After checking for the presence of the element, Cannot read property?

Question

JavaScript

Here they suggested how to check for the presence of an element, the error was resolved, but a new one appeared, here is the full code of the script:

window.onload = function () {
    var day = new Date,
        md = (new Date(day.getFullYear(), day.getMonth() + 1, 0, 0, 0, 0, 0)).getDate(),
        $month_name = "января февраля марта апреля мая июня июля августа сентября октября ноября декабря".split(" ");

        function set_select(a, c, d, e) {
            var el = document.getElementsByName(a)[0];

            if (!el) {
                console.warn('Element "' + a + '" not found');
                return;
            }

            for (var b = el.options.length = 0; b < c; b++) {
                el.options[b] = new Option(a == 'month' ? $month_name[b] : b + d, b + d);
            }

            el.options[e] && (el.options[e].selected = !0)
        }
    set_select("day", md, 1, day.getDate() - 1);
    set_select("month", 12, 1, day.getMonth());
    set_select("year", 50, day.getFullYear()-50, 50);


    var year = document.getElementById('year');
    var month = document.getElementById("month");

    function check_date() {
        var a = year.value | 0,
            c = month.value | 0;
        md = (new Date(a, c, 0, 0, 0, 0, 0)).getDate();
        a = document.getElementById("day").selectedIndex;
        set_select("day", md, 1, a)
    };

    if (document.addEventListener) {
        year.addEventListener('change', check_date, false);
        month.addEventListener('change', check_date, false);

    } else {
        year.detachEvent('onchange', check_date);
        month.detachEvent('onchange', check_date);
    }

}

Mistake:

Uncaught TypeError: Cannot read property 'addEventListener' of null

Answer 1

It's the same - you don't check if there are elements with id year and month on the page before attaching handlers to them.
The solution algorithm is very simple:
1. Look for all the places where the addEventListener function is used (well, it’s more correct, of course, to look at a specific file and line directly in the console).
2. See under what conditions the variable on which this method is called can be null.
3. Add the necessary checks.