const counter = (function () {
  let privateCounter = 0;
  function changeBy(val) {
    privateCounter += val;
  }

  return {
    increment() {
      changeBy(1);
    },

    decrement() {
      changeBy(-1);
    },

    value() {
      return privateCounter;
    },
  };
})();

console.log(counter.value()); // 0.

counter.increment();
counter.increment();
console.log(counter.value()); // 2.

counter.decrement();
console.log(counter.value()); // 1.