How to write a function to select the value of an option in a dropdown list?

I am using the following code to make a button that consists of VF pages as a dropdown list and a filter criteria button for searching various properties along with pages:

 <div id="vp_page_dd">
    <span></span>
    <button type="button"  id="btnSort" >
        <table width="100%">
            <tr>
                <td width="81%" id="BtnDropDown_test">
                    <select id= "VFPageListsDropDown123" class="menuDropDown"> </select>
                </td>
                <td width="19%"> <span class="caret"></span></td>
            </tr>
        </table>
   </button>
   <span>
       <!--  <div id="dropdowns-1" class="dropdowns dropdowns-tip">
           <ul class="dropdowns-menu" id="VFPageListsDropDown"></ul>
       </div> -->
   </span>
</div>
                                <!-- SELECT VISUAL FORCE PAGE DROP DOWN Ends -->
                                <!-- Fillter Criteria Starts -->
                                <div id="div_FilterCriteria" align="right">
                                <button type="button" class="btn-sample" onclick="showPopUpModalForFilterCriteria()">Filter Criteria</button>
                                </div>
                                <!-- Criteria -->    
                <div class="modal fade" id="myModal1"  tabindex="-1" role="dialog" aria-labelledby="myModalLabel" aria-hidden="true" width="100%"   >
                    <div class="modal-dialog  modal-lg">
                        <div class="modal-content">
                            <div class="modal-header" id="titlePopup1" >
                                <button type="button" class="close" data-dismiss="modal" aria-hidden="true">&times;</button>
                            </div>
                            <div class="modal-body">
                                <center>
                                <!-- put content of the pop up window ... -->
                                    <div class="dt-example" >
                                        <div  id="filterCriteria" style="overflow:auto">
                                            <table>
                                                <tr>
                                                    <td style="width:25%;"></td>
                                                    <td style="width:70%;">
                                                         <b>Filter Criteria: </b>
                                                         <select id="mainSelect" class="mainSelectDropDown" align="left">
                                                            <option value="Pages" >Pages</option>
                                                            <option value="webservices" >WebServices</option>
                                                            <option value="batch" >Batch</option>
                                                             <option value="complexcity" >Complexcity</option>
                                                        </select>
                                                        <br/><br/>
                                                        <b>Fillter Values: </b>
                                                            <select id="filterSubSelect" class="subSelectDropDown" align="left">
                                                            </select>
                                                    </td>

Below are the functions for the purpose of page search for the filter criteria button:

  $('#mainSelect').change(function(){

                        $('#filterSubSelect').html('');


                        var selectedOption= $('#mainSelect').val();

                        if(selectedOption==="Pages"){

                        getPages();            

                        }else{

                         getobjects();

                        }

                        });


                         /*  'Select Visual Force Pages' DROP DOWN - click on it will */
                   $('#filterSubSelect').change(function() {

                        console.log("sub filter changed");
                        var selectedMainOption= $('#mainSelect').val();

                        var selectedSubOption= $( "#filterSubSelect option:selected" ).text();



                        console.log("selectedSubOption::"+selectedSubOption);

                        if(selectedMainOption==="Pages"){

                           var selectedVFPage=selectedSubOption;


                           setSelectedVFPageActive(selectedVFPage);

                           console.log("changed one drop down to: " + document.getElementById('SelectedVFPageActive').value); //VFPageListsDropDown
                           console.log(" inside drop down click(): " + jsonfileURL);

                           getOneVFpage(jsonfileURL, selectedVFPage);


                         }else{


                         }

setSelectedVFPage function:

function setSelectedVFPageActive(selectedVFPage) {
    document.getElementById("SelectedVFPageActive").value = selectedVFPage;
    console.log("current Active VF page set to 2 (setSelectedVFPageActive) : " + document.getElementById("SelectedVFPageActive").value);
    return selectedVFPage;

What I want is a similar function for the id='VFPageListsDropDown123'.I have tried to write the function in the same way as the above written two functions, so that when I select a page from the dropdown list, it comes to display,but it is not happening.Can anyone tell how to write the function?