Start with the idea

Use const when the binding will not be reassigned and let when it will. A const object can still have its properties changed; const protects the binding, not every value inside it.

Step-by-step lesson notes

Variables in JavaScript

Variables are containers for storing data values. In modern JavaScript (ES6+), we have three ways to declare variables: var, let, and const.

var, let, and const

// var — function-scoped, avoid in modern code
var name = "Alice";

// let — block-scoped, can be reassigned
let score = 100;
score = 200; // ✅ Allowed

// const — block-scoped, cannot be reassigned
const PI = 3.14159;
// PI = 3; // ❌ TypeError: Assignment to constant variable

Data Types

JavaScript is dynamically typed — variables can hold any type of data, and the type can change at runtime.

// Primitive Types
let text = "Hello";          // String
let age = 25;                 // Number
let price = 19.99;            // Number (no separate float)
let isActive = true;          // Boolean
let nothing = null;           // Null
let notDefined;               // Undefined
let id = Symbol("id");        // Symbol (ES6)
let bigNum = 9007199254740991n; // BigInt (ES2020)

// Reference Types
let arr = [1, 2, 3];          // Array
let obj = { name: "Bob" };    // Object
let fn = () => {};            // Function

console.log(typeof text);     // "string"
console.log(typeof age);      // "number"
console.log(typeof isActive); // "boolean"
console.log(typeof nothing);  // "object" (historical quirk!)
console.log(typeof notDefined); // "undefined"

Variable Naming Rules

• Must start with a letter, underscore (_), or dollar sign ($)
• Can contain letters, digits, underscores, and dollar signs
• Case-sensitive (myVar and myvar are different)
• Cannot use reserved keywords (let, const, return, etc.)
• Use camelCase by convention: myVariableName

let firstName = "Alice";    // ✅ camelCase (preferred)
let _private = 42;           // ✅ underscore prefix
let $price = 9.99;           // ✅ dollar sign

// let 2fast = true;         // ❌ starts with digit
// let my-name = "Bob";      // ❌ hyphens not allowed

A complete example

Run this complete index.js program in the JavaScript console editor below. The earlier reference fragments may need a browser page or other surrounding setup.

const learner = "Kavi";
let completed = 1;
completed = completed + 2;
console.log(learner + ": " + completed);

Kavi: 3

Make one change. See what happens.

Load the example, change a value, predict the output, and run it again. Use the console output to check your reasoning.

Wait for the editor to load, then load this example and press Run.

Powered by the free OneCompiler editor. Code you run is sent to that external service. Keep personal data and secret keys out of your examples.

Put the idea to work

Your learning checkpoint

• Explain the example in your own words.
• Complete the challenge before revealing its answer.
• Describe the common mistake and how to avoid it.