Rudimentary Empty Field Validator

<script type="text/javascript"><!--

function Validate_form(form) {

var fields = [];

this.add = function(element, err) {
if ( !('__validate' in form[element]) ) {
form[element].__validate = function() {
if (this.value=='') {
alert(err);
this.focus();
return false;
}
}

}
fields.push(form[element]);
}

this.check = function() {
var len = fields.length;
for (var i=0; i<len; i++)
if ( !fields[i].__validate() )
return false;

return true;
}
}

function form_onsubmit(form) {
var vform = new Validate_form(form);
vform.add('domain_name', 'The domain field is empty');
return(vform.check());
}
--></script>
<form name="signup" method="post" action="" onsubmit="return form_onsubmit();">
<input type="text" name="domain_name" />
<input type="submit" />
</form>

This is nothing special. just a basic empty field validator. it's probably not as efficient as it could be because it assigns the function to the various elements. It also only supports text fields. I was thinking of allowing a 3rd argument on Validate_form.add to accept something like Validate_form.stock.email/number/etc (functions). ah well..