Trying To Write A Generic Form Validator

I'm trying to write a generic/reusable form validator in Javascript...
just something that checks to make sure required fields have a value. By
generic I mean I don't want to explicitly reference the name/id of the
form or the name of any of the data fields within a "validation"
function.

My first shot seems to have some errors in it:

FieldsToValidateByForm = {};
FieldsToValidateByForm['contact'] = ["FirstName",
"LastName","State","Email"];

function validate(form)
{
problemFields = new Array();
returnval = true;
FieldsToValidate = FieldsToValidateByForm[form.id];

for(i=0; i < FieldsToValidate.length; i++) {
fieldInQuestion = form[FieldsToValidate[i]];
if(fieldInQuestion.value.length < 1) //problem spot?
problemFields.push(FieldsToValidate[i]);
}

if(problemFields.length > 0) {
returnval = false;
warn(problemFields); /* tells user they're missing a field,
that's all */
}

return returnval;
}


What I think is happening (not sure) is that the expression
form[fieldsToValidate[i]] is not giving me what I want: a reference to
the object corresponding to the form field with the same name. In
otherwords, I must have some fundamental misunderstanding of how the DOM
works here. Unfortunately, I can't seem to find a good enough reference
to set me straight....