I have used both and prefer TypeScript. Flow is fine, but it's lack of Editor support and performance make for me an easy choose. Typescript has a better architecture and tooling than Flow, a bigger community and a more vibrant ecosystem. Microsoft do it expectionally good this time. Same thoughts goes for Atom editor. Now that Github was adquired by Microsoft, there is no reason to divide forces in two editors. Atom is really overpassed in this race.
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