Static & Private Class Fields

class StateMachine {
  #state;
  static #validTransitions = new Map([
    ['idle', ['running', 'error']],
    ['running', ['idle', 'error']],
    ['error', ['idle']]
  ]);
  
  constructor() {
    this.#state = 'idle';
  }
  
  transition(newState) {
    const validTransitions = StateMachine.#validTransitions.get(this.#state);
    if (!validTransitions?.includes(newState)) {
      throw new Error('Invalid state transition');
    }
    this.#state = newState;
  }
}