Multicombobox not showing when clicking back button

Question

Multicombobox not showing when clicking back button

I am trying to develop an online booking system that has different options for each course. When a course is selected, a combo box opens in which you select the week in which you want to take the course. There is a function to check the correctness of certain fields. If you need to go back to filling out the form, the dropdown will not be visible. Is there a way to make the combo box visible?

html as follows 1 course course 2 course 3 Choose your course ...

The selection is unloaded from Wordpress like this:

<input type="hidden" name= "course1a" value="<?php echo $title;?>,<?php echo     the_field('cost_of_course',$week1a); ?>" />
<select name="date1a" id="date1a"> 
 <option value="">Select Week</option> 
  <?php 

  $categories =get_the_category($week1a); 
  foreach ($categories as $category) {
  if($category->name !== '6 +') 
  if($category->name !== '7+') 
  if($category->name !== '8+') 
  if($category->name !== '9+')
  if($category->name !== '10 +')
  if($category->name !== '11+')
  if($category->name !== 'All')


 $option = '<option value="'.$category->cat_name.'">';
 $option .= $category->cat_name;

 $option .= '</option>';
echo $option;
}
?>

The Javascript handling this looks like this:

function showWeek(str) {
if (str == "") {
    document.getElementById("txtHint").innerHTML = "";
    return;
} else { 
    if (window.XMLHttpRequest) {
        // code for IE7+, Firefox, Chrome, Opera, Safari
        xmlhttp = new XMLHttpRequest();
    } else {
        // code for IE6, IE5
        xmlhttp = new ActiveXObject("Microsoft.XMLHTTP");
    }
    xmlhttp.onreadystatechange = function() {
        if (xmlhttp.readyState == 4 && xmlhttp.status == 200) {
            document.getElementById("txtHint").innerHTML = xmlhttp.responseText;
        }
    }
    xmlhttp.open("GET","dates.php?week="+str,true);
    xmlhttp.send();
}
}