JSFiddle - React, Tailwind, and code Playground

JavaScript

// these are labels for the days of the week
cal_days_labels = ['Mon', 'Tue', 'Wed', 'Thu', 'Fri', 'Sat', 'Sun'];

// these are human-readable month name labels, in order
cal_months_labels = ['January', 'February', 'March', 'April',
    'May', 'June', 'July', 'August', 'September',
    'October', 'November', 'December'];

// these are the days of the week for each month, in order
cal_days_in_month = [31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31];

// this is the current date
cal_current_date = new Date();

function Calendar(month, year) {
    this.month = (isNaN(month) || month == null) ? cal_current_date.getMonth() : month;
    this.year = (isNaN(year) || year == null) ? cal_current_date.getFullYear() : year;
    this.html = '';
}

Calendar.prototype.generateHTML = function () {

    // get first day of month
    var firstDay = new Date(this.year, this.month, 1);
    var startingDay = firstDay.getDay();

    // find number of days in month
    var monthLength = cal_days_in_month[this.month];

    // compensate for leap year
    if (this.month == 1) { // February only!
        if ((this.year % 4 == 0 && this.year % 100 != 0) || this.year % 400 == 0) {
            monthLength = 29;
        }
    }

    // do the header
    var monthName = cal_months_labels[this.month]
    var html = '<table class="calendar-table">';
    html += '<tr><th colspan="7">';
    html += monthName + "&nbsp;" + this.year;
    html += '</th></tr>';
    html += '<tr class="calendar-header">';
    for (var i = 0; i <= 6; i++) {
        html += '<td class="calendar-header-day">';
        html += cal_days_labels[i];
        html += '</td>';
    }
    html += '</tr><tr>';

    // fill in the days
    var day = 1;
    // this loop is for is weeks (rows)
    for (var i = 0; i < 9; i++) {
        // this loop is for weekdays (cells)
        for (var j = 1; j <= 7; j++) {
            html += '<td class="calendar-day">';
            if (day <= monthLength && (i > 0 || j >= startingDay)) {
         ...