I've worked with both. Typescript is (and feels) bolted-on, and can fail in dozens of ways. ReasonML's type checker is 100% airtight. It also has pattern match

I've worked with both. Typescript is (and feels) bolted-on, and can fail in dozens of ways. ReasonML's type checker is 100% airtight. It also has pattern matching and piping, to help make your code more concise. Typescript is easy: it's just ES6. ReasonML is (and feels) very much unpolished. Conflicts and overlaps between ReasonML's and Bucklescript's and JS's standard libraries, poor documentation, very flexible but very unintuitive JS interop syntax, frequent updates that break your code, etc. One big advantage is that ReasonML's compiler is lighting fast. As in, it compiles a big project in an instant. I don't know how it works behind the scenes, but the sheer speed changes how you develop. With JS/TS, you hit save, wait a few seconds for the iterative recompilation, refresh your browser, check your work. With Reason, you code, hit save, immediately get a helpful error message, rinse and repeat, and eventually you open up your browser and everything works 100%. It's pretty cool.