Defective Stopwatch

This is an example of a JavaScript solution to Exercise[20].

New stopwatch!

// CODE DUMP

/*
 * Timer is a constructor function that may be
 * used to produces instances of objects of 
 * type Timer. An object of type Timer has
 * methods start() and stop() that start
 * and stop a timer, a method called reset
 * that resets a timer, and a method called
 * elaspedTimeInMs that returns an amount of
 * elapsed time. If a timer has been started
 * but not stopped, then elapsedTimeInMs
 * returns the amount of time (in milliseconds)
 * that has elapsed since the timer was started.
 * If a timer has been started and stopped,
 * then elapsedTimeInMs returns the amount of
 * time that elapsed between the time the
 * timer was started and stopped.
 */
function Timer() {
  this.reset();
}
Timer.prototype = {
  reset: function() {
    this.timeStarted = null;
    this.timeStopped = null;
  },
  start: function() {
    this.timeStarted = new Date();
  },
  stop: function() {
    this.timeStopped = new Date();
  },
  elapsedTimeInMs() {
    if (this.timeStarted != null) {
      if (this.timeStopped != null) {
        return this.timeStopped - this.timeStarted;
      }
      else {
        return new Date() - this.timeStarted;
      }
    }
    else {
      return undefined;
    }
  }
};

/*
 * pause does nothing for timeInMs millisends
 * before returning control to the code that
 * called pause. Internally, pause uses a
 * Timer object to wait the specified
 * amount of time before completing.
 */
function pause(timeInMs) {
  var timer = new Timer();
  timer.start();
  while (timer.elapsedTimeInMs() < timeInMs);
}

var stopwatch = new Timer();
var stopwatchIsRunning = false;

function output(message) {
  console.log(message);
  var display = document.getElementById("display");
  display.innerHTML = message;
}

function showCode() {
  var code = document.getElementById("code");
  code.setAttribute("style", "display:inline")
}

function hideCode() {
  var code = document.getElementById("code");
  code.setAttribute("style", "display:none")
}

function startStopwatch() {
  if (stopwatchIsRunning) {
    output("3");
    pause(1000);
    output("2");
    pause(1000);
    output("1");
    pause(1000);
    output("Snap!");
    showCode();
    stopwatchIsRunning = false;
  }
  else {
    hideCode();
    stopwatch.start();
    stopwatchIsRunning = true;
    output("Stopwatch is running...");
  }
}

function stopStopwatch() {
  stopwatch.stop();
  stopwatchIsRunning = false;
  output("Stopwatch has been stopped.");
}

function resetStopwatch() {
  hideCode();
  stopwatch.reset();
  stopwatchIsRunning = false;
  output("Stopwatch has been reset.");
}

function displayElapsedTime() {
  hideCode();
  output("Elapsed time (ms): " + stopwatch.elapsedTimeInMs());
}