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Comment by planede | Hacker News Reader

It seems to be stem from `Record` being contravariant in its Key type, right?

  const taskCounts = { work: 10, home: 5, personal: 3 };

This probably has the deduced type of `Record<"work" | "home" | "personal", number>`.

I wonder if there is a hole in the type system that could be filled with an `ExactKeyTypeRecord<K, V>`, which is invariant in K and is a subtype of `Record<K, V>`. Maybe this could have Object.keys() defined as expected.

I have no experience with TS, but I believe Python's TypedDict has a bunch of similar holes in the type system that could be mitigated with a bit more elaborate type hierarchies.

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Flowtype works like this out of the box. In my opinion, it's a much better default.

Huh, nice! Looking at the documentation[1], I assume this is the difference between "exact object types" and "inexact object types". I though about bringing this up for Python TypedDict, so I'm glad there is an actual precedent.

https://flow.org/en/docs/types/objects/

I believe that would be covariant: `'home' | 'work'` is a subtype of `string`, and `Record<'home' | 'work', number>` is assignable to `Record<string, number>` so the subtype relationship is preserved.

I don't follow how this parameter works then in Record:

  const taskCounts = { work: 10, home: 5, personal: 3 };

  const what : Record<string, number> = taskCounts; // OK, covariance?
  const the : Record<"work" | "home", number> = taskCounts; // Also OK, contravariance?
  const hell : Record<"work" | "home" | "personal" | "hello", number> = taskCounts; // This is an error

https://www.typescriptlang.org/play?#code/MYewdgzgLgBFCGEDWB...

I think you were originally correct and that `Record<string, number>` was not a good example. I would guess that union literals and primitives are handled differently by the compiler in this case, perhaps because a record with keys of every possible string is impossible it may have a special condition to skip some typing rules. In fact, `Record<number, number>` is assignable to `Record<string, number>`.

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