No, but you can slowly Reason-ify a javascript (or typescript!) app at the interfaces using gentype[1], which emits javascript/typescript/flow interfaces for the reason side. It works perfectly well to ship a slowly-becoming-reason frontend app.

But you do have to have the compiler give the thumbs up before it will compile the reason side of the project.

You can also checkout the tutorial for slowly converting a single file here, although I find myself doing this sort of thing very rarely: https://reasonml.github.io/docs/en/converting-from-js

[1] https://github.com/cristianoc/genType