The Keyof Type Operator shows inconsistent behavior.

[13OnTheCode]

🔎 Search Terms

The Keyof Type Operator shows inconsistent behavior.

🕗 Version & Regression Information

⏯ Playground Link

https://www.typescriptlang.org/play?#code/C4TwDgpgBAKgjFAvFA3lA2gDwFxQM7ABOAlgHYDmAurgK6kDWpA9gO6lQC+AsAFAD0fKEIB6Afl69QkKACUEyehBBMAZrDi8BQqGIk8p0GACYksiAGMmhACYAeAiQoAaKHUatSAPk2CR4npLg0DImCkqqsEY+2ro8QA

💻 Code

type T1 = { [x: string]: unknown }
//   ^? type T1 = { [x: string]: unknown; }

type R1 = keyof T1
//   ^? type R1 = string | number

type T2 = Record<string, unknown>
//   ^? type T2 = { [x: string]: unknown; }

type R2 = keyof T2
//   ^? type R2 = string

🙁 Actual behavior

According to the type hint, the types of T1 and T2 are the same, but the results they get through keyof are not the same. According to the document, they should both get string | number .

Document link: https://www.typescriptlang.org/docs/handbook/2/keyof-types.html#handbook-content

🙂 Expected behavior

Infer the correct type.

Additional information about the issue

In addition, it may also be related to #56888.

[MartinJohns]

This is working as intended, see also #55706. Duplicate of #49540.

Record<> is implemented as a mapped type, it behaves different than an indexed type.

Used search terms: keyof record in:title

[fatcerberus]

Yeah, it's confusing that Record<string, T> is displayed as an index-signature type in type hints. It's actually a mapped type over string, which, while they behave similarly most of the time, is not the same thing as a string index signature.