Animal.elm

module Animal where

data Either a b = Left a | Right b

-- Left 42      <=>  { "tag":"Left", "contents":[42] }
-- Right "cat"  <=>  { "tag":"Right", "contents":["cat"] }

port animal : Signal (Either Int String)
port animal = always (Right "cat") <~ every second

Animal.js

var animal = Elm.worker(Elm.Animal);

animal.ports.animal.subscribe(function(either) {
    switch (either.tag) {
      case "Left":
        errorCode = either.contents[0];
        break;
      case "Right":
        animal = either.contents[0];
        break;
    }
});