Variables & Data Types

// const — value cannot be reassigned (use this by default)
const name = "Nelson";
const pi   = 3.14159;

// let — value can change (use when you know it will change)
let score = 0;
score = score + 10;  // valid
score = 50;           // valid

// var — the old way. Avoid it. Has confusing scoping behaviour.
var old = "don't use this";

// You cannot reassign a const
const age = 26;
age = 27;  // TypeError: Assignment to constant variable

// String — text
const firstName = "Nelson";
const message   = 'Single quotes work too';
const greeting  = `Template literal — embed: ${firstName}`;  // backticks

// Number — integers AND decimals (one type for both)
const age    = 26;
const price  = 9.99;
const bigNum = 1_000_000;  // underscore for readability

// Boolean
const isLoggedIn = true;
const isPremium  = false;

// null — intentional absence of a value
const selectedCourse = null;

// undefined — a variable declared but not given a value
let username;  // undefined
console.log(username);  // undefined

// typeof — check what type a value is
typeof "hello"     // "string"
typeof 42          // "number"
typeof true        // "boolean"
typeof null        // "object" (a famous JS bug — null is not an object)
typeof undefined   // "undefined"
typeof {}          // "object"
typeof []          // "object"
typeof function(){} // "function"