html - javascript form validation, before submitting help? -

i have following form:

<select name="size">     <option value="">please select</option>     <option value="1">medium</option>     <option value="2">large</option> </select> <select name="material">     <option value="">please select</option>     <option value="9">wood</option>     <option value="8">metal</option> </select>    <input type="submit" value="submit"> 

i creating javascript, when above form submitted form verified values have been selected, , gets submitted. if values have not been selected alert popups saying either of following:

1. "size needs selected , material needs selected",
2. "size needs selected", or
3. "material needs selected"

thanks

$("form").submit(function(){     if ($("#size").val() == "" && $("#material").val() == "") { alert("size , material  needs selected"); return false; }     elseif ($("#size").val() == "") { alert("size needs selected"); return false; }     elseif ($("#material").val() == "") { alert("material needs selected"); return false;}     $(this).submit(); });