What Is a Variable in Programming?

Variables are one of the most fundamental concepts in programming. They let your code remember and work with information—like a player's name, a game score, or whether someone is logged in.

A variable is a name your program attaches to a value, so it can read that value later and replace it with a different one.

Think of a variable as a labeled box that holds information. The label is the variable's name, and what's inside the box is its value.

Just like you might have a box labeled "Photos" that contains your pictures, you might have a variable called score that contains the number 100.

📦

Real-World Analogy

Imagine a storage locker facility. Each locker has a unique number (the variable name), and you can put things inside (the value). You can also take things out and replace them with something else—variables work the same way!

Creating Variables in JavaScript

In JavaScript, you create a variable using the let keyword, followed by the name you want to give it:

let score = 100;

Let's break this down:

let — Tells JavaScript "I'm creating a variable"
score — The name you chose for this variable
= — The assignment operator (puts the value in the box)
100 — The value being stored
; — Marks the end of the statement

Try It Yourself

Naming Variables

Variable names have rules and best practices:

Rules (Must Follow)

Must start with a letter, underscore (_), or dollar sign ($)
Can contain letters, numbers, underscores, and dollar signs
Cannot contain spaces or hyphens
Cannot be reserved keywords (like let, if, function)
Are case-sensitive (Score and score are different)

Best Practices (Should Follow)

Use descriptive names: playerScore instead of x
Use camelCase for multiple words: firstName, totalAmount
Start with lowercase letters
Make names meaningful but not too long

✓ Good Names

userName
totalPrice
isLoggedIn
itemCount

✗ Bad Names

x
data
thing
2fast

let, const, and var

JavaScript has three ways to declare variables:

let — Use this for variables that will change. Most common choice.
const — Use this for values that should never change (constants).
var — The old way. Avoid using this in modern code.

let score = 0;        // Can change later
const maxScore = 100;  // Should never change
score = 50;            // ✓ This works
maxScore = 200;        // ✗ Error! Can't change a const

Common Mistakes

Using a variable before declaring it:
```
console.log(name);
let name = "Sam";
// ReferenceError: Cannot access 'name' before initialization
```
A let or const variable exists from the top of its block, but you are not allowed to read it until the declaration line runs. That gap is called the temporal dead zone.
Forgetting to declare with let/const:
```
score = 100;
console.log(score);  // 100 — it works, but 'score' is now a global
```
Assigning to a name you never declared creates a global variable (and is an outright error in strict mode and inside JavaScript modules). Declare it instead:
```
let score = 100;
console.log(score);  // 100
```
Note that you cannot write both versions in the same block — a second let score for a name already in use is a SyntaxError: Identifier 'score' has already been declared.

Trying to change a const:

const pi = 3.14;
pi = 3.14159;
// TypeError: Assignment to constant variable.

Quick Quiz

Question 1 of 3Score: 0/0

Variables Quiz