Calendar Kata in JavaScript.

calendar.js

module.exports = Calendar;

function Calendar(year, month, startOn) {
  var year = year;
  var month = month;
  var startOn = startOn || 0;
  var date = new Date(year, month - 1, 1);

  function title() {
    return '        ' + months()[date.getMonth()] + ' ' + date.getFullYear();
  }

  function months() {
    return ['January', 'February', 'March', 'April', 'May', 'June', 'July', 'August', 'September',
      'October', 'November', 'December'];
  }

  function header() {
    var dD = daysDays().slice(startOn, startOn+7);
    return ' ' + dD.join(' ');
  }

  function daysDays() {
    return daysOfWeek().concat(daysOfWeek());
  }

  function daysOfWeek() {
    return ['Sun', 'Mon', 'Tue', 'Wed', 'Thu', 'Fri', 'Sat'];
  }

  function days() {
    var noSpaces = (7 + date.getDay() - startOn) % 7;
    var offsettedDays = offsetDaysWithSpaces(daysThisMonth(), noSpaces);

    offsettedDays = offsettedDays.map(function(day) {
      return fillSpaces(day, 4);
    })

    var actualDays = daysToWeeks(offsettedDays);
    return actualDays.join('\n');
  }

  function daysThisMonth() {
    var lastDayOfMonth = new Date(year, month, 0);

    return buildOutput(1, lastDayOfMonth.getDate() + 1, function(day) {
      return day;
    });
  }

  function offsetDaysWithSpaces(days, offset) {
    return buildOutput(0, offset, function(offset) {
      return ' ';
    }).concat(days);
  }

  function daysToWeeks(days) {
    return buildOutput(0, Math.ceil(days.length / 7), function(week) {
      var daysThisWeek = week * 7;
      return days.slice(daysThisWeek, daysThisWeek + 7).join('');
    });
  }

  function fillSpaces(input, spacesCount) {
    var start = String(input).length;

    return buildOutput(start, spacesCount, function() {
      return ' ';
    }).concat(input).join('')
  }

  function buildOutput(start, end, fn) {
    var output = [], result;

    for (var i = start; i < end; i++) {
      result = fn.call(this, i);
      output.push(result);
    }

    return output;
  }

  this.toString = function() {
    return [
      title(),
      header(),
      days()
    ].join('\n');
  }
}


describe('Calendar', function() {
  it('July 2014', function() {
    var calendar = new Calendar(2014, 7)
    var july = "        July 2014\n" +
               " Sun Mon Tue Wed Thu Fri Sat\n" +
               "           1   2   3   4   5\n" +
               "   6   7   8   9  10  11  12\n" +
               "  13  14  15  16  17  18  19\n" +
               "  20  21  22  23  24  25  26\n" +
               "  27  28  29  30  31"

    expect(calendar.toString()).toEqual(july)
  })

  it('August 2014', function() {
    var calendar = new Calendar(2014, 8)
    var august = "        August 2014\n" +
                 " Sun Mon Tue Wed Thu Fri Sat\n" +
                 "                       1   2\n" +
                 "   3   4   5   6   7   8   9\n" +
                 "  10  11  12  13  14  15  16\n" +
                 "  17  18  19  20  21  22  23\n" +
                 "  24  25  26  27  28  29  30\n" +
                 "  31"

    expect(calendar.toString()).toEqual(august)
  })

  it('Feb 2008', function() {
    var calendar = new Calendar(2008, 2, 3)
    var feb = "        February 2008\n" +
              " Wed Thu Fri Sat Sun Mon Tue\n" +
              "           1   2   3   4   5\n" +
              "   6   7   8   9  10  11  12\n" +
              "  13  14  15  16  17  18  19\n" +
              "  20  21  22  23  24  25  26\n" +
              "  27  28  29"

    expect(calendar.toString()).toEqual(feb)
  })
})