class StateMachine {
  constructor() {
    this.state = 'idle';
    this.transitions = {
      idle: { start: 'running' },
      running: { pause: 'paused', stop: 'stopped' },
      paused: { resume: 'running', stop: 'stopped' },
      stopped: { reset: 'idle' }
    };
  }
  
  transition(action) {
    const next = this.transitions[this.state]?.[action];
    if (next) this.state = next;
    return this.state;
  }
}

const sm = new StateMachine();
console.log(sm.transition('start'));
console.log(sm.transition('invalid'));
console.log(sm.transition('pause'));
console.log(sm.transition('resume'));
console.log(sm.transition('stop'));
console.log(sm.transition('reset'));