JSFiddle - React, Tailwind, and code Playground

HTML

<select id="aggRoom">
<option value=""/></OPTION>
<option value="A,B,C,D">Agg Room A,B,C,D</OPTION>
<option value="C,D"/>Agg Room C,D</OPTION>
<option value="B,C"/>Agg Room B,C</OPTION>
<option value="A,B"/>Agg Room A,B</OPTION>
    <option value="B,C,D"/>Agg Room B,C,D</OPTION>
</select>
<br><br>
<input type="checkbox" name="singleRoom" value="A">Room A</br>
<input type="checkbox" name="singleRoom" value="B">Room B</br>
<input type="checkbox" name="singleRoom" value="C">Room C</br>
<input type="checkbox" name="singleRoom" value="D">Room D</br>

JavaScript

$('#aggRoom').change(function() {
    var rooms = ($('#aggRoom').val()).split(",");
    $('input:checkbox').removeAttr('checked'); 

    for (i=0;i<rooms.length;i++){             
        $('input:checkbox[value='+rooms[i]+']').prop('checked', 'checked');
    }
});