Exactly. Flow even has tagged unions now, and `maybe` types. TypeScript is also a very mature solution to this problem. A true professional works with whatever language he needs to solve his/her problem, and shores up that language with the best tools available.

In this case, I half agree, because I think mobile development is best done using native platforms. But if you're doing web development, JavaScript is a must. And React is an elegant way to handle browser UI (Flow/Redux is also quite nice way to handle browser state, if a bit verbose).

I do share his concerns about the patent clause, and about Facebook's long-term commitment to the project, but at the level of personal projects, these concerns have not been enough to scare me away.