Elm (programming language)

The best teacher on this list for front-end developers. Elm does one job, browser apps, and does it with a strict but helpful compiler that promises no runtime errors in practice and explains its complaints in plain sentences. The Elm Architecture, a model, an update function and a view, is the clearest demonstration you will find of why pure functions and immutable state make UI bugs easier to reason about. Plenty of people learn Elm, then carry the habits back to TypeScript and React. That is a fine outcome, because the language itself is deliberately small and changes rarely, and talking to JavaScript goes through ports rather than direct calls, which can feel restrictive on a large app.