Javascript's FOR and CL's LOOP disagree about how to interpret end-tests and step-forms. In JS, all of the step forms are evaluated before any of the end tests are, while in LOOP, the end-test for one :FOR clause is evaluated (and, if it fails, the loop terminates) before the step-form for the next :FOR clause is processed. The syntax of LOOP strongly implies the latter behavior. This was leading to unwanted surprises, since the generated JS FOR loop wasn't always terminating when expected. After a dozen or more experiences of JS code crashing because of this contradiction, I bit the bullet and made LOOP generate different JS code when multiple :FOR clauses are specified. This makes the generated code more verbose, because end-tests and step-forms can't be inlined into the FOR block anymore. But now the code does what LOOP implies.
This patch has been pushed (along with a couple of very minor tweaks to special-forms.lisp).
Daniel