multiselect detect

hi all. i want to ask how can i detect if multipleselect is selected or it is empty upon the form is submitted? thank you.

if value is empty generate the below messgae:
alert message("you must select at least one category");

i have tried everything i know but still i can't get the javascript to work. thank you for any idea or solution you can give me.

on my form the code looks like this-->>
<select multiple size="10" name='jobcategory[]'>
<?php
$job_list=array('&nbsp;Actuarial Science / Statistics','&nbsp;Advertising Client Servicing / Media Planning','&nbsp;Agriculture / Forestry / Fisheries','&nbsp;Architecture / Interior Design','&nbsp;Arts / Creative / Graphics Design','&nbsp;Aviation');
$vcnt=1;
foreach ($job_list as $job_name){
echo "<option value='$vcnt'>$job_name</option>n";
$vcnt++;
}
?>
</select>

and on the function formPreCheck(formobj){..........
}else if (document.appform.jobcategory.length <= 0){
alert("Please select at least one job category!");
//document.appform.language.focus();
return false;
}

please help.

Try using the selectedIndex property.

Change:
}else if (document.appform.jobcategory.length <= 0){
to:
}else if (document.appform.jobcategory.selectedIndex >= 0){

.length only returns the number of options in the select, not how many are selected