If you will start a project from scratch I recommend to use TypeScript. But, If you work with legacy projects written in JavaScript I recommend Flow (JS). Both tools have the same objective: reduce the bad code (which create illegible code, generate bugs e problems to maintenance). Flex helps you to avoid fall in bad codes, but TypeScript prevent you to c you to create bad codes. I believe cause this some JavaScript fans don't like TS, because TS block you to write some types o code. This is the fundamental difference between TS and Flow: Flow avoid problems, but no force. TS force you to prevent problems.