyword

• function Keyword: The word that tells JavaScript we are defining a function.
• Function Name: The name you choose for the function so you can call it later (e.g., greet).
• Parameters: The input variables the function needs to do its job (e.g., name).
• Function Body: The code inside the { ... } that makes up the function's logic.
• return Statement: The value that the function sends back after it's done. (This is optional).

🗂️ Ways to Define a Function

In modern JavaScript, there are 3 primary ways we define functions:

1. Function Declaration

This is the classic, most fundamental way to define a function.

Syntax:

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

// Calling the function
let result = calculateSum(5, 10); // result is 15

• Key Feature: Declarations are Hoisted. This means the browser knows about them before it executes the code. You can call a function declaration before you define it in the code!

2. Function Expression

With this method, we define a function (usually an anonymous one) and assign it to a variable.

Syntax: