Closures for private state

closures.js

const makeCounter = () => {
  let count = 0;
  
  return {
    get() {
      return count;
    },
    increment() {
      count += 1;
    },
    decrement() {
      count -= 1;
    }
  }
}

const counter1 = makeCounter();

counter1.get(); // 0
counter1.increment();
counter1.get(); // 1

const counter2 = makeCounter();
counter2.get(); // 0
counter2.decrement(); // -1

// note that counter1 hasn't changed. They have separate values