code: function createActivity(activityMessage) {
    if (activityMessage === undefined ||
        activityMessage === undefined ||
        typeof activityMessage !== 'string') {
      throw new Error('Creating a new activity requires an activity message.');
    }
  
    var activityId = uuid++;
    var activity = {
      id: activityId,
      message: activityMessage,
      timestamps: []
    };
  
    activities[activityId] = activity;
  
    return activityId;
  }
docs: Create a new Activity object with its own unique ID. Does not start the
  activity.
  @param {string} activityMessage The activity message to use.
  @return {number} Activity ID.