JavaScript code snippet code review and refactoring. Snippet DID NOT have tests. NO guarantee that changes doesn't break it.

viewable_dates.js

set_viewable_dates: function (task, month_to_check, now) {
  /*
     it will set:
     start_to_show, end_to_show (and postponed_to_show)  | <-- moment() dates |
     variables to be used in list template,
     so that original start, end (and postponed) values of a task wouldn't be affected

     if a tasks start, end, postponed values span over month_to_check,
     start, end, postponed will be reduced to only contain values which cover month_to_check
     so that start_to_show <= month_to_check >= end_to_show
     */
  var tmp = {};
  tmp.task_start = moment(_.cloneDeep(task.start));
  tmp.task_end = moment(_.cloneDeep(task.end));


  // start date which will be used in list view only
  // set start to be the first day of current month,
  // if tasks begins before the start of the month in question
  if (tmp.task_start.month() < month_to_check) {
    task.start_to_show =
      moment()
      .year(tmp.task_start.year())
      .month(month_to_check)
      .date(1)
      .hour(0)
      .minute(0)
      .second(0);
  }
  else {
    task.start_to_show = moment(_.cloneDeep(tmp.task_start));
  }
  // change start to today
  // if task has begun before today
  //if (task.start_to_show.diff(now, 'days') < 0) {
  //    task.start_to_show = now;
  //    task.start_to_show.hour(0).minute(0).second(0);
  //    task.set_to_add_all_day = true; // set task to all day, as it has begun earlier than today
  //}

  /*
     TODO: check if postponed exists -
     TODO: probably could set it to end_to_show without going through the two following ifs
     as postponed > end
     */
  // end date which will be used in list view only
  // set end to be num of days of current month,
  // if end spans over current month
  if (tmp.task_end.month() > month_to_check) {
    task.end_to_show =
      moment()
      .year(tmp.task_end.year())
      .month(month_to_check)
      .date(service.get_days_in_month(tmp.task_end.year(), month_to_check))
      .hour(23)
      .minute(59)
      .second(59);
  }
  else {
    task.end_to_show = moment(_.cloneDeep(tmp.task_end));
  }
  // postponed date which will be used in list view only
  // set postponed to be num of days of current month,
  // if postponed spans over current month
  if (task.postponed) {
    tmp.task_postponed = moment(_.cloneDeep(task.postponed));

    if (tmp.task_postponed.month() > month_to_check) {
      task.postponed_to_show =
        moment()
        .year(tmp.task_postponed.year())
        .month(month_to_check)
        .date(service.get_days_in_month(tmp.task_postponed.year(), month_to_check))
        .hour(23)
        .minute(59)
        .second(59);
    }
    else {
      task.postponed_to_show = moment(_.cloneDeep(tmp.task_postponed));
    }
    task.end_to_show = service.get_task_end(task);
  }

  return task;
},

  list_contains_date: function (output_task_list, end_d) {
    if (output_task_list.length) {
      for (var day = 0; day < output_task_list.length; day++) {
        if (output_task_list[day].date.year() === end_d.year() &&
            output_task_list[day].date.month() === end_d.month() &&
            output_task_list[day].date.date() === end_d.date()) {
          return true;
        }
      }
    }
    return false;
  },

  check_and_create_date: function (output_task_list, date_to_add) {
    if (!service.list_contains_date(output_task_list, date_to_add)) {
      output_task_list.push(
          {
            'date': moment(date_to_add),
            'tasks': [],
            'tasks_all_day': []
          }
          );
    }
  },

  add_task_to_single_day_list: function (output_task_list, task, end) {
    for (var j = 0; j < output_task_list.length; j++) {
      // compare only y/m/d
      if (output_task_list[j].date.year() === end.year() &&
          output_task_list[j].date.month() === end.month() &&
          output_task_list[j].date.date() === end.date()) {

        output_task_list[j].tasks.push(task);
        break;
      }
    }
  },

  add_task_to_all_day_list: function (output_task_list, task, end) {
    for (var j = 0; j < output_task_list.length; j++) {
      // compare only y/m/d
      if (moment(output_task_list[j].date).get('year') === moment(end).get('year') &&
          moment(output_task_list[j].date).get('month') === moment(end).get('month') &&
          moment(output_task_list[j].date).get('date') === moment(end).get('date')) {


        output_task_list[j].tasks_all_day.push(task);
        break;
      }
    }
  },

  get_days_in_month: function (year, month) {
    // month 0 by default returns January
    // if day is set to 0, month 1 will return January as well
    return new Date(year, month + 1, 0).getDate();
  },

  get_task_end: function (task) {
    var p = task.postponed_to_show ? task.postponed_to_show : task.postponed;
    var e = task.end_to_show ? task.end_to_show : task.end;

    if (p) {
      var max = moment.max(p, e);
      return moment(_.cloneDeep(max));
    }
    else {
      return moment(_.cloneDeep(e));
    }

  },

  get_task_start: function (task) {
    return task.start_to_show ? task.start_to_show : task.start;
  },

  is_task_multi_day: function (task) {
    if (task.is_multi_day) {
      return true;
    }
    else {
      // should be checking original start date to correctly determine duration of task
      task.is_multi_day = moment(task.start).diff(moment(service.get_task_end(task)), 'days') < 0;
      return task.is_multi_day;
    }
  },

  is_task_in_current_month: function (task, month_to_check) {
    return moment(task.start).month() <= month_to_check && month_to_check <= moment(task.end).month();
  },

  add_all_day_task: function (output_task_list, task, date_to_add) {
    service.check_and_create_date(output_task_list, date_to_add);
    service.add_task_to_all_day_list(output_task_list, task, date_to_add);
  },

  add_task_end: function (output_task_list, task, date_to_add) {
    //console.log('TASK IN ADD TASK END: ', JSON.stringify(task, null, 2));
    service.check_and_create_date(output_task_list, date_to_add);
    if (task.allday) {
      service.add_all_day_task(output_task_list, task, date_to_add);
    }
    else {
      service.add_task_to_single_day_list(output_task_list, task, date_to_add);
    }
  },

  add_task_start: function (output_task_list, task) {

    // TODO: if first day of task - set flag, which indicates just to show the START time
    var new_end = _.cloneDeep(task.start_to_show);
    //console.log('start to show before:', JSON.stringify(task.start_to_show, null, 2));
    //console.log('new_end before:', JSON.stringify(new_end, null, 2));
    var tmp_task = _.cloneDeep(task);
    tmp_task.end_to_show = moment(new_end).hour(23).minute(59).second(59);
    tmp_task.postponed_to_show = moment(_.cloneDeep(task.end_to_show));

    service.check_and_create_date(output_task_list, tmp_task.end_to_show);

    var tmp_diff = moment(tmp_task.start_to_show).diff(moment(tmp_task.start), 'days');
    // if dates are on, e.g., 01.03 and 02.03, diff will give result of 1
    // if dates are on, e.g., 31.03 and 01.04, diff will output 0
    if (moment(tmp_task.start).month === moment(tmp_task.start_to_show).month()) {
      tmp_diff += 1;
    }

    if (tmp_diff > 0) {
      //if start_to_show is after start, it means
      // that the start of a task should be put as an all day task
      service.add_task_to_all_day_list(output_task_list, tmp_task, tmp_task.end_to_show);
    }
    else {
      service.add_task_to_single_day_list(output_task_list, tmp_task, tmp_task.end_to_show);
    }
  }
};

viewable_dates_comments.js

// XXX: function toooo long
set_viewable_dates: function (task, month_to_check, now) {
  /*
     it will set:
     start_to_show, end_to_show (and postponed_to_show)  | <-- moment() dates |
     variables to be used in list template,
     so that original start, end (and postponed) values of a task wouldn't be affected

     if a tasks start, end, postponed values span over month_to_check,
     start, end, postponed will be reduced to only contain values which cover month_to_check
     so that start_to_show <= month_to_check >= end_to_show
     */
  var tmp = {};
  tmp.task_start = moment(_.cloneDeep(task.start));
  tmp.task_end = moment(_.cloneDeep(task.end));


  // start date which will be used in list view only
  // set start to be the first day of current month,
  // if tasks begins before the start of the month in question
  if (tmp.task_start.month() < month_to_check) {
    // XXX: pull out as separate function
    task.start_to_show =
      moment()
      .year(tmp.task_start.year())
      .month(month_to_check)
      .date(1)
      .hour(0)
      .minute(0)
      .second(0);
  }
  else {
    task.start_to_show = moment(_.cloneDeep(tmp.task_start));
  }
  // XXX: Use VCS to keep history
  // change start to today
  // if task has begun before today
  //if (task.start_to_show.diff(now, 'days') < 0) {
  //    task.start_to_show = now;
  //    task.start_to_show.hour(0).minute(0).second(0);
  //    task.set_to_add_all_day = true; // set task to all day, as it has begun earlier than today
  //}

  /*
     TODO: check if postponed exists -
     TODO: probably could set it to end_to_show without going through the two following ifs
     as postponed > end
     */
  // end date which will be used in list view only
  // set end to be num of days of current month,
  // if end spans over current month
  if (tmp.task_end.month() > month_to_check) {
    // XXX: pull out as separate function
    task.end_to_show =
      moment()
      .year(tmp.task_end.year())
      .month(month_to_check)
      .date(service.get_days_in_month(tmp.task_end.year(), month_to_check))
      .hour(23)
      .minute(59)
      .second(59);
  }
  else {
    task.end_to_show = moment(_.cloneDeep(tmp.task_end));
  }
  // postponed date which will be used in list view only
  // set postponed to be num of days of current month,
  // if postponed spans over current month
  if (task.postponed) {
    tmp.task_postponed = moment(_.cloneDeep(task.postponed));

    if (tmp.task_postponed.month() > month_to_check) {
      // XXX: pull out as separate function
      task.postponed_to_show =
        moment()
        .year(tmp.task_postponed.year())
        .month(month_to_check)
        .date(service.get_days_in_month(tmp.task_postponed.year(), month_to_check))
        .hour(23)
        .minute(59)
        .second(59);
    }
    else {
      task.postponed_to_show = moment(_.cloneDeep(tmp.task_postponed));
    }
    task.end_to_show = service.get_task_end(task);
  }

  return task;
},

  list_contains_date: function (output_task_list, end_d) {
    // XXX: Useles check. arrays of zero length don't get iterated.
    // XXX: Use forEach
    if (output_task_list.length) {
      for (var day = 0; day < output_task_list.length; day++) {
        // XXX: Pull out separate function
        if (output_task_list[day].date.year() === end_d.year() &&
            output_task_list[day].date.month() === end_d.month() &&
            output_task_list[day].date.date() === end_d.date()) {
          return true;
        }
      }
    }
    return false;
  },

  // XXX: check what? Maybe `add_new` would be more appropriate.
  check_and_create_date: function (output_task_list, date_to_add) {
    if (!service.list_contains_date(output_task_list, date_to_add)) {
      output_task_list.push(
          {
            'date': moment(date_to_add),
            'tasks': [],
            'tasks_all_day': []
          }
          );
    }
  },

  add_task_to_single_day_list: function (output_task_list, task, end) {
    for (var j = 0; j < output_task_list.length; j++) {
      // compare only y/m/d
      // XXX: Pull out separate function
      if (output_task_list[j].date.year() === end.year() &&
          output_task_list[j].date.month() === end.month() &&
          output_task_list[j].date.date() === end.date()) {

        output_task_list[j].tasks.push(task);
        break;
      }
    }
  },

  add_task_to_all_day_list: function (output_task_list, task, end) {
    for (var j = 0; j < output_task_list.length; j++) {
      // compare only y/m/d
      // XXX: Pull out separate function
      if (moment(output_task_list[j].date).get('year') === moment(end).get('year') &&
          moment(output_task_list[j].date).get('month') === moment(end).get('month') &&
          moment(output_task_list[j].date).get('date') === moment(end).get('date')) {


        output_task_list[j].tasks_all_day.push(task);
        break;
      }
    }
  },

  // XXX: skip `get` prefix. It's function, it's supposed to return something.
  get_days_in_month: function (year, month) {
    // month 0 by default returns January
    // if day is set to 0, month 1 will return January as well
    return new Date(year, month + 1, 0).getDate();
  },

  // XXX: skp `get` prefix.
  get_task_end: function (task) {
    // XXX: use descriptive naming
    var p = task.postponed_to_show ? task.postponed_to_show : task.postponed;
    var e = task.end_to_show ? task.end_to_show : task.end;

    if (p) {
      var max = moment.max(p, e);
      return moment(_.cloneDeep(max));
    }
    else {
      return moment(_.cloneDeep(e));
    }

  },

  // XXX: skip `get` prefix
  get_task_start: function (task) {
    return task.start_to_show ? task.start_to_show : task.start;
  },

  // XXX: use modules as namespace. Then you can skip `task` prefix.
  is_task_multi_day: function (task) {
    if (task.is_multi_day) {
      return true;
    }
    else {
      // should be checking original start date to correctly determine duration of task
      task.is_multi_day = moment(task.start).diff(moment(service.get_task_end(task)), 'days') < 0;
      return task.is_multi_day;
    }
  },

  // XXX: correct naming. This funtion tests if task is in given month
  is_task_in_current_month: function (task, month_to_check) {
    return moment(task.start).month() <= month_to_check && month_to_check <= moment(task.end).month();
  },

  add_all_day_task: function (output_task_list, task, date_to_add) {
    service.check_and_create_date(output_task_list, date_to_add);
    service.add_task_to_all_day_list(output_task_list, task, date_to_add);
  },

  // XXX: Separate service for all day tasks and single day tasks
  // XXX: Maybe we can move some logic to task class it self!?
  add_task_end: function (output_task_list, task, date_to_add) {
    //console.log('TASK IN ADD TASK END: ', JSON.stringify(task, null, 2));
    service.check_and_create_date(output_task_list, date_to_add);
    if (task.allday) {
      service.add_all_day_task(output_task_list, task, date_to_add);
    }
    else {
      service.add_task_to_single_day_list(output_task_list, task, date_to_add);
    }
  },

  add_task_start: function (output_task_list, task) {
    // TODO: if first day of task - set flag, which indicates just to show the START time
    var new_end = _.cloneDeep(task.start_to_show);
    //console.log('start to show before:', JSON.stringify(task.start_to_show, null, 2));
    //console.log('new_end before:', JSON.stringify(new_end, null, 2));
    var tmp_task = _.cloneDeep(task);
    tmp_task.end_to_show = moment(new_end).hour(23).minute(59).second(59);
    tmp_task.postponed_to_show = moment(_.cloneDeep(task.end_to_show));

    service.check_and_create_date(output_task_list, tmp_task.end_to_show);

    var tmp_diff = moment(tmp_task.start_to_show).diff(moment(tmp_task.start), 'days');
    // if dates are on, e.g., 01.03 and 02.03, diff will give result of 1
    // if dates are on, e.g., 31.03 and 01.04, diff will output 0
    if (moment(tmp_task.start).month === moment(tmp_task.start_to_show).month()) {
      tmp_diff += 1;
    }

    // XXX: Cool explicit else
    if (tmp_diff > 0) {
      // XXX: This logic should go somewhere else. Task class?
      // add_task_to_all_day? These conditinals are spread out through all
      // codebase.

      //if start_to_show is after start, it means
      // that the start of a task should be put as an all day task
      service.add_task_to_all_day_list(output_task_list, tmp_task, tmp_task.end_to_show);
    }
    else {
      service.add_task_to_single_day_list(output_task_list, tmp_task, tmp_task.end_to_show);
    }
  }
};