Valeriu Cuțebov

<div id="calendar"></div>

<script src='https://npmcdn.com/react@15.3.0/dist/react.min.js'></script>
<script src='https://npmcdn.com/react-dom@15.3.0/dist/react-dom.min.js'></script>
<script src='https://cdnjs.cloudflare.com/ajax/libs/moment.js/2.14.1/moment-with-locales.min.js'></script>

<script>
'use strict';

const Heading = ({ date, changeMonth, resetDate }) => /*#__PURE__*/
React.createElement("nav", { className: "calendar--nav" }, /*#__PURE__*/
React.createElement("a", { onClick: () => changeMonth(date.month() - 1) }, "\u2039"), /*#__PURE__*/
React.createElement("h1", { onClick: () => resetDate() }, date.format('MMMM'), " ", /*#__PURE__*/React.createElement("small", null, date.format('YYYY'))), /*#__PURE__*/
React.createElement("a", { onClick: () => changeMonth(date.month() + 1) }, "\u203A"));



const Day = ({ currentDate, date, startDate, endDate, onClick }) => {
  let className = [];

  if (moment().isSame(date, 'day')) {
    className.push('active');
  }

  if (date.isSame(startDate, 'day')) {
    className.push('start');
  }

  if (date.isBetween(startDate, endDate, 'day')) {
    className.push('between');
  }

  if (date.isSame(endDate, 'day')) {
    className.push('end');
  }

  if (!date.isSame(currentDate, 'month')) {
    className.push('muted');
  }

  return /*#__PURE__*/(
    React.createElement("span", { onClick: () => onClick(date), currentDate: date, className: className.join(' ') }, date.date()));

};

const Days = ({ date, startDate, endDate, onClick }) => {
  const thisDate = moment(date);
  const daysInMonth = moment(date).daysInMonth();
  const firstDayDate = moment(date).startOf('month');
  const previousMonth = moment(date).subtract(1, 'month');
  const previousMonthDays = previousMonth.daysInMonth();
  const nextsMonth = moment(date).add(1, 'month');
  let days = [];
  let labels = [];

  for (let i = 1; i <= 7; i++) {
    labels.push( /*#__PURE__*/React.createElement("span", { className: "label" }, moment().day(i).format('ddd')));
  }

  for (let i = firstDayDate.day(); i > 1; i--) {
    previousMonth.date(previousMonthDays - i + 2);

    days.push( /*#__PURE__*/
    React.createElement(Day, { key: moment(previousMonth).format('DD MM YYYY'), onClick: date => onClick(date), currentDate: date, date: moment(previousMonth), startDate: startDate, endDate: endDate }));

  }

  for (let i = 1; i <= daysInMonth; i++) {
    thisDate.date(i);

    days.push( /*#__PURE__*/
    React.createElement(Day, { key: moment(thisDate).format('DD MM YYYY'), onClick: date => onClick(date), currentDate: date, date: moment(thisDate), startDate: startDate, endDate: endDate }));

  }

  const daysCount = days.length;
  for (let i = 1; i <= 42 - daysCount; i++) {
    nextsMonth.date(i);
    days.push( /*#__PURE__*/
    React.createElement(Day, { key: moment(nextsMonth).format('DD MM YYYY'), onClick: date => onClick(date), currentDate: date, date: moment(nextsMonth), startDate: startDate, endDate: endDate }));

  }

  return /*#__PURE__*/(
    React.createElement("nav", { className: "calendar--days" },
    labels.concat(),
    days.concat()));


};

class Calendar extends React.Component {
  constructor(props) {
    super(props);

    this.state = {
      date: moment(),
      startDate: moment().subtract(5, 'day'),
      endDate: moment().add(3, 'day') };

  }

  resetDate() {
    this.setState({
      date: moment() });

  }

  changeMonth(month) {
    const { date } = this.state;

    date.month(month);

    this.setState(
    date);

  }

  changeDate(date) {
    let { startDate, endDate } = this.state;

    if (startDate === null || date.isBefore(startDate, 'day') || !startDate.isSame(endDate, 'day')) {
      startDate = moment(date);
      endDate = moment(date);
    } else if (date.isSame(startDate, 'day') && date.isSame(endDate, 'day')) {
      startDate = null;
      endDate = null;
    } else if (date.isAfter(startDate, 'day')) {
      endDate = moment(date);
    }

    this.setState({
      startDate,
      endDate });

  }

  render() {
    const { date, startDate, endDate } = this.state;

    return /*#__PURE__*/(
      React.createElement("div", { className: "calendar" }, /*#__PURE__*/
      React.createElement(Heading, { date: date, changeMonth: month => this.changeMonth(month), resetDate: () => this.resetDate() }), /*#__PURE__*/

      React.createElement(Days, { onClick: date => this.changeDate(date), date: date, startDate: startDate, endDate: endDate })));


  }}


ReactDOM.render( /*#__PURE__*/
React.createElement(Calendar, null),
document.getElementById('calendar'));
</script>


<style>
body {
  font-family: "Roboto", sans-serif;
  background-color: #f8f7fa;
  color: #333;
}

.calendar {
  position: absolute;
  top: 50%;
  left: 50%;
  margin-top: -225px;
  margin-left: -195px;
  width: 360px;
  padding: 15px;
  box-shadow: 1px 1px 20px 0 rgba(0, 0, 0, 0.4);
  border-radius: 12px;
  overflow: hidden;
}
.calendar--nav {
  margin: -15px -15px 15px;
  padding: 0 15px;
  background-color: #b670f4;
  color: #fff;
  height: 70px;
  position: relative;
}
.calendar--nav a {
  position: absolute;
  cursor: pointer;
  left: 10px;
  font-size: 32px;
  line-height: 1;
  top: 16px;
  width: 30px;
  text-align: center;
  display: inline-block;
  color: rgba(255, 255, 255, 0.4);
  -webkit-user-select: none;
     -moz-user-select: none;
      -ms-user-select: none;
          user-select: none;
}
.calendar--nav a:hover {
  color: #fff;
}
.calendar--nav a:last-child {
  left: auto;
  right: 10px;
}
.calendar--nav h1 {
  margin: 0;
  position: absolute;
  left: 40px;
  right: 40px;
  text-align: center;
  cursor: pointer;
  font-weight: 400;
  font-size: 30px;
  line-height: 66px;
  -webkit-user-select: none;
     -moz-user-select: none;
      -ms-user-select: none;
          user-select: none;
}
.calendar--nav small {
  font-weight: 300;
  font-size: 60%;
}
.calendar--days {
  font-size: 0;
}
.calendar--days span {
  width: 14.28571%;
  display: inline-block;
  text-align: center;
  -webkit-user-select: none;
     -moz-user-select: none;
      -ms-user-select: none;
          user-select: none;
  cursor: pointer;
  margin: 8px 0;
  line-height: 34px;
  position: relative;
  font-size: 16px;
}
.calendar--days span.label {
  text-transform: uppercase;
  font-weight: 700;
  color: rgba(0, 0, 0, 0.3);
  font-size: 14px;
  cursor: initial;
}
.calendar--days span.active {
  font-weight: 700;
  background-color: rgba(182, 112, 244, 0.05);
  border-radius: 12px;
}
.calendar--days span.muted {
  color: rgba(0, 0, 0, 0.3);
}
.calendar--days span.between {
  border-radius: 0;
}
.calendar--days span.start, .calendar--days span.between, .calendar--days span.end {
  background-color: #b670f4;
  color: #fff;
}
.calendar--days span.start {
  border-radius: 12px 0 0 12px;
}
.calendar--days span.end {
  border-radius: 0 12px 12px 0;
}
.calendar--days span.start.end {
  border-radius: 12px;
}
.calendar--days span.between:nth-child(7n):after, .calendar--days span.start:nth-child(7n):after {
  content: "";
  position: absolute;
  top: 0;
  bottom: 0;
  left: 100%;
  background-color: #b670f4;
  width: 20px;
}
.calendar--days span.between:nth-child(7n+1):after, .calendar--days span.end:nth-child(7n+1):after {
  content: "";
  position: absolute;
  top: 0;
  bottom: 0;
  right: 100%;
  background-color: #b670f4;
  width: 20px;
}
.calendar--days span.start.end:after {
  display: none;
}
</style>