Limit dates based on user selection

I have Leave request form where the employees are able to request different type of leaves such as paid leave, non paied leave, leave for blood donation etc.
Those types are stored in Mysql table and i am visualising them in dropdown via this code:

<div class="form-group">
  <label for="leavetype">Leave type*:</label>
  <select name="leavetype" id="leavetype" required>
    <option disabled selected value="">-- Select--</option> <?php
            include "dbConn.php";
            $records = mysqli_query($db, "SELECT leavetype FROM leavestypes");
            while($data = mysqli_fetch_array($records))
            {
              echo "
		<option value='".$data['leavetype'] ."'>" .$data['leavetype'] ."</option>";
            }
          ?>
  </select>

What i want is to limit allowed days, for example if the user select Leave for blood donation to be able to use only two days in the date pickers. Here is how the user pick dates:

<div class="form-group">
  <label for="startdate">Start:*</label>
  <input required="" class="form-control" name="startdate" id="startdate" type="date">
</div>
<div class="form-group">
  <label for="tilldate">End:*</label>
  <input required="" class="form-control" name="tilldate" id="tilldate" type="date">
</div>

I am trying to limit it via script in the end of the file:

<script>
const form = document.querySelector('#leave-form');
form.addEventListener('submit', function(event) {
            const startDate = new Date(document.querySelector('#startdate').value);
            const endDate = new Date(document.querySelector('#tilldate').value);
            const leavetype = document.querySelector('#leavetype').value.trim();
            if (endDate < startDate) {
                alert('end date cannot be before start date');
                event.preventDefault();
            } else if (
                leavetype === 'leave for blood donation - two days allowed' ||
                leavetype === 'test2 - two days allowed' ||
                (leavetype === 'leave for marrage - two days allowed ' && ((endDate - startDate) / (1000 * 60 * 60 * 24) + 1 > 2)) {
                    alert('You can use only two days for this type of leave');
                    event.preventDefault();
                }
            });
</script>

I was trying using includes() method instead of === but not working again.
The problem is that when I have this script if I select some of those type of leaves its always in alert no matter how many days I select in the date picker.

You need to group the "OR" with a parenthesis in your else-if statement

Because in your example, it triggers when :

• leavetype === 'leave for blood donation - two days allowed'
• leavetype === 'test2 - two days allowed'
• (leavetype ==='leave for marrage - two days allowed ' && ((endDate - startDate) / (1000 * 60 * 60 * 24) + 1 > 2)

I'm guessing what you wanted to do is this:

else if (
    (leavetype === 'leave for blood donation - two days allowed' || 
     leavetype === 'test2 - two days allowed' ||
     leavetype ==='leave for marrage - two days allowed ') &&
    ((endDate-startDate) / (1000 * 60 * 60 * 24) + 1 > 2)
) {
    alert('You can use only two days for this type of leave');
    event.preventDefault();
  }

Problem fixed. If someone looking here this is the solution I used:

    const form = document.querySelector('#leave-form');
form.addEventListener('submit', function(event) {
  const startDate = new Date(document.querySelector('#startdate').value);
  const endDate = new Date(document.querySelector('#tilldate').value);
  const leavetype = document.querySelector('#leavetype').value.trim(); // remove any leading or trailing whitespace
  const daysDiff = (endDate - startDate) / (1000 * 60 * 60 * 24) + 1;
  if (endDate < startDate) {
    alert('Крайната дата не може да бъде преди началната');
    event.preventDefault();
  } else if (daysDiff > 2 && (leavetype === 'leave for blood donation - two days allowed' || 
  leavetype === 'test2-  two days allowed' ||
  leavetype === 'leave for marrage - two days allowed ')) {
    alert('Можете да поискате максимум 2 дни за този тип отсъствие');
    event.preventDefault();
  }
});