Super simple JS two level multi level linkage simple example

Super simple JS linkage code, but to cooperate with jquery to use, you can also modify their own not optional JS library code

<tr>
  <th> The primary classification </th><td>
    <select name="subsidiary_cat_id" class="subsidiary_cat_id" onchange="linkage('.subsidiary_cat_id','.cat_id');">
      <option> Please select a </option>
    </select>
    <script language="javascript">
    var linkage_data={$list_file_category};//Initialize the cascading data, {$list_file_category} is the data of PHP's JSON_ENCODE, the array has 3 data, id,parent_id,name
    function linkage(parent_dom,son_dom){
        var parent_id=0;
        if(parent_dom){
            parent_id=$(parent_dom).val();    
        }
        $(son_dom).empty();//So let's clear the drop down
        var html='<option> Please select a </option>';
        $.each(linkage_data,function(key,value){
            if(value.parent_id==parent_id){
                html+="<option value='"+value.id+"'>"+value.name+"</option>";
            }
        });
        $(son_dom).append(html);
    }
    linkage('',".subsidiary_cat_id")//Do not pass the parent node, represented as the first level of data
    </script>
    </td></tr>
<tr><th> The secondary directory </th><td>
  <select name="cat_id" class="cat_id">
    <option> Please select a </option>
  </select></td></tr>