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());

const str = "JavaScript is Awesome!";

const result = str
  .split(" ")
  .map((word, i) => 
    i % 2 === 0
      ? word.toUpperCase()
      : word.toLowerCase()
  )
  .join("-");

const reversed = result
  .split("")
  .reduce((acc, char) => char + acc, "");

console.log(reversed);

const delay = (ms, val) => new Promise(res => setTimeout(res, ms, val));

async function* asyncGen() {
  yield await delay(10, "alpha");
  yield await delay(10, "beta");
  yield await delay(10, "gamma");
}

async function run() {
  const results = [];

  const gen = asyncGen();
  const [first, , third] = await Promise.all([
    gen.next(),
    gen.next(),
    gen.next()
  ]);

  results.push(first.value, third.value);

  const p1 = Promise.resolve("x").then(v => v + "1");
  const p2 = Promise.reject("err").catch(e => e + "2");

  results.push(...(await Promise.all([p1, p2])));
  console.log(results);
}

run();