I once worked with an engineer who implemented the card game hearts in every language he learned. After the first implementation, you don't have to spend so much time thinking about the problem domain and can focus on finding the most idiomatic solution in the new language. The other benefit is that the project always has a clear endpoint.
This will help you develop a better sense of how F# and JavaScript differ as languages, what ideas are natural in one but unnatural in another, the conventions and idioms around tackling certain problems, and so on.