JavaScript Variables & Data Types

Declaring data with let, const, and understanding primitive types

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Variables are containers for storing data (storing data values). JavaScript variables can be declared using let, const, or var.

let vs const vs var

const: Used for variables whose values should never change (block-scoped). Always use const by default unless you know the value will be reassigned.

let: Used for variables whose values will change over time (block-scoped).

var: The older declaration keyword (function-scoped). Rarely used in modern ES6+ code.

Variables in JavaScript

Try It Yourself »

// Declaring constants
const siteName = "Codexia";
const maxScore = 100;

// Declaring reassignable variables
let userXp = 40;
let isCompleted = false;

// Performing arithmetic
userXp += 25; // userXp is now 65

console.log("Welcome to", siteName);
console.log("Current XP:", userXp);

Data Type	Description	Example
String	Textual characters wrapped in quotes	"Hello World"
Number	Integers and floating point numbers	42, 3.14
Boolean	Logical true or false	true, false
Object	Key-value dictionary structure	{ name: "John", age: 30 }
Array	Ordered list of items	[1, 2, 3, 4]
Undefined	Variable declared without a value	let x;
Null	Explicit representation of empty or nothing	null

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Test Yourself With Exercise

+15 XP

Which keyword should you use to declare a variable whose reference should NOT be reassigned?

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