Infer type from union type key

Asked Nov 16 '21 at 08:00

Active Nov 16 '21 at 08:00

Viewed 67 times

I have a type alias that uses a union of literal types for the key:

type Type = {
  [key in 'a' | 'b']: string
}

However, TypeScript fails to infer the type correctly when I do this:

const object: Type = {'a': 'aaa', 'b': 'bbb'}

Object.keys(object).forEach(key => ...)

The type of key above is inferred as string, although I would've expected it to be inferred as 'a' | 'b'.

Here's a reproduction of the above.

asked Nov 16 '21 at 08:00

artooras

Use `(Object.keys(object) as Array).forEach(key => ...)` – captain-yossarian from Ukraine Nov 16 '21 at 08:14
Thanks for the link. So, this is by design? Is my only option then, If I want to have type validations inside my `forEach` function, is to cast the type manually again, as in `.forEach((key: 'a' | 'b') => ...)`? – artooras Nov 16 '21 at 08:28
1

You should not do `'a' | 'b'` since after `(Object.keys(object) as Array)` forEach is aware about `key` type – captain-yossarian from Ukraine Nov 16 '21 at 08:31

0 Answers0