Parent list id="select1" or class="select1"
<select id="select1" size="5">
<option value="group-1">select 1</option>
<option value="group-2">select 2</option>
<option value="group-3">select 3</option>
</select>Add in dependent list class="dependent-list" and selector data-depends="#select1" or data-depends=".select1"
<select size="5" class="dependent-list" data-depends="#select1" id="select2">
<option value="1" data-group="group-1">select 1 -> 1</option>
<option value="2" data-group="group-2">select 1 -> 2</option>
</select>Ok, and just init
var dependentSelects = document.querySelectorAll('.dependent-list');
new DependentList(dependentSelects);