Variables & Data Types

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

name = "Bob";         // Reassigning is allowed
console.log(name);    // => "Bob"

const birthYear = 1990;
console.log(birthYear);  // => 1990

// birthYear = 1991;     // ❌ TypeError: Assignment to constant variable

// BUT: const objects/arrays CAN be mutated
const person = { name: "Alice" };
person.name = "Bob";     // ✅ This works — the object itself is not frozen
person.age = 30;         // ✅ Adding properties is allowed
console.log(person);     // => { name: "Bob", age: 30 }

var oldWay = "avoid this";

// ✅ Valid names
let userName = "Alice";         // camelCase (JavaScript convention)
let $price = 9.99;             // $ is allowed
let _count = 42;               // _ is allowed
let tempInCelsius = 25;        // Descriptive names

// ❌ Invalid names
// let 1stPlace = "gold";      // Can't start with a digit
// let my-var = "test";        // Hyphens not allowed
// let class = "math";         // Reserved keyword

// Case-sensitive
let myVar = 1;
let myvar = 2;                 // Different variable!

// 1. String
let greeting = "Hello";
let name = 'Alice';
let template = `Hello, ${name}!`;  // Template literal
console.log(template);              // => "Hello, Alice!"

// 2. Number (includes integers and decimals)
let integer = 42;
let decimal = 3.14;
let negative = -10;
let scientific = 1.5e6;            // 1,500,000

// 3. Boolean
let isActive = true;
let isComplete = false;

// 4. Undefined (variable declared but not assigned)
let something;
console.log(something);             // => undefined

// 5. Null (intentional absence of value)
let empty = null;

// 6. Symbol (unique identifier, advanced)
const id1 = Symbol("id");
const id2 = Symbol("id");
console.log(id1 === id2);           // => false (each Symbol is unique)

// 7. BigInt (very large integers)
const bigNumber = 9007199254740991n;
const anotherBig = BigInt("12345678901234567890");

console.log(typeof "hello");       // => "string"
console.log(typeof 42);            // => "number"
console.log(typeof true);          // => "boolean"
console.log(typeof undefined);     // => "undefined"
console.log(typeof null);          // => "object"  (this is a JS bug, null is primitive)
console.log(typeof Symbol("id"));  // => "symbol"
console.log(typeof 123n);          // => "bigint"
console.log(typeof console.log);   // => "function"

let value = "Hello";
console.log(typeof value);     // => "string"

value = 42;
console.log(typeof value);     // => "number"

value = true;
console.log(typeof value);     // => "boolean"

// Implicit coercion
console.log("5" + 3);      // => "53"     (number converted to string for +)
console.log("5" - 3);      // => 2        (string converted to number for -)
console.log("5" * "2");    // => 10       (both strings converted to numbers)
console.log(5 + true);     // => 6        (true converted to 1)
console.log(5 + false);    // => 5        (false converted to 0)

// Loose equality (==) — uses coercion
console.log(5 == "5");     // => true     (coerces string to number)

// Strict equality (===) — no coercion (ALWAYS USE THIS)
console.log(5 === "5");    // => false    (different types)
console.log(5 === 5);      // => true

// String to number
console.log(Number("42"));          // => 42
console.log(Number("hello"));       // => NaN (Not a Number)
console.log(parseInt("42px"));      // => 42
console.log(parseFloat("3.14em"));  // => 3.14

// Number to string
console.log(String(42));            // => "42"
console.log((42).toString());       // => "42"

// To boolean
console.log(Boolean(1));            // => true
console.log(Boolean(0));            // => false
console.log(Boolean(""));           // => false
console.log(Boolean("hello"));      // => true

const name = "Alice";
const age = 30;

// String interpolation
console.log(`My name is ${name} and I am ${age} years old.`);
// => "My name is Alice and I am 30 years old."

// Expressions inside ${}
console.log(`Next year I will be ${age + 1}.`);
// => "Next year I will be 31."

// Multi-line strings
const message = `
  Hello ${name},
  Welcome to our platform.
  We're glad to have you!
`;

// 1. Declare a const with your name (string)
// 2. Declare a let with your age (number)
// 3. Create a const that stores whether you're a student (boolean)
// 4. Log them all using template literals
// 5. Check the type of each variable using typeof
// 6. Try: what is '10' - 5? What is '10' + 5? Why are they different?

const myName = "Your Name";
let myAge = 25;
const isStudent = true;

console.log(`Name: ${myName}, Age: ${myAge}, Student: ${isStudent}`);
console.log(typeof myName);   // => "string"
console.log(typeof myAge);    // => "number"