创建DOM节点的JS方法不起作用

Question

我正在使用Javascript构建日历。这是我的第一个项目，我偶然发现了一个问题。 日历正文，即行和列，不会显示。我使用DOM在HTML中构建表格。

我的经验很少，但我无法弄清楚为什么Calendar.prototype.generateHTML（）不起作用。

我真的很感激任何帮助，我已经坚持了一段时间。

window.onload = function () {
    initMonths();
    createYears();
    initPrevButton();
    Calendar(0,1990);
    Calendar.prototype.generateHTML();
}


//globals

let currentDate = new Date();
let WEEK_DAYS = ['Mon', 'Tue', 'Wed', 'Thu', 'Fri', 'Sat', 'Sun'];
let MONTHS = ['January', 'February', 'March', 'April', 'May', 'June', 'July', 'August', 'September', 'October', 'November', 'December'];
let MONTHS_DURATIONS = [31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31];
let START_YEAR = 1990;
let END_YEAR = 2020;

let thead = document.getElementsByTagName('thead');

// Header

function initMonths() {
    let select = document.getElementsByTagName('select')[0];

    MONTHS.forEach(function(current, index, arr) {
        let option = document.createElement('option');

        option.innerHTML = current;

        select.appendChild(option);
    });
}


function createYears() {
    let select = document.getElementsByTagName('select')[1];

    for (var i = START_YEAR; i <= END_YEAR; i++) {
        let option = document.createElement('option');
        option.innerHTML = i;

        select.appendChild(option);
    }
}

function initPrevButton() {
    let prevButton = document.getElementsByTagName('button')[0];

    prevButton.addEventListener('click', function(index) {
        MONTHS[index] = MONTHS[index - 1];
    });
}



// body

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

Calendar.prototype.generateHTML = function() {
    let firstDay = new Date(this.year, this.month, 1);
    let startingDay = firstDay.getDay();
    let monthLength = MONTHS_DURATIONS[this.month];

    if (this.month == 1) {
        if (this.year % 4 === 0 && this.year % 100 != 0 || this.year % 400 === 0) {
            monthLength = 29;
        }
    }

    function generateRows() {
        let day = 1;
        //rows
        for (let i = 0; i < 5; j++) {
            //columns
            for (let j = 0; j <= 6; j++) { 
                let tbody = document.getElementById('tableBody');
                let createRow =  document.createElement('tr');
                let createCell = document.createElement('td');
                if (day <= monthLength && (i > 0 && j >= startingDay)) {
                    createCell.innerHTML = day;
                    createRow.appendChild(createCell);
                    day++;
                }
                tbody.appendChild(createRow);

                if(day > monthLength) {
                    break;
                }
            }
        }
    }

}