Variables

Quick Summary

Variables are named containers for data. In JavaScript, use const for values that should not be reassigned and let for values that will change. Initialize variables when you declare them to avoid undefined, and choose clear, descriptive names so your code reads like a sentence. Most of the time, use const by default and switch to let only when reassignment is required.

Understanding Variables

Think of a variable as a labeled box that holds information. The label (variable name) helps you find and use the information later.

Declaring Variables

JavaScript offers three ways to declare variables:

const - For values that won't change (immutable binding):

const PI = 3.14159;
const apiUrl = "https://api.example.com";
const maxRetries = 3;

let - For values that will change:

let score = 0;
let currentUser = null;
let isLoading = true;

var - The old way (avoid in modern code):

var oldStyleVariable = "not recommended";

Naming Conventions

Good variable names make code self-documenting:

| Bad | Good | Why | |-----|------|-----| | x | userAge | Descriptive | | data | productList | Specific | | temp | previousValue | Meaningful | | flag | isAuthenticated | Clear intent |

camelCase Convention

JavaScript uses camelCase for variable names:

First word lowercase, subsequent words capitalized
firstName, totalAmount, isUserLoggedIn

const vs let Decision Tree

Will this value ever need to change?
- No → Use const
- Yes → Use let
Default to const — it prevents accidental reassignment and signals intent

Try It Yourself

Here's a practical example you can try. Copy this code and run it in your browser's console (press F12 to open developer tools) or in the Code Playground.

// Use const for values that won't change const appName = "TaskManager"; const maxFileSize = 5 * 1024 * 1024; // 5MB in bytes // Use let for values that will change let currentStep = 1; let userInput = ""; let isFormValid = false; // Variables can be reassigned with let currentStep = 2; isFormValid = true; // But const prevents reassignment // appName = "NewName"; // Error! // Initialize with meaningful defaults let items = []; let errorMessage = null; let retryCount = 0;