Yes, ports or custom elements are the recommended options, https://github.com/elm-community/js-integration-examples

There are a bunch of other options/workarounds/hacks depending on the need. E.g. using getters or creating proxy objects https://github.com/anmolitor/intl-proxy, or event listeners, or postprocessing of the generated JS code, but those shouldn't be the first idea to reach for.