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.

Yes the answer is always ports when this topic comes up. Unfortunately one can only pass some basic types through ports. Passing a RegExp or an Intl.DateTimeFormat is not possible. It needs a wrapper on the Elm side, and the Elm people decided I can't be trusted to write this wrapper for me.

Back then if I wanted to search a list of strings for entries that match a regex supplied at run-time, I'd have to pass the whole list through a port, filter it outside, then pass it back in. Rather than just using the filter function. Ports are asynchronous messaging which means I have to restructure the whole code and wait for a state change when the filtered list is returned.

Let me cite the Elm docs on ports[1]: "Definitely do not try to make a port for every JS function you need."

So! Where does that leave me? Unsupported, that's where. Because I need a JS function. In 0.18 unsupported was fine. They broke it for 0.19 and the project died. Maybe it was dying of other causes anyway, but that one action sure drove people away.

[1] https://guide.elm-lang.org/interop/ports