Variables and Data Types.

 That's great! Let's dive right into the very first steps of writing JavaScript: Variables and Data Types.

📝 Variables: Storing Information

A variable is essentially a named container for a value. In modern JavaScript, we use two main keywords to declare a variable:

Keyword	Purpose	Example
const	Declares a constant. The value cannot be changed after it's assigned. Use this by default!	const PI = 3.14159;
let	Declares a variable whose value can be changed (reassigned).	let score = 0;

Example:

// Declaring a constant (preferred)

const userName = "Alice";

// Declaring a variable that can be changed

let age = 25;

age = 26; // This is allowed!

// This would cause an error because PI is a constant!

// const PI = 3.14; 

// PI = 3.14159; 

🔢 Data Types: The Kinds of Values

Every value in JavaScript has a type. Here are the most common primitive data types:

 * String: Used for text. Always enclosed in quotes.

   const greeting = "Hello, world!";

const letter = 'A';

 * Number: Used for both integers (whole numbers) and floating-point numbers (decimals).

   const answer = 42;

const price = 19.99;

 * Boolean: Used for logic; represents only one of two values: true or false.

   let isAdult = true;

const isOverdue = false;

 * Undefined: Means the variable has been declared but has not been assigned a value yet.

   let city; // city is currently 'undefined'

 * Null: Means the variable has been explicitly assigned to represent "no value" or "nothing."

   const activeUser = null; 

Checking the Type

You can check the type of any value using the typeof operator:

console.log(typeof "Bob");    // Output: "string"

console.log(typeof 100);      // Output: "number"

console.log(typeof true);     // Output: "boolean"

console.log(typeof city);     // Output: "undefined"

💻 Activity: Try It Out!

 * Open your browser's Developer Tools (usually by pressing F12 or Ctrl+Shift+I/Cmd+Option+I).

 * Go to the Console tab.

 * Type and execute the following lines of code:

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const favoriteColor = "blue";

let year = 2025;

let isCodingFun = true;

console.log(favoriteColor);

console.log(year);

year = year + 1; // Change the value of the 'let' variable

console.log(year);