"keyof" infers type incorrectly when it is applied to interface that extends Record<string, any>

[nicky1038]

🔎 Search Terms

keyof interface extends record number

🕗 Version & Regression Information

• This is the behavior in every version I tried

⏯ Playground Link

Link

💻 Code

interface EmptyInterface {};
type StringRecord = Record<string, string>;
interface Interface extends StringRecord {};

type EIFKey = keyof EmptyInterface // never
type SRKey = keyof StringRecord // string
type IFKey = keyof Interface // number | string, why not string?

🙁 Actual behavior

IFKey is number | string

🙂 Expected behavior

IFKey should be string

[Andarist]

In general, what is indexable by string is also indexable by number:

const stringRecord: StringRecord = {};
const test1 = stringRecord["foo"];
const test2 = stringRecord[100];

const interface_: Interface = {};
const test3 = interface_["foo"];
const test4 = interface_[100];

The difference in keyof results here is surprising to me though.

[fatcerberus]

I believe the difference in behavior comes down to, Record is implemented as a mapped type. keyof MappedTypeOverString is string. Interfaces can’t be mapped types though (AFAIK), so extending a Record<string, T> demotes it to an index signature, and keyof StringIndexSignatureType is string | number, by design.

[typescript-bot]

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