Defining an object. Function vs Classes

stopWatch.js

function Stopwatch() {
  let isStarted = false;
  let startTime;
  let stopTime;

  Object.defineProperty(this, "duration", {
    get: () => {
      if(startTime && stopTime) {
        return (stopTime - startTime) / 1000;
      }
      if(startTime) {
        return (new Date().getTime() - startTime) / 1000;
      }
      throw new Error("You haven't initiate the timer.");
    },
    set: () => {
      throw new Error("You cannot set the duration by yourself.");
    },
  });

  this.reset = () => {
    if (isStarted) {
      this.stop();
    }
    stopTime = undefined;
    startTime = undefined;
  };

  this.start = () => {
    if (!isStarted) {
      isStarted = true;
      startTime = new Date().getTime();
    } else {
      throw new Error("This has already started.");
    }
  };

  this.stop = () => {
    if (isStarted) {
      isStarted = false;
      stopTime = new Date().getTime();
    } else {
      throw new Error("This has already stopped.");
    }
  };
}

class StopwatchClass {
  #isStarted = false;
  #startTime;
  #stopTime;

  constructor() {}

  get duration() {
    if(this.#startTime && this.#stopTime) {
      return (this.#stopTime - this.#startTime) / 1000;
    }
    if(startTime) {
      return (new Date().getTime() - this.#startTime) / 1000;
    }
    throw new Error("You haven't initiate the timer.");
  }

  set duration(_) {
    throw new Error("You cannot set the duration by yourself.");
  }

  reset() {
    if (this.#isStarted) {
      stop();
    } else {
      throw new Error("Cannot reset. This's already stopped.");
    }
    this.#stopTime = undefined;
    this.#startTime = undefined;
  }

  start() {
    if (!this.#isStarted) {
      this.#isStarted = true;
      this.#startTime = new Date().getTime();
    } else {
      throw new Error("This has already started.");
    }
  }

  stop() {
    if (this.#isStarted) {
      this.#isStarted = false;
      this.#stopTime = new Date().getTime();
    } else {
      throw new Error("This has already stopped.");
    }
  }
}