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

class Pipeline {
  #value;
  #log = [];

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

  map(fn) {
    this.#value = fn(this.#value);
    this.#log.push(`map:${this.#value}`);
    return this;
  }

  filter(fn) {
    if (Array.isArray(this.#value)) {
      this.#value = this.#value.filter(fn);
      this.#log.push(`filter:${this.#value}`);
    }
    return this;
  }

  reduce(fn, init) {
    if (Array.isArray(this.#value)) {
      this.#value = this.#value.reduce(fn, init);
      this.#log.push(`reduce:${this.#value}`);
    }
    return this;
  }

  result() {
    console.log(this.#log.join(' | '));
    return this.#value;
  }
}

new Pipeline([1, 2, 3, 4, 5, 6])
  .filter(n => n % 2 === 0)
  .map(arr => arr.map(n => n ** 2))
  .reduce((acc, n) => acc + n, 0)
  .result();

const name = "Taylor";
const age = 28;
const role = "engineer";

const user = {
  name,
  age,
  role,
  greet() {
    return `Hi, I'm ${this.name}, age ${this.age}`;
  },
  get info() {
    return `${this.role.toUpperCase()} @ ${this.age}`;
  }
};

const { name: userName, info, greet } = user;

console.log(userName);
console.log(info);
console.log(greet.call({ name: "Jordan", age: 35 }));

const handler = {
  get(target, prop, receiver) {
    if (prop in target) {
      const value = Reflect.get(target, prop, receiver);
      return typeof value === "number" ? value * 2 : value;
    }
    return `Property "${prop}" not found`;
  },
  set(target, prop, value) {
    if (typeof value !== "number") {
      throw new TypeError("Only numbers allowed");
    }
    Reflect.set(target, prop, value ** 2);
    return true;
  },
  has(target, prop) {
    return prop.startsWith("_") ? false : prop in target;
  },
};

const store = new Proxy({ _secret: 99, score: 5 }, handler);

store.level = 3;

console.log(store.score);
console.log(store.level);
console.log("_secret" in store);
console.log("score" in store);
console.log(store._secret);

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(","));

const createModule = (() => {
  const privateCache = new WeakMap();

  return function(name) {
    const state = { name, version: 1, active: true };
    privateCache.set(state, { accessCount: 0 });

    return {
      getInfo() {
        const meta = privateCache.get(state);
        meta.accessCount++;
        return `${state.name}@v${state.version}`;
      },
      getAccessCount() {
        return privateCache.get(state).accessCount;
      },
      upgrade() {
        state.version++;
        return this;
      }
    };
  };
})();

const mod = createModule("auth");
mod.upgrade().upgrade();
console.log(mod.getInfo());
console.log(mod.getAccessCount());