Validate list box

[sandy1028]

I have a search page where there are 3 options to search. The query string is passed along with file name to 3 types of search.

I am not able to validate the field for src_type="nsearch";

<script>
function validate(){
         if((document.searchType.os.value == "--") && (document.searchType.type.value=="--")){
          alert("Select Criteria");
                document.searchType.ostype.focus();
          return false;
                }
        }

</script>

if(src_type=="topn"){
<form id=searchType name=searchType method="get" onSubmit="return validate()" action="<?=$_SERVER['PHP_SELF']?>">

<td><a href=search.php?src_type=topn>Top N</a></td>

<div style=width:980px align=center>
        <table border=0 cellpadding=0 cellspacing=8>
          <tr>
            <td align="left"><label><font color="#006699" face="Arial">Operating System </font></label></td>
            <td align=left>
                <select id="os" name='os'>
                <option value="--" selected>--
                <option value="AIX">AIX
                <option value="Linux">Linux
                <option value="Solaris">Solaris
                </select>
   <td align=left>
                <select id="type" name='type'>
                <option value="--" selected>--
                <option value="A">A
                <option value="L">L
                <option value="S">S
                </select>
}

Here I am not able to validate the list box because of the query string. How to validate the box

[Kudose]

document.getElementById('os').options[selectedIndex].value

document.getElementById('type').options[selectedIndex].value

document.getElementById('os').focus()