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sidebar_position 2
title Variables: Giving JS a Memory
sidebar_label Variables
description Learn how to store and manage data using let and const.

Imagine you are moving into a new house. You have a lot of stuff, so you put your things into boxes and write labels on them (like "Kitchen Supplies" or "Books").

In JavaScript, a Variable is just a box with a label that holds a piece of information.

How to Create a Variable

To create a variable, we use a keyword, give it a name, and assign a value to it using the = sign.

let playerName = "Ajay";
let playerScore = 100;

The Three Parts of a Variable:

  1. The Keyword: (let or const) This tells the computer you are creating a new "box."
  2. The Label (Name): This is how you will refer to the data later.
  3. The Value: The actual data you are storing inside the box.

let vs const (The Golden Rule)

In modern JavaScript (2026), we use two main keywords. Choosing the right one is very important!

1. let (The Changeable Box)

Use let when the information inside the box will change later (like a score in a game or a timer).

let score = 0;
score = 10; // This is allowed!

2. const (The Locked Box)

Short for "Constant." Use const for information that should never change (like your birthday or a fixed setting). Pro Tip: If you aren't sure, use const first!

const birthYear = 2000;
birthYear = 2005; // ❌ ERROR! You can't change a constant.

Interactive Playground

Let's see how variables work in real-time. In this CodePen, we have a "Counter" app. One variable keeps track of the number!

Challenge Tasks:

  1. In the JS tab, look for let count = 0;. Change the 0 to 100. Notice the starting number changes!
  2. Try changing the keyword let to const. Click the "Increase" button. Does it still work? (Hint: Check your console for an error!)

Naming Rules (The "Dos and Don'ts")

You can't just name your variables anything. JavaScript has a few rules:

  • Do use camelCase: Start with a small letter, and every new word starts with a Capital (e.g., userProfilePicture).
  • Do use descriptive names: let price = 10; is better than let p = 10;.
  • Don't start with a number: let 1stPlace = "Gold"; will break your code.
  • Don't use spaces: let user name = "Ajay"; is not allowed.

Summary Checklist

  • [x] I understand that a variable is a container for data.
  • [x] I know that let is for values that change.
  • [x] I know that const is for values that stay the same.
  • [x] I practiced naming variables using camelCase.