JavaScript

class EventEmitter {
  #listeners = new WeakMap();
  #registry = new FinalizationRegistry((label) => {
    console.log(`Cleaned up: ${label}`);
  });

  subscribe(target, callback) {
    if (!this.#listeners.has(target)) {
      this.#listeners.set(target, []);
    }
    this.#listeners.get(target).push(callback);
    this.#registry.register(target, target.name ?? "unknown");
  }

  emit(target) {
    const cbs = this.#listeners.get(target);
    if (cbs) cbs.forEach(cb => cb());
  }
}

const emitter = new EventEmitter();
let obj1 = { name: "sensor" };
let obj2 = { name: "timer" };

const ws = new WeakSet([obj1, obj2]);

emitter.subscribe(obj1, () => console.log("sensor fired"));
emitter.subscribe(obj2, () => console.log("timer fired"));
emitter.subscribe(obj1, () => console.log("sensor logged"));

emitter.emit(obj1);

console.log(ws.has(obj1));
obj1 = null;
console.log(ws.has({ name: "sensor" }));

function Vehicle(make, model, year) {
  this.make = make;
  this.model = model;
  this.year = year;
  this.describe = function () {
    return `${this.year} ${this.make} ${this.model}`;
  };
}

Vehicle.prototype.age = function (currentYear) {
  return currentYear - this.year;
};

const car = new Vehicle("Toyota", "Supra", 1998);
const bike = new Vehicle("Harley", "Sportster", 2005);

console.log(car.describe());
console.log(bike.age(2025));
console.log(car.constructor === Vehicle);
console.log(Object.getPrototypeOf(car) === Vehicle.prototype);

const p1 = new Promise((resolve) => {
  console.log("A");
  resolve("B");
});

const p2 = p1.then((val) => {
  console.log(val);
  return "C";
});

p2.then((val) => {
  console.log(val);
});

console.log("D");

class AppError extends Error {
  constructor(message, statusCode) {
    super(message);
    this.name = this.constructor.name;
    this.statusCode = statusCode;
  }
}

class ValidationError extends AppError {
  constructor(message) {
    super(message, 400);
    this.fields = [];
  }
}

function riskyOperation(value) {
  if (value === null) throw new ValidationError("Null value");
  if (value < 0) throw new AppError("Negative value", 422);
  return value * 2;
}

const results = [];

for (const val of [10, null, -5, 3]) {
  try {
    results.push(riskyOperation(val));
  } catch (e) {
    if (e instanceof ValidationError) {
      results.push(`Validation:${e.statusCode}`);
    } else if (e instanceof AppError) {
      results.push(`App:${e.statusCode}`);
    } else {
      results.push("Unknown");
    }
  }
}

console.log(results.join(" | "));

const p1 = new Promise((resolve) => {
  console.log("A");
  resolve("X");
});

const p2 = p1.then((val) => {
  console.log("B");
  return val + "Y";
});

const p3 = p2.then((val) => {
  console.log("C:", val);
});

console.log("D");

"use strict";

function createCounter() {
  let count = 0;

  return {
    increment() { count++; },
    decrement() { count--; },
    getCount() { return count; },
    reset: () => { count = 0; }
  };
}

const counter = createCounter();
counter.increment();
counter.increment();
counter.increment();
counter.decrement();

const { getCount, reset } = counter;

console.log(getCount());
reset();
console.log(counter.getCount());

const person = { name: "Carlos", scores: [10, 20, 30] };
const clone = { ...person };

clone.name = "Diana";
clone.scores.push(40);

const snapshot = Object.assign({}, person);
snapshot.name = "Elena";
snapshot.scores.push(50);

console.log(person.name);
console.log(person.scores.length);
console.log(clone.name);
console.log(clone.scores === person.scores);