Map type is defined as follows in the codebase: type Map<K, T> = { +[key: K]: T };

Ah, I got confused here... I think we should rename this type (see inline comment).

Issues:

1. I can not iterate over obj1 or obj2 using for..in, reason: string [1] is incompatible with K [2]. [incompatible-type].
2. I can not use Object.keys() or Object.entries() properly as they return string, not generic K as key type. See the definition in core.js
3. I can not assign differences to result object, with any method, reason: Cannot assign computed property using K [1]. [invalid-computed-prop] or Cannot use in because on the left-hand side, K [1] must be a string or number. [invalid-in-lhs].
4. I can not call this function recursive without casting via any: Cannot call deepDiff with obj2[key] bound to obj2 because T [1] is incompatible with Map [2]. [incompatible-call]

Thanks for explaining!! I think option 2 is the easiest to solve. It's a common issue in our codebase, and we have a solution in the very objects.js file you're testing here. (We basically solve it with an any-cast... sometimes those are necessary, but like unsafe blocks in Rust, we try to be very "targeted" with how we use those, and isolate it to reusable utilities instead of leaving it in business logic.)

(By the way, that Flow issue has a GitHub issue, where this commit is mentioned, which solves the problem starting with Flow 0.196, which will be available in React Native 0.72 – currently in release candidate phase.)

I don't think using Object (or even any) is a serious issue in this context. I feel like we trying to type some runtime things and it will never be accurate and always will require some casting, e.g. via any. Other function in this file also are based on Object and seems fine (findMaximumDepth or hash). Even if someone will call this function with some weird values, e.g. deepDiff(undefined, 3.14) the result will be {} which for me is suitable with a definition: returns an object with properties from obj1 not included in obj2. (there is a bug in this code that makes this function not return proper result for deepDiff([undefined, 4], 3.14) - but that's my oversight)

Requesting review to see your point of view.

I don't disagree. But without seeing everything you tried, it's hard for a reviewer to give advice. Sometimes there is an "easy" solution available that lets us type things better, and by pushing on this it might give you more context on how to resolve issues like this in the future, where it might be more of an issue.