Flow supports importing type definitions for third party untyped libs. And in fact did a better job at being integrated in current projects. Flow lost because

Flow supports importing type definitions for third party untyped libs. And in fact did a better job at being integrated in current projects. Flow lost because the compiler was in really bad shape, slow and frequently crashing. Also their equivalent repository to DefinitelyTyped would ignore PRs for months and years and afaik still does. It's like it was somebody's toy project and its author eventually lost interest. It's a pitty because TypeScript still has unsound generics. But Microsoft know how to make dev tools and maintain them.