Thanks for your answer. About the in-code documentation: even if you don't use Intellisense, it helps: you can get an idea about how to use a library much faster with type annotations.
As about the transitioning Even if the legacy project doesn't use babel, but plain ES5, maybe with jQuery, you can still add some type comments like /* : string */ (or even let flow's type inference do its magic; it will usually cover ~50% of your project without you doing anything) and enjoy at least a bit of type safety without changing anything else.
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