Ver.2 実行月の日曜始まり5週カレンダーを出力するJS / 週単位で配列に入れるようにした

calendar.js

function ADate(year, month, date) {
  this.year = year;
  this.month = month;
  this.date = date;
}

var today = new Date();
var year = today.getFullYear();
var month = today.getMonth() +1;
var lastMonth = new Date(year, month-1, 0);
var thisMonth = new Date(year, month, 0);
var thisMonthFirst = new Date(year, month-1, 1);
var nextMonthFirst = new Date(year, month, 1);

var preDates = [];
var dates = [];
var postDates = [];

for (var i=0,len=thisMonthFirst.getDay(); i<len; i++) {
  preDates[i] = new ADate(lastMonth.getFullYear(), lastMonth.getMonth() +1, lastMonth.getDate() -len +1 +i);
}

for (var i=0,len=thisMonth.getDate(); i<len; i++) {
  dates[i] = new ADate(thisMonth.getFullYear(), thisMonth.getMonth() +1, i+1);
}

for (var i=0,len=6-nextMonthFirst.getDay()+1; i<len; i++) {
  postDates[i] = new ADate(nextMonthFirst.getFullYear(), nextMonthFirst.getMonth() +1, i+1);
}

var weeks = {};
var prlen = preDates.length;
var thlen = dates.length;
var pslen = postDates.length;
weeks['dateCount'] = prlen + thlen + pslen;
weeks['weekCount'] = weeks['dateCount'] / 7;
for (var i=0; i<weeks['weekCount']; i++) {
  var ds = [];
  for (var j=0; j<7; j++) {
    var d = i*7 + j;
    var date = d < prlen ? preDates[d] : (d -prlen < thlen ? dates[d -prlen] : postDates[d -prlen -thlen]);
    ds[j] = date;
  }
  weeks[i] = ds;
}

console.log(weeks);

window.onload = function() {
  var table = document.createElement('table');

  for (var i=0; i<weeks['weekCount']; i++) {
    var weektr = document.createElement('tr');

    for (var j=0; j<7; j++) {
      var date = weeks[i][j];
      var datetd = document.createElement('td');
      var mn = date.month;
      if (mn < 10) mn = '0' + mn;

      var dt = date.date;
      if (dt < 10) dt = '0' + dt;

      datetd.dataset.date = date.year + '-' + mn + '-' + dt;
      datetd.innerText = date.date;
      weektr.appendChild(datetd);
    }

    table.appendChild(weektr);
  }

  document.body.appendChild(table);
};

表示は同様。
出力にデータセットで日付を入れるように
<td data-date="2017-06-23">23</td>