An infinite list has a length, it just happens to be (countable) infinite.

Of course, as JS doesn't do (guaranteed) tail call optimisation, even finite lists will have "no length" ;)

(and first rewriting `length` to be tail recursive (or using a loop with mutation):

    function length(l) {
      function go(l, acc) {
        if (l == null) {
          return acc;
        } else {
          return go(l.rest, acc + 1);
        }
      }
      return go(l, 0);
    }

What length does a cyclical list have?

  (define l '(1 2 3))
  (set-cdr! (cddr l) l)