let weekdays = ['Sun', 'Mon', 'Tue', 'Wed', 'Thur', 'Fri', 'Sat'],

    months = ["January", "February", "March", "April", "May", "June", "July", "August", "September", "October", "November", "December"],

    lastDayInMonth = [31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31],

    currentDate = new Date(),

    currentMonth = currentDate.getMonth(),

    day = currentDate.getDate(),

    year = currentDate.getFullYear();

​

function makeCalendar(month) {

    let nextDate = new Date(month + 1 + ' 1 ,' + year),

        firstDay = nextDate.getDay(),

        firstDay2 = firstDay,

        daysInMonth = lastDayInMonth[month],

        calendarView = "",

        i = 1;

​

    if (month == 1) {

        if ((year % 100 !== 0) && (year % 4 === 0) || (year % 400 === 0)) {

            lastDayInMonth[month] = 29;

        }

    }

​

    while (firstDay > 0) {

        calendarView += "<td class='premonth'></td>";

        firstDay--;

    }

​

​

    while (i <= daysInMonth) {

​

​

        if (firstDay2 > 6) {

            firstDay2 = 0;

            calendarView += "</tr><tr>";

        }

​

​

​

        if (i == day && month == currentMonth) {

            calendarView += "<td class='currentday'>" + i + "</td>";

        } else {

            calendarView += "<td class='currentmonth'>" + i + "</td>";

​

        }

​

        firstDay2++;

        i++;

    }

​

    var calendarTable = "<table class='calendar'> <tr class='month'><th class='previous' id='prev'>&lt;</th><th class = 'monthname' colspan='5'>" + months[month] + " " + 2017 + "</th><th class='next' id='next'>&gt;</th></tr>";

    calendarTable += "<tr class='weekdays'>";

    for (let i = 0; i < weekdays.length; i++) {

        calendarTable += "<td>" + weekdays[i] + "</td>";

    }

    calendarTable += "</tr>";

    calendarTable += "<tr>";

    calendarTable += calendarView;

    calendarTable += "</tr></table>";

​

    return calendarTable;

​

}

​

let view = document.getElementById("calendar-div"),

    cal = makeCalendar(currentMonth);

​

view.innerHTML = cal;

​

let previous = document.getElementById("prev").addEventListener('click', function() {

    let prevMonth = currentMonth -= 1;

    cal = makeCalendar(prevMonth);

})