// JavaScript Document
<script>
function isEmpty(str) 
{
// Check whether string is empty.
for (var intLoop = 0; intLoop < str.length; intLoop++)
if (" " != str.charAt(intLoop))
return false;

return true;
}

function checkRequired(f) 
{
var strError = "";
for (var intLoop = 0; intLoop < f.elements.length; intLoop++)
if (null!=f.elements[intLoop].getAttribute("required")) 
if ((f.elements[intLoop].type == "checkbox" && !f.elements[intLoop].checked) || (f.elements[intLoop].type != "checkbox" && isEmpty(f.elements[intLoop].value)))
strError += "  " + f.elements[intLoop].name + "\n";
if ("" != strError) {
alert("Los siguientes datos requeridos son necesarios:\n" + strError);
return false;
} 
else
return true       
}</script>