Lesson 2: Variables – var, let, and const

Learning Objectives

By the end of this lesson, learners will:

• Understand what variables are and why they’re used
• Know how to declare variables using var, let, and const
• Learn the differences between the three
• Understand the concepts of scope and reassignment
• Learn best practices for declaring variables

What Is a Variable?

A variable is a container that holds a value so you can use it or change it later in your program.

Think of it like a labeled box:
You write the label (userName), and you put something inside (like "Alice").

Declaring a Variable in JavaScript

There are three keywords to declare a variable in modern JavaScript:

Using let (Modern and Recommended)

let age = 25;
console.log(age); // 25

age = 26;
console.log(age); // 26

• You can reassign the value of a let variable.
• It is block-scoped, meaning it only works within the { } block it’s defined in.

Using const (For Fixed Values)

const birthYear = 2000;
console.log(birthYear); // 2000

// birthYear = 2001; ❌ This would cause an error!

• const variables cannot be reassigned.
• Great for values that should never change, like configuration settings or constants.

Using var (Old and Avoided)

var name = "Alice";
console.log(name); // Alice

var name = "Bob";
console.log(name); // Bob

• You can redeclare var variables — this can lead to bugs.
• It is function-scoped, not block-scoped.

✅ Best practice: Avoid var in modern JavaScript.

Examples of Variable Declarations

let city = "Berlin";          // string
const pi = 3.14159;           // number
let isStudent = true;         // boolean
let temperature;              // undefined

Variables must be declared before use

console.log(message); // ❌ ReferenceError
let message = "Hello!";

JavaScript reads top-down, so declare your variables before you try to use them.

Variable Naming Rules

1. Must start with a letter, underscore _, or dollar sign $
2. Can contain letters, numbers, _, and $
3. Cannot start with a number
4. Cannot be a reserved keyword (like let, return, function, etc.)

✅ Valid:

let userName;
let $price;
let _totalAmount;

❌ Invalid:

let 1stPlace;    // Cannot start with number
let let = 10;    // Cannot use reserved keyword

Scope: Where Variables Live

• Block scope: Inside {} — used by let and const
• Function scope: Entire function body — used by var

{
  let x = 10;
  console.log(x); // 10
}
console.log(x); // ❌ Error: x is not defined

Best Practices

Practice Exercise

Try this in your browser console:

let user = "Alice";
const year = 2025;

console.log(user); // Alice
user = "Bob";
console.log(user); // Bob

// year = 2030; ❌ Error: Assignment to constant variable

Summary Table