class Range {
  constructor(start, end, step = 1) {
    this.start = start;
    this.end = end;
    this.step = step;
  }

  [Symbol.iterator]() {
    let current = this.start;
    const { end, step } = this;
    return {
      next() {
        if (current <= end) {
          const value = current;
          current += step;
          return { value, done: false };
        }
        return { value: undefined, done: true };
      }
    };
  }
}

const range = new Range(1, 10, 3);
const result = [...range].map(n => n ** 2);
console.log(result);

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 EventEmitter {
  #listeners = new Map();

  on(event, listener) {
    if (!this.#listeners.has(event)) {
      this.#listeners.set(event, []);
    }
    this.#listeners.get(event).push(listener);
    return this;
  }

  emit(event, ...args) {
    const handlers = this.#listeners.get(event) ?? [];
    handlers.forEach(fn => fn(...args));
    return this;
  }
}

const emitter = new EventEmitter();
const log = [];

emitter
  .on("data", val => log.push(`A:${val}`))
  .on("data", val => log.push(`B:${val * 2}`))
  .on("end",  ()  => log.push("done"));

emitter.emit("data", 5).emit("data", 3).emit("end");

console.log(log.join(","));