JavaScript Variables

In the previous lesson, we learned the different ways to run JavaScript. Now let’s learn how to store data using variables.

📦 What is a Variable?

A variable is a named container that stores a value. You can think of it like a labeled box: you put a value inside, give the box a name, and use that name whenever you need the value again.

For example, instead of writing your score everywhere in your code, you store it once in a variable and refer to it by name.

✍️ Declaring a Variable

In modern JavaScript, you create a variable using const or let, followed by a name, an equals sign, and a value.

const userName = "Alex";
let score = 0;

Let’s break this down:

Once a variable is created, you can use its name anywhere in your code:

const userName = "Alex";
console.log(userName); // prints: Alex

Here we declare and then read the variable back:

• The first line creates userName and stores the value "Alex" in it.
• The second line passes userName to console.log, which looks up the stored value and prints Alex.

🔁 Changing a Variable’s Value

A variable created with let can be given a new value later. This is called reassigning.

let score = 0;
console.log(score); // prints: 0

score = 10;
console.log(score); // prints: 10

Let’s walk through what happens line by line:

• We declare score with let and set it to 0.
• The first console.log reads score and prints 0.
• We reassign score to 10. Because there is no let here, we are updating the existing variable, not creating a new one.
• The second console.log reads score again and prints the new value, 10.

A variable created with const cannot be reassigned. Trying to change it causes an error.

const userName = "Alex";
userName = "Sam"; // ❌ Error: cannot reassign a const

We create userName with const on the first line, then try to give it a new value on the second line. Because const values are locked once set, JavaScript stops with an error instead of changing it.

🏷️ Rules for Naming Variables

Variable names follow a few rules:

• Start with a letter, _, or $ - names cannot start with a number
• No spaces - use camelCase to join words, like firstName
• Case-sensitive - score and Score are different variables
• No reserved words - you cannot use keywords like const or let as names

// ✅ Correct
const firstName = "Alex";
const totalScore = 100;
const _count = 5;

// ❌ Wrong
const 1stName = "Alex";   // starts with a number
const first name = "Alex"; // contains a space

Here is what separates the valid names from the broken ones:

• firstName, totalScore, and _count all start with a letter or _ and contain no spaces, so they are valid.
• 1stName is invalid because it starts with a number.
• first name is invalid because the space splits it into two pieces JavaScript cannot read as one name.

⚠️ Common Mistakes to Avoid

🔧 Try It Yourself!

🧩 What You’ve Learned

• ✅ A variable is a named container that stores a value
• ✅ You create variables using const or let
• ✅ let variables can be reassigned, const variables cannot
• ✅ Variable names have rules and should be clear and descriptive
• ✅ JavaScript is case-sensitive, so naming matters

🚀 What’s Next?

You now know how to store data in variables. Next, we will look more closely at the difference between let and const and when to use each. Let’s continue to let vs const.