javascript if statement if is always true

September 30, 2022

I am making a calendar and the if statement is supposed to see if the last day of the month is 30, 31, or anything else. When I test the code, the statement always affects the if statement even when the condition is not true. Here is my html code: ( all of this is correst)

<!DOCTYPE html>
<html>
<head>
<title>Calendar</title>
<link rel="stylesheet" href="calendar.css">
<script src="calendar.js"></script>
</head>
<body>
<main>
    <h1><span id="month_year">&nbsp;</span></h1>

    <table id="calendar">
        <tr><th>Sun</th><th>Mon</th><th>Tue</th><th>Wed</th><th>Thu</th><th>Fri</th> 
<th>Sat</th></tr>
        
    </table>
</main>
</body>
</html>

For my javascript, I have set the last day of the month to be 30 to test the else if. (lastMonthDay = 30)
here is my JavaScript code:
var $ = function (id) { return document.getElementById(id); };

window.onload = function () {

var d = new Date();
var month = d.getMonth();
var year =  new Date().getFullYear();
$("month_year").innerHTML = getMonthText(month) + " " + year;

var lastMonthDay = 30;
var calendarNum = 1;
var calendarDate = 1;
var tbodyRef = document.getElementById("calendar").getElementsByTagName("tbody")[0];

if(lastMonthDay = 31){
    for ( i = 0; i < 5; ++i){
    newRow = tbodyRef.insertRow();
        for ( n = 0; n < 7; ++n){
        newCell = newRow.insertCell()
            if(calendarNum != 35 && calendarNum != 34
                && calendarNum != 1 && calendarNum != 2 ){
                var newText = document.createTextNode(calendarDate);
                newCell.appendChild(newText);
                calendarDate = calendarDate + 1;
            }
            else{
                var newText = document.createTextNode("");
                newCell.appendChild(newText);
            }
        calendarNum = calendarNum + 1;
        }
    }
}
else if(lastMonthDay = 30){
    for ( i = 0; i < 5; ++i){
    newRow = tbodyRef.insertRow();
        for ( n = 0; n < 7; ++n){
        newCell = newRow.insertCell()
            if(calendarNum != 35 && calendarNum != 34 && calendarNum != 33
                && calendarNum != 1 && calendarNum != 2 ){
                var newText = document.createTextNode(calendarDate);
                newCell.appendChild(newText);
                calendarDate = calendarDate + 1;
            }
            else{
                var newText = document.createTextNode("");
                newCell.appendChild(newText);
            }
        calendarNum = calendarNum + 1;
        }
    }
}
else{   
    for ( i = 0; i < 5; ++i){
    newRow = tbodyRef.insertRow();
        for ( n = 0; n < 7; ++n){
        newCell = newRow.insertCell()
            if(calendarNum != 35 && calendarNum != 34 && calendarNum != 33 && 
calendarNum != 34
                && calendarNum != 1 && calendarNum != 2 ){
                var newText = document.createTextNode(calendarDate);
                newCell.appendChild(newText);
                calendarDate = calendarDate + 1;
            }
            else{
                var newText = document.createTextNode("");
                newCell.appendChild(newText);
            }
        calendarNum = calendarNum + 1;
        }
    }
}
};

Any explanation or link to a similar question is appreciated. The only code that does not work is the if statements.