A stopwatch-like timer

timer.js

const Events = new Map();

function startEvent() {
  return {
    start: Date.now(),
    end: null,
  };
}

function endEvent(event) {
  if (!event || event.end !== null) {
    throw new Error('Unable to end event.');
  }

  return {
    ...event,
    end: Date.now(),
  };
}

class Timer {
  static IDLE = Symbol('IDLE');
  static ACTIVE = Symbol('ACTIVE');
  static PAUSED = Symbol('PAUSED');
  static STOPPED = Symbol('STOPPED');

  /**
   *
   */
  get state() {
    const events = Events.get(this);

    if (events.length === 0) {
      return Timer.IDLE;
    }

    const event = events[events.length - 1];

    if (event === null) {
      return Timer.STOPPED;
    }

    if (event.end === null) {
      return Timer.ACTIVE;
    }

    return Timer.PAUSED;
  }

  /**
   *
   */
  get active() {
    return this.state === Timer.ACTIVE;
  }

  /**
   *
   */
  get paused() {
    return (
      this.state === Timer.IDLE ||
      this.state === Timer.PAUSED ||
      this.state === Timer.STOPPED
    );
  }

  /**
   *
   */
  get stopped() {
    return this.state === Timer.STOPPED;
  }

  /**
   *
   */
  get time() {
    const events = Events.get(this);
    const total = events.reduce((subtotal, event) => {
      if (event === null) return subtotal;

      const end = event.end === null ? Date.now() : event.end;
      const duration = end - event.start;
      return subtotal + duration;
    }, 0);
    return total;
  }

  /**
   *
   */
  constructor() {
    Events.set(this, []);
  }

  /**
   *
   */
  start() {
    if (this.stopped || this.state === Timer.ACTIVE) return;

    const events = Events.get(this);
    const event = startEvent();
    Events.set(this, [...events, event]);
  }

  /**
   *
   */
  pause() {
    if (this.paused) return;

    const events = Events.get(this);
    const event = endEvent(events[events.length - 1]);
    Events.set(this, [...events.slice(0, -1), event]);
  }

  /**
   *
   */
  stop() {
    if (this.stopped) return;

    if (!this.paused) {
      this.pause();
    }

    const events = Events.get(this);
    const event = null;
    Events.set(this, [...events, event]);
  }
}