function StopWatch() {

    let duration = 0;

    Object.defineProperty(this, 'duration', {
        get: function () {
            return duration;
        }
    });

    let status;

    let interval;


    this.start = function () {
        if (status == 'started')
            throw Error('Already started!');
        interval = setInterval(timer, 1000);
        status = 'started';
    }

    const timer = function () {
        duration++;
    }

    this.stop = function () {
        if (status == 'stopped' || (typeof status) == 'undefined')
            throw Error('Already stopped!')
        clearInterval(interval);
        status = 'stopped';
    }
}
const sw = new StopWatch();