Classes & Inheritance (ES6+)

8. Practice Problems

1. Write a Rectangle class with width, height and an area() method.
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   a1.js

   class Rectangle {
       constructor(w, h) { this.w = w; this.h = h; }
       area() { return this.w * this.h; }
   }
   console.log(new Rectangle(5, 3).area());

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2. Add a static square(side) factory method.
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   a2.js

   class Rectangle {
       constructor(w, h) { this.w = w; this.h = h; }
       area() { return this.w * this.h; }
       static square(s) { return new Rectangle(s, s); }
   }
   console.log(Rectangle.square(5).area());

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3. Make Cube extend Rectangle: add depth and override area to return surface area.
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   a3.js

   class Rectangle {
       constructor(w, h) { this.w = w; this.h = h; }
       area() { return this.w * this.h; }
   }
   class Cube extends Rectangle {
       constructor(s) { super(s, s); this.d = s; }
       area() { return 6 * super.area(); }
   }
   console.log(new Cube(3).area());   // 54

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4. Build a Stopwatch class with start/elapsed methods.
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   a4.js

   class Stopwatch {
       #t0 = null;
       start() { this.#t0 = Date.now(); }
       elapsed() { return Date.now() - this.#t0; }
   }
   const sw = new Stopwatch();
   sw.start();
   for (let i = 0; i < 1e6; i++);
   console.log("ms:", sw.elapsed());

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5. Show that calling this before super() in a derived constructor errors.
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   a5.js

   class A {}
   try {
       class B extends A {
           constructor() { this.x = 1; super(); }
       }
       new B();
   } catch (e) {
       console.log("err:", e.message);
   }

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6. Build a Queue with enqueue, dequeue, and a size getter.
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   a6.js

   class Queue {
       #items = [];
       enqueue(x) { this.#items.push(x); }
       dequeue() { return this.#items.shift(); }
       get size() { return this.#items.length; }
   }
   const q = new Queue();
   q.enqueue("a"); q.enqueue("b");
   console.log(q.size, q.dequeue(), q.size);

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7. Define a getter fullName on a Person class.
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   a7.js

   class Person {
       constructor(f, l) { this.f = f; this.l = l; }
       get fullName() { return `${this.f} ${this.l}`; }
   }
   console.log(new Person("Arif", "Hossain").fullName);

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8. Use static method fromJSON to build an instance from a string.
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   a8.js

   class User {
       constructor(name, age) { this.name = name; this.age = age; }
       static fromJSON(s) { const { name, age } = JSON.parse(s); return new User(name, age); }
   }
   console.log(User.fromJSON('{"name":"Arif","age":22}'));

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9. Inherit and override toString for a Money class.
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   a9.js

   class Money {
       constructor(n) { this.n = n; }
       toString() { return `৳${this.n}`; }
   }
   console.log(`Total: ${new Money(99)}`);

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10. Build a Logger hierarchy (Logger → DebugLogger).
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    a10.js

    class Logger {
        log(m) { console.log("[INFO]", m); }
    }
    class DebugLogger extends Logger {
        log(m) { console.log("[DEBUG]", m); }
    }
    new DebugLogger().log("hi");

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11. Why is composition often preferred over inheritance?
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    Answer: Inheritance creates rigid trees: a change in the base ripples through every descendant; behaviours are coupled to the hierarchy. Composition gives the same reuse with looser bonds: a class holds a collaborator as a field and forwards calls, so each piece can be changed independently. Modern frameworks (React hooks, dependency injection) are largely composition-first.

12. Use a private field to enforce a non-negative balance.
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    a12.js

    class Wallet {
        #b = 0;
        deposit(n) { if (n < 0) throw new Error("negative"); this.#b += n; }
        get balance() { return this.#b; }
    }
    const w = new Wallet();
    w.deposit(100);
    console.log(w.balance);
    try { w.deposit(-10); } catch (e) { console.log(e.message); }

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13. Test that a class instance is also an instance of Object.
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    a13.js

    class A {}
    const x = new A();
    console.log(x instanceof A, x instanceof Object);

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14. Class expression: assign a class to a variable and instantiate it.
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    a14.js

    const Box = class {
        constructor(v) { this.v = v; }
    };
    console.log(new Box(42).v);

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15. Use extends to add a method to all instances of Array… by subclassing.
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    a15.js

    class SafeArr extends Array {
        first() { return this[0]; }
        last()  { return this.at(-1); }
    }
    const a = SafeArr.from([10, 20, 30]);
    console.log(a.first(), a.last());

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16. Show that typeof Class === "function".
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    a16.js

    class A {}
    console.log(typeof A);   // "function"

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