function Stopwatch () {
    var startTime = 0;
    var endTime = 0;
    var running = 0;
    var duration = 0;

    this.start = function() {
        if (running) {
            throw new Error('Stopwatch has alredy started');
        }

        startTime = new Date();
        running = true;
    };

    this.stop = function() {
        if (!running) {
            throw new Error('Stopwatch is not started');
        }

        endTime = new Date();
        running = false;
        duration = endTime.getTime() - startTime.getTime();
        this.duration = duration / 1000;
    };

    this.reset = function() {
        startTime = 0;
        endTime = 0;
        running = 0;
        duration = 0;
        this.duration = 0;
    };

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

        set: function(val) {
            duration = val;
        }
    })
}