ক্লাস এবং ওওপি (Classes & OOP)

class Pet {
  constructor(name, type, age) {
    this.name = name;
    this.type = type;
    this.age = age;
    this.energy = 100;
  }

  // Method
  eat(food) {
    this.energy += 20;
    console.log(`${this.name} eats ${food}. Energy: ${this.energy}`);
  }

  sleep() {
    this.energy += 50;
    console.log(`${this.name} takes a nap. Energy: ${this.energy}`);
  }

  info() {
    return `${this.name} is a ${this.age}-year-old ${this.type}`;
  }
}

// Create instances with 'new'
const cat = new Pet("Mochi", "cat", 3);
const dog = new Pet("Pixel", "dog", 5);

console.log(cat.info());
cat.eat("tuna");
dog.sleep();
console.log(cat instanceof Pet); // true

Mochi is a 3-year-old cat
Mochi eats tuna. Energy: 120
Pixel takes a nap. Energy: 150
true

class Temperature {
  #celsius; // private field (starts with #)

  constructor(celsius) {
    this.#celsius = celsius;
  }

  // Getter — access like a property, runs like a function
  get fahrenheit() {
    return this.#celsius * 9/5 + 32;
  }

  // Setter — validates when you assign
  set celsius(value) {
    if (value < -273.15) {
      throw new RangeError("Below absolute zero!");
    }
    this.#celsius = value;
  }

  get celsius() {
    return this.#celsius;
  }

  // Static method — called on the class, not instances
  static fromFahrenheit(f) {
    return new Temperature((f - 32) * 5/9);
  }
}

const water = new Temperature(100);
console.log(water.fahrenheit); // 212 (getter, looks like a property)
console.log(water.celsius);    // 100

water.celsius = 0;             // setter runs validation
console.log(water.fahrenheit); // 32

// Static method — called on the CLASS
const body = Temperature.fromFahrenheit(98.6);
console.log(body.celsius.toFixed(1)); // 37.0

212
100
32
37.0

class Character {
  constructor(name, health) {
    this.name = name;
    this.health = health;
  }

  describe() {
    return `${this.name} (HP: ${this.health})`;
  }

  takeDamage(amount) {
    this.health -= amount;
    console.log(`${this.name} takes ${amount} damage! HP: ${this.health}`);
  }
}

class Warrior extends Character {
  constructor(name, health, weapon) {
    super(name, health); // call parent constructor FIRST
    this.weapon = weapon;
  }

  attack(target) {
    let damage = Math.floor(Math.random() * 20) + 10;
    console.log(`${this.name} swings ${this.weapon}!`);
    target.takeDamage(damage);
  }
}

class Mage extends Character {
  constructor(name, health, mana) {
    super(name, health);
    this.mana = mana;
  }

  // Override parent method
  describe() {
    return `${super.describe()} | Mana: ${this.mana}`;
  }

  castSpell(target) {
    if (this.mana < 10) {
      console.log(`${this.name} is out of mana!`);
      return;
    }
    this.mana -= 10;
    target.takeDamage(30);
    console.log(`${this.name} casts fireball! Mana left: ${this.mana}`);
  }
}

const knight = new Warrior("Roland", 100, "greatsword");
const wizard = new Mage("Elara", 70, 50);

console.log(knight.describe());
console.log(wizard.describe());
wizard.castSpell(knight);

Roland (HP: 100)
Elara (HP: 70) | Mana: 50
Roland takes 30 damage! HP: 70
Elara casts fireball! Mana left: 40