And it works in both directions. You can "easily" (I mean, this is JS afterall so this heavily depends on the degree of toolchain hell chosen) move to the TS toolchain and rename your files to have the .ts extension and you have a valid TS project and benefit from the inferred types for your code. Type annotations can then be added later on as you go. And the syntax everybody already knows stays valid. This is a huge benefit compared to something like ReScript because it makes adoption really easy. With the obvious downside of not having nice features like pattern matching.