Stopwatch in JS (Constructor Function)

const Stopwatch = function () {
    let states = { "reset": 0, "started": 1, "stopped": 2 }
    let startTime, endTime, duration, currentState

    this.start = function () {
        if (currentState === states["started"])
            throw new Error("Stopwatch has already started.")

        currentState = states["started"]
        startTime = Date.now()

        return "started..."
    }

    this.stop = function () {
        if (currentState !== states["started"])
            throw new Error("Stopwatch is not started.")

        currentState = states["stopped"]
        endTime = Date.now()

        return "stopped..."
    }

    this.reset = function () {
        startTime = undefined
        endTime = undefined
        duration = undefined
        currentState = states["reset"]

        return "reset..."
    }

    Object.defineProperty(this, "duration", {
        get: function () {
            if (currentState === states["stopped"])
                return (endTime - startTime) / 1000
            else if (currentState === states["started"])
                throw new Error("Clock has not been stopped.")
            else
                throw new Error("Clock has not been started yet.")
        }
    })
}