BEHOLD! I give you JavaScript magic like Ruby haseth!

example.js

(function() {
  Date.prototype.today = function() {
    var date = this;

    var beginningOfMonth = function() {

      var bom = new Date(date.getFullYear(), date.getMonth(), 1);

      var beginningOfWeek = function() {
        var day = bom.getDay();
        var diff = bom.getDate() - day - (day === 0 ? -7 : 0);
        return new Date(bom.setDate(diff));
      };

      return {
        beginningOfWeek: beginningOfWeek
      };
    };

    var endOfMonth = function() {

      var eom = new Date(date.getFullYear(), date.getMonth() + 1, 0);

      var endOfWeek = function() {
        var day = eom.getDay();
        var diff = eom.getDate() + day + (day === 0 ? 6 : -1);
        return new Date(eom.setDate(diff));
      };

      return {
        endOfWeek: endOfWeek
      };
    };

    return {
      beginningOfMonth: beginningOfMonth,
      endOfMonth: endOfMonth
    };
  };

  var firstCalendarDay = new Date().today().beginningOfMonth().beginningOfWeek();
  var lastCalendarDay  = new Date().today().endOfMonth().endOfWeek();

  console.log(firstCalendarDay);
  console.log(lastCalendarDay);
}());