Introduction to
JavaScript

// Your very first JavaScript program
console.log("Hello, World!");
console.log("Welcome to JavaScript 🚀");

const name = "Alice";      // string — won't change
let age = 25;              // number — might change
let job = "Developer";    // can be reassigned

job = "Engineer";         // reassigning let is fine

console.log(name);
console.log(age);
console.log(job);

const greeting = "Hello";        // String
const score     = 42;             // Number
const isReady   = true;           // Boolean
let   nothing;                     // undefined (no value assigned)
const empty     = null;            // null (intentionally empty)
const colors    = ["red", "blue"]; // Array (a type of Object)

console.log(typeof greeting);  // "string"
console.log(typeof score);     // "number"
console.log(typeof isReady);   // "boolean"
console.log(typeof nothing);   // "undefined"
console.log(typeof empty);     // "object" (historical quirk!)
console.log(typeof colors);    // "object"

const name = "Alice";
const age  = 25;

// Old way (messy):
// "Hello, " + name + "! You are " + age + " years old."

// Modern way (clean):
const message = `Hello, ${name}! You are ${age} years old.`;
console.log(message);

const a = 10, b = 5;

console.log(a + b);   // 15  — addition
console.log(a - b);   // 5   — subtraction
console.log(a * b);   // 50  — multiplication
console.log(a / b);   // 2   — division
console.log(a % b);   // 0   — modulo (remainder) → 10%5=0, 10%3=1
console.log(a ** b);  // 100000 — exponentiation (10^5)

console.log(10 >  5);        // true  — greater than
console.log(10 <  5);        // false — less than
console.log(10 >= 10);       // true  — greater than or equal

// === checks value AND type (always prefer this)
console.log(5  === 5);       // true
console.log(5  === "5");     // false! number ≠ string
console.log(5  ==  "5");     // true  — == coerces types (avoid!)

// These 6 values are FALSY — everything else is truthy
const falsyValues = [0, "", false, null, undefined, NaN];

falsyValues.forEach(v => {
  console.log(`${v} is falsy`);
});

console.log("---");

// Truthy examples — watch out for these surprises!
const truthyValues = [42, "hello", [], {}];
truthyValues.forEach(v => {
  console.log(`${JSON.stringify(v)} is TRUTHY`);
});

console.log("--- AND (&&) ---");
// && returns the FIRST falsy value, or the last value if all truthy
console.log(false  && "hello");   // false  — stops at first falsy
console.log("hello" && "world");   // "world" — both truthy, returns last

console.log("--- OR (||) ---");
// || returns the FIRST truthy value, or the last value if all falsy
console.log("hello" || "world");   // "hello" — first truthy wins
console.log(0       || "Guest");   // "Guest" — 0 is falsy, falls through
console.log(""      || "Guest");   // "Guest" — "" is falsy, falls through

// || for default values (classic pattern)
const greet = (name) => console.log(`Hello, ${name || "Guest"}!`);
greet("Alice");   // Hello, Alice!
greet("");        // Hello, Guest!  — empty string is falsy

// && for conditional execution — "do this only if that is true"
const user = { name: "Alice", isAdmin: true };
user.isAdmin && console.log("User is an admin");

const loggedIn = false;
loggedIn && console.log("Dashboard loaded");
console.log("(nothing logged — user not logged in)");

const scores = { alice: 10, bob: 0 };  // Bob scored zero — valid!

console.log("--- Using || (problem) ---");
console.log(scores.alice || "N/A");   // 10  ← fine
console.log(scores.bob   || "N/A");   // "N/A" ← BUG! 0 is falsy

console.log("--- Using ?? (solution) ---");
console.log(scores.alice ?? "N/A");   // 10  ← fine
console.log(scores.bob   ?? "N/A");   // 0   ← correct!
// ?? only triggers for null / undefined — not 0 or ""

const users = [
  { name: "Alice", address: { city: "Warsaw" } },
  { name: "Bob"   }   // no address!
];

// Without optional chaining — would CRASH on Bob:
// console.log(users[1].address.city) → TypeError!

// With ?. — safe, returns undefined instead of crashing
console.log(users[0]?.address?.city);   // "Warsaw"
console.log(users[1]?.address?.city);   // undefined — no crash!
console.log(users[5]?.address?.city);   // undefined — index out of bounds, no crash!

console.log("--- Combined with ?? ---");
// ?. and ?? are best friends — access safely, then provide fallback
console.log(users[0]?.address?.city ?? "Unknown city");  // Warsaw
console.log(users[1]?.address?.city ?? "Unknown city");  // Unknown city

// Also works for optional method calls
const phone = users[1]?.contact?.getPhone?.() ?? "no phone on file";
console.log(phone);

const score = 87;

if (score >= 90) {
  console.log("Excellent! You got an A.");
} else if (score >= 75) {
  console.log("Great score! You passed with distinction.");
} else if (score >= 50) {
  console.log("You passed.");
} else {
  console.log("Try again next time.");
}

const day = "Monday";

switch (day) {
  case "Monday":
    console.log("Start your week strong! 💪");
    break;
  case "Friday":
    console.log("Almost the weekend! 🎉");
    break;
  default:
    console.log("Just another day.");
}

// Classic for loop: init ; condition ; increment
for (let i = 1; i <= 5; i++) {
  console.log(`Lap ${i}`);
}

// while loop — runs as long as condition is true
let count = 3;
while (count > 0) {
  console.log(`Countdown: ${count}`);
  count--;
}
console.log("Blast off! 🚀");

console.log("---");

// for...of — best way to loop over arrays
const fruits = ["Apple", "Banana", "Cherry"];
for (const fruit of fruits) {
  console.log(fruit);
}

// Define once...
function greet(name) {
  return `Hello, ${name}!`;
}

// ...call many times
console.log(greet("Alice"));
console.log(greet("Bob"));

// Traditional function:
// function square(n) { return n * n; }

// Arrow function — same thing, shorter:
const square = (n) => n * n;

// With multiple lines, use curly braces + return:
const circleArea = (radius) => {
  const pi = 3.14159;
  return pi * radius ** 2;
};

console.log(square(5));
console.log(circleArea(5).toFixed(2));