get multi selected options value

<select multiple onchange="assignCategory(this.id)" style="width:120px;" id="category_0" name="category">
<option value="0">select category</option>
<option value="5">Banners</option>

<option value="16">Out door banner stands</option>
<option value="6">Window signs</option>
<option value="26">&nbsp; &nbsp;Window sign (perforated)</option>
<option value="27">Window sign (opaque)</option>
<option value="28">Window cling. (opaque)</option>
<option value="7">Side walk signs</option>
<option value="11">Banner accessories</option>
<option value="29">Faqs</option>
</select>



<script>

function assignCategory(objThisId){

var InvForm=document.getElementById(objThisId).options.length;

var optL=document.getElementById(objThisId).options;

var x=0;

var selBranchVal='';

for (x=0;x<=InvForm;x++)

         {

//alert("op:"+optL[x].value+'opsel:'+optL[x].selected);

// alert($(optL[x]).selected);

           if(optL[x]){

if($(optL[x]).selected)

{

//alert('jha'+optL[x].value);

//alert(InvForm.SelBranch[x].value);

selBranchVal= selBranchVal+optL[x].value+",";

// alert("sds"+selBranchVal);

} 

}

         }

alert(selBranchVal);

}
</script>