Hello World with UHC javascript backend

gistfile1.hs

type JSString = PackedString
foreign import prim "primStringToPackedString" toJS :: String -> JSString

data Document
foreign import js "document" getDocument :: IO Document
foreign import js "%1.write(%2)" write :: Document -> JSString -> IO ()

main = do
  doc <- getDocument
  write doc $ toJS "Hello, World!"