Allows you to send new state to JS every time state is changed in Elm program.

State.elm

withOnStateChangeSub : (model -> Cmd msg) -> (msg -> model -> ( model, Cmd msg )) -> msg -> model -> ( model, Cmd msg )
withOnStateChangeSub statePort update msg model =
    let
        ( updatedModel, cmd ) =
            update msg model

        updatedCmd : Cmd msg
        updatedCmd =
            if updatedModel == model then
                cmd
            else
                Cmd.batch [ statePort updatedModel, cmd ]
    in
        ( updatedModel, updatedCmd )


-- Use 

port statePort : Model -> Cmd msg


update : Msg -> Model -> ( Model, Cmd Msg )
update msg model =
  ...


main =
  program
    { ...
    , update = withOnStateChangeSub statePort update
    }