Simple Javascript Stopwatch constructor

stopwatch.js

function Stopwatch() {
    let start, stop, interval;
    let duration = 0;

    this.start = function() {
        if (start) {
            throw new Error('already started');
        }

        interval = setInterval(() => {
            duration += 100;
        }, 100);

        start = true;
        stop = false;
    }

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

        if (stop) {
            throw new Error('already stopped');
        }

        clearInterval(interval);
        stop = true;
        start = false;
    }

    this.reset = function() {
        interval = null;
        start = false;
        stop = false;
        duration = 0;
    }

    Object.defineProperty(this, 'duration', {
        get: function() {
            return duration / 1000;
        }
    })
};