function createCounter() {
  let count = 0;
  return {
    increment: () => ++count,
    decrement: () => --count,
    getValue: () => count
  };
}

const counter1 = createCounter();
const counter2 = createCounter();
counter1.increment();
counter1.increment();
counter2.increment();
console.log(counter1.getValue(), counter2.getValue());
counter1.decrement();
console.log(counter1.getValue(), counter2.getValue());

const wm = new WeakMap();
const obj1 = { name: 'first' };
const obj2 = { name: 'second' };
const obj3 = obj1;

wm.set(obj1, 'value1');
wm.set(obj2, 'value2');
wm.set(obj3, 'value3');

console.log(wm.get(obj1));
console.log(wm.get(obj2));
console.log(wm.get(obj3));
console.log(wm.has(obj1));
console.log(wm.size);

class CustomError extends Error {
  constructor(message) {
    super(message);
    this.name = 'CustomError';
  }
}

try {
  throw new CustomError('Something went wrong');
} catch (e) {
  console.log(e instanceof Error);
  console.log(e instanceof CustomError);
  console.log(e.constructor.name);
  console.log(e.name);
}

class EventEmitter {
  constructor() {
    this.events = {};
  }
  
  on(event, callback) {
    this.events[event] = this.events[event] || [];
    this.events[event].push(callback);
    return this;
  }
  
  emit(event, data) {
    if (this.events[event]) {
      this.events[event].forEach(cb => cb(data));
    }
    return this;
  }
}

class Logger {
  log(msg) { console.log(`[LOG]: ${msg}`); }
}

class DataProcessor {
  constructor(emitter, logger) {
    this.emitter = emitter;
    this.logger = logger;
    this.emitter.on('process', (data) => {
      this.logger.log(data.toUpperCase());
    });
  }
  
  process(data) {
    this.emitter.emit('process', data);
  }
}

const emitter = new EventEmitter();
const logger = new Logger();
const processor = new DataProcessor(emitter, logger);

processor.process('hello world');
emitter.emit('process', 'composition rocks');

const obj = { count: 0 };
const arr = [obj, obj, obj];

function increment(item) {
  item.count++;
  return item;
}

const results = arr.map(increment);
console.log(obj.count);
console.log(results[0] === results[1]);
console.log(results.length);
console.log(arr[0].count);