Dynamically Determine Current Field Element?

Is it possible to dynamically determine the current field element in a
form?

I ask because I have a 'tab' button on a form, and when a user clicks
on it I need to be able to shift the focus to the next field, and also
if the current field is the last field on the form, essentially to do
nothing.

Conceptually, I guess I'm trying to do something like:

function clicktab(){

variable = current.field.element.number;
if (variable < n) {
variable++;
document.forms[0].element[variable].focus;
}}

In the above function, "n" would be the total number of fields in the
form.