beginner

JavaScript Variables

Learn to declare variables using let, const, and var in JavaScript.

Variables are the building blocks of JavaScript applications. They store data that your program can use and manipulate. Modern JavaScript offers three ways to declare variables: let, const, and var, each with different behaviors and use cases. Understanding when to use each one is crucial for writing clean, bug-free code. This exercise teaches you the fundamentals of JavaScript variable declaration.

📚 Concepts & Theory

Variable Declaration:

const - For values that won't change:

const PI = 3.14159;

const API_URL = "https://api.example.com";

// PI = 3.14; // Error! Cannot reassign

let - For values that will change:

let score = 0;

score = 10;  // OK!

let count = 1;

count++;     // Now count is 2

var - Old style (avoid in modern code):

var name = "Alice"; // Function-scoped

// Has hoisting issues - prefer let/const

Best Practices:

Use const by default

Use let when you need to reassign

Avoid var in modern JavaScript

Naming Conventions:

// camelCase for variables

let userName = "John";

let totalPrice = 99.99;

// UPPER_SNAKE_CASE for constants

const MAX_SIZE = 100;

const API_KEY = "abc123";

🎯 Your Challenge

Declare a constant called `SITE_NAME` with the value "CodeStarter". Then declare a variable with `let` called `visitorCount` and set it to 0. Finally, increase `visitorCount` by 1.

📝 Starter Code

JavaScript

// Declare a constant for site name


// Declare a variable for visitor count


// Increase visitor count by 1

Use const for values that never change
Use let for values that will change
UPPER_SNAKE_CASE is convention for constants
++ adds 1 to a variable

Solution

JavaScript

const SITE_NAME = "CodeStarter";

let visitorCount = 0;

visitorCount++;

Explanation

We use const for SITE_NAME because it should never change. We use let for visitorCount because it will be updated. The ++ operator is shorthand for adding 1 to a variable (equivalent to visitorCount = visitorCount + 1).

⚠️ Common Mistakes to Avoid

Using var instead of let/const
Trying to reassign a const
Forgetting to use camelCase
Not initializing variables before use

❓ Frequently Asked Questions

let is block-scoped and doesn't have hoisting issues, while var is function-scoped and gets hoisted. Modern JavaScript best practices recommend using let over var.

Yes! const prevents reassignment of the variable, but object properties can still be modified. Only the reference is constant, not the contents.