[Bug Report] Wrong keyof type with string index signature

[meteorlxy]

TypeScript Version: 3.9.x

Search Terms: keyof, index signature

Code

interface Test1 {
 [key: string]: string;
}

type KeyofTest1 = keyof Test1

interface Test2 {
 [key: number]: string;
}

type KeyofTest2 = keyof Test2

Expected behavior:

• KeyofTest1 should be string
• KeyofTest2 should be number

Actual behavior:

• KeyofTest1 is string | number (wrong)
• KeyofTest2 is number (correct)

Playground Link: https://www.typescriptlang.org/play/?ssl=1&ssc=1&pln=12&pc=1#code/JYOwLgpgTgZghgYwgAgCoQM5gIzIN4BQyxyA2gNYQCeAXMllKAOYC6dDzA3AQL4EFgqABxQBpagHsY6LLgC8ySlSlpMOfqEixEKGWABM+IiQrU6IAK4BbAEbQ29MIxBNufAcLGTpawwqUqevoEQA

Related Issues:

[IllusionMH]

This is by design
https://www.typescriptlang.org/docs/handbook/release-notes/typescript-2-9.html#support-number-and-symbol-named-properties-with-keyof-and-mapped-types

Given an object type X, keyof X is resolved as follows:

If X contains a string index signature, keyof X is a union of string, number, and the literal types representing symbol-like properties, otherwise

[meteorlxy]

Andrii Dieiev (@IllusionMH) Thanks for your quick reply. ❤️

What's the underlay reason of this "by design" ? 🤔

It looks odd and causes some strange type checking error in some third party packages

[IllusionMH]

I think it's because there's no difference between obj[123] and obj["123"] in JS, and TS won't error in case of orders[orderId] if it's orderId is number (which would be annoying).
Any number converted to string will be valid string, however not every string will be valid number therefore same is not applied for number index signature.

I guess it should be OK to use Extract<keyof T, string> if you don't expect to use numbers there.

But I can be wrong here.

[meteorlxy]

Yeah a possible solution. I'll trying to submit a PR to that package.

Thanks again~ 😄

[ExE-Boss]

Arguably, now that we have #40336 and #40598, keyof should report `${number}` for numeric index signatures.