JS Data Types

// String — text
const name = "Alice";
const greeting = `Hello, ${name}!`;  // template literal

// Number — integers and decimals (one type)
const age   = 25;
const price = 9.99;
const big   = 1_000_000;  // underscores for readability

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

// Undefined — declared but not assigned
let x;
console.log(x);  // undefined

// Null — intentional absence of value
const data = null;

// BigInt — integers too large for Number
const huge = 9007199254740993n;

// Symbol — unique identifier (advanced)
const id = Symbol("id");

// Object
const user = { name: "Alice", age: 25 };

// Array (is an object)
const colours = ["red", "green", "blue"];

// Function (is an object)
function add(a, b) { return a + b; }

typeof "hello"    // "string"
typeof 42         // "number"
typeof true       // "boolean"
typeof undefined  // "undefined"
typeof null       // "object"  ← famous JS quirk
typeof []         // "object"
typeof {}         // "object"
typeof function(){} // "function"

// Better check for arrays:
Array.isArray([1, 2, 3])  // true

"5" + 3    // "53"  (number coerced to string)
"5" - 3    // 2     (string coerced to number)
"5" * "2"  // 10
Boolean("")        // false
Boolean("hello")   // true
Boolean(0)         // false
Boolean(1)         // true