What are the tradeoffs between ReasonML (https://reasonml.github.io/) and TypeScript (https://www.typescriptlang.org/)?
The are very different.
If you want to write typesafe code both are excellent choices.
If you want to write typesafe JavaScript, then TypeScript is the option.
If you want to write typesafe some language that compiles down to JavaScript then ReasonML is one of many options. The some language in ReasonML's case is OCAML.
My biased opinion : https://medium.com/@basarat/typescript-won-a4e0dfde4b08