JavaScript Variables

Variables are containers for storing data. JavaScript has three ways to declare variables: let, const, and var.

let — Reassignable Variable

let name = "Alice";
console.log(name);  // "Alice"

name = "Bob";       // Can be reassigned
console.log(name);  // "Bob"

const — Constant (Cannot Reassign)

const PI = 3.14159;
console.log(PI);  // 3.14159

PI = 3;  // ERROR! Cannot reassign a const

Use const by default. Only use let when you need to reassign.

var — The Old Way (Avoid)

var age = 25;
age = 26;  // Works

// var has quirks:
// - Function-scoped (not block-scoped)
// - Can be redeclared
// - Gets "hoisted"
// Use let and const instead!

Data Types

JavaScript variables can hold different types of data:

let name = "Alice";        // String
let age = 25;              // Number
let isStudent = true;      // Boolean
let score = null;          // Null (intentionally empty)
let address;               // Undefined (not assigned)
let items = [1, 2, 3];    // Array
let person = {             // Object
    name: "Alice",
    age: 25
};

Naming Rules

• Must start with a letter, _, or $
• Can contain letters, numbers, _, $
• Cannot use reserved words (let, function, return, etc.)
• Are case-sensitive