Test

<form id="queryForm">

   <label><input type="checkbox" name="category" value="Category:Purp"> Purp</label>

   <label><input type="checkbox" name="category" value="Category:Arch"> Arch</label>

   <label><input type="checkbox" name="category" value="Category:Source"> Source</label>

   <input type="button" onclick="buildQuery()" value="Abfrage erstellen">

</form>

<script> function buildQuery() {

   var checkboxes = document.querySelectorAll('input[name="category"]:checked');
   var query = "Die Abfrage enthält eine leere Bedingung.";
   
   document.getElementById("queryResult").innerHTML = query;

} </script>