A private seat counter

A private #available field prevents external code from editing the count directly. Public methods enforce a rule: no more reservations once seats run out. A getter reads the state safely.

class SeatCounter {
  #available;
  constructor(seats) { this.#available = seats; }
  reserve() {
    if (this.#available === 0) return false;
    this.#available -= 1;
    return true;
  }
  get available() { return this.#available; }
}
const counter = new SeatCounter(1);
console.log(counter.reserve());
console.log(counter.reserve());
console.log(counter.available);

What to notice

Try starting with two seats. Writing counter.#available outside the class would be a syntax error; do not put it in runnable code. Next, build a stack with a private array.