Other people's comments explain the details well, but I wanted to say that I have a large (real-world) project where the bulk of the app and all the business lo

Other people's comments explain the details well, but I wanted to say that I have a large (real-world) project where the bulk of the app and all the business logic is in Elm, and all the more fiddly UI bits are in Typescript. The two languages communicate with each other over typesafe Elm ports and it all works fairly wonderfully.