JavaScript Data Types

// String - text data
const name = "Alice";
const greeting = 'Hello';
const template = `Hi, ${name}!`;

// Number - integers and decimals
const age = 25;
const price = 19.99;
const negative = -10;
const infinity = Infinity;
const notANumber = NaN;

// Boolean - true or false
const isActive = true;
const isLoggedIn = false;

// Undefined - declared but not assigned
let user;
console.log(user); // undefined

// Null - intentionally empty
const data = null;

// Symbol - unique identifier (ES6)
const id = Symbol("userId");

// BigInt - large integers (ES2020)
const huge = 9007199254740991n;

// undefined - not assigned
let x;
console.log(x);  // undefined

function noReturn() {}
console.log(noReturn());  // undefined

const obj = {};
console.log(obj.missing);  // undefined

// null - intentionally empty
let user = null;  // "no user yet"

// Checking for both
if (value == null) {
  // true for both null and undefined
}

if (value === null) {
  // true only for null
}

if (value === undefined) {
  // true only for undefined
}

// Nullish coalescing (??)
const name = null ?? "Default";  // "Default"
const age = 0 ?? 18;            // 0 (0 is not nullish)

// Object - key-value pairs
const person = {
  name: "Alice",
  age: 25,
  isStudent: false
};

// Array - ordered list (special object)
const colors = ["red", "green", "blue"];
const mixed = [1, "two", true, null];

// Function - callable object
function greet(name) {
  return "Hello, " + name;
}

// Date, RegExp, Map, Set are also objects
const now = new Date();
const pattern = /hello/i;
const map = new Map();
const set = new Set([1, 2, 3]);

// Primitive Types
const str = "Hello, World!";     // String
const num = 42;                   // Number
const float = 3.14;               // Number (no separate float type)
const bool = true;                // Boolean
const nothing = null;             // Null
let notDefined;                   // Undefined
const sym = Symbol("id");         // Symbol
const bigNum = 9007199254740991n; // BigInt

console.log("String:", str, "| typeof:", typeof str);
console.log("Number:", num, "| typeof:", typeof num);
console.log("Float:", float, "| typeof:", typeof float);
console.log("Boolean:", bool, "| typeof:", typeof bool);
console.log("Null:", nothing, "| typeof:", typeof nothing);
console.log("Undefined:", notDefined, "| typeof:", typeof notDefined);
console.log("Symbol:", sym.toString(), "| typeof:", typeof sym);
console.log("BigInt:", bigNum, "| typeof:", typeof bigNum);

// Reference Types
const arr = [1, 2, 3];            // Array
const obj = { name: "Alice" };    // Object
const func = function() {};       // Function

console.log("\nArray:", arr, "| typeof:", typeof arr);
console.log("Object:", obj, "| typeof:", typeof obj);
console.log("Function:", func, "| typeof:", typeof func);

// typeof operator
typeof "hello"     // "string"
typeof 42          // "number"
typeof true        // "boolean"
typeof undefined   // "undefined"
typeof null        // "object" (bug!)
typeof {}          // "object"
typeof []          // "object"
typeof function(){} // "function"

// Array.isArray() for arrays
Array.isArray([1, 2, 3])  // true
Array.isArray("hello")    // false

// instanceof for objects
[] instanceof Array       // true
{} instanceof Object      // true
new Date() instanceof Date // true

// Checking for null
const value = null;
value === null  // true (use strict equality)

// typeof for primitives
console.log(typeof "hello");     // "string"
console.log(typeof 42);          // "number"
console.log(typeof true);        // "boolean"
console.log(typeof undefined);   // "undefined"
console.log(typeof Symbol());    // "symbol"
console.log(typeof 123n);        // "bigint"

// typeof quirks
console.log("\n--- typeof quirks ---");
console.log(typeof null);        // "object" (historical bug!)
console.log(typeof [1, 2, 3]);   // "object" (arrays are objects)
console.log(typeof {});          // "object"
console.log(typeof function(){}); // "function"

// Better type checking
console.log("\n--- Better type checking ---");
console.log(Array.isArray([1, 2, 3]));  // true
console.log(Array.isArray("hello"));    // false
console.log(null === null);             // true
console.log([1,2] instanceof Array);    // true
console.log({} instanceof Object);      // true

// Implicit type coercion
console.log("5" + 3);      // "53" (number to string)
console.log("5" - 3);      // 2 (string to number)
console.log("5" * "2");    // 10 (both to numbers)
console.log(true + 1);     // 2 (true becomes 1)
console.log(false + 1);    // 1 (false becomes 0)

// Truthy and falsy values
// Falsy: false, 0, "", null, undefined, NaN
// Everything else is truthy

if ("hello") console.log("Truthy!");  // Runs
if (0) console.log("Falsy!");         // Doesn't run

// Explicit conversion
String(123)       // "123"
Number("42")      // 42
Boolean(1)        // true
parseInt("42px")  // 42
parseFloat("3.14") // 3.14