The package contains a module with data structure to communicate with JavaScript over ports.
Usually one would set one incoming port and one outgoing port with generic data passing through.
import JsData
{-| Outgoing port
-}
port msgToJs : JsData.JsData -> Cmd msg
{-| Incoming port
-}
port msgFromJs : (JsData.JsData -> msg) -> Sub msg
For more inspiration take a look at great talk by Murphy Randle.