keyof { [x: string]: unknown } unexpectedly includes number

[otonashixav]

Bug Report

Types with an index signature for strings have number included in keyof, that is that keyof { [x: string]: unknown } = string | number. I find this unexpected. A possible explanation for this behaviour is that number can index such types be being implicitly coerced to a string:

const a: { [x: string]: unknown; } = {};
a[0] = "example"; // this is fine

However, consider that Record<string, unknown> is equally capable of such behaviour, and keyof Record<string, unknown> does not include number:

const b: Record<string, unknown> = {};
b[0] = "inconsistency"; // this is also fine

In that case, does it make sense for number to appear, considering it is not a prerequisite for numbers to implicitly index string-indexed types? Or alternatively, does Record<string, unknown> incorrectly not report number?

More examples are provided in the playground link.

🔎 Search Terms

• keyof string index signature
• keyof index signature
• index signature
• string index signature includes number

🕗 Version & Regression Information

This is the behavior in every version I tried, and I reviewed the FAQ for entries about ctrl-f "keyof"

⏯ Playground Link

Playground link with relevant code

💻 Code

This is a short snippet of the playground example with the main inconsistent example.

type A = { [x: string]: unknown; }
type KA = keyof A;
//   ^? - type KA = string | number
const a: A = {};
a[0]; // indexable by number therefore number in keyof A?

type B = Record<string, unknown>
type KB = keyof B;
//   ^? - type KB = string
const b: B = {};
b[0]; // indexable by number without number in keyof B

🙁 Actual behavior

keyof { [x: string]: unknown } unexpectedly includes number, whereas keyof Record<string, unknown> does not, which is inconsistent.

🙂 Expected behavior

keyof { [x: string]: unknown } should not include number.

I find this more reasonable than the alternative possibly expected behaviour: keyof Record<string, unknown> and all types which number can index via being implicitly coerced to a string should include number as a key.

[MartinJohns]

This is working as intended: #23592 & Release Notes 2.9

[otonashixav]

If it is working as intended, should keyof Record<string, unknown> also be string | number, seeing as numbers have no issue indexing it? Or should numbers not be allowed to index Record<string, unknown>, or otherwise is { [x in string]: unknown } somehow distinct from { [x: string]: unknown }?

I'm assuming this description of keyof is how it is intended to work in this scenario: #23994 (comment)

keyof is the set of "known" keys for a type. in other words, set of keys the compiler will let you index into the type without a no-implicit-any-error. so if a type has a number index signature, it can be only indexed with number; if it has string index signature, it can be indexed with number or with string.

[fatcerberus]

Record<string, unknown> behaves differently than the equivalent index signature because it's constructed using a mapped type. Why the behavior is different I don't know, but either way it's still legal to index the record type with a numeric key, so it seems like the keyof behavior might be itself a bug:
https://www.typescriptlang.org/play?#code/MYewdgzgLgBAZiEAuGAlApqATgEwDzRYCWYA5gDQwCuYA1mCAO5gB8MAvDAN4C+A3AChQkWACMAhlhRcYAbQAeKQiVIBdFDXpMwMHh278BQ8NHiIA0ugCeKWtZBwYUKwAd0DsyH0AiBCAlY3oIA9MEw4TAAegD8xiIwAZY2MHZWHs5uHgH6ACwATCFhETFGfrIADKr6ABwAjAUCARVVnPXl1YJAA

[RyanCavanaugh]

Keeping keyof Record<string, unknown> as string means it has proper variance behavior.

The inconsistency can't be eliminated, just moved around, since Record<"0", unknown> is not ever going to be "0" | 0

[justingrant]

Keeping keyof Record<string, unknown> as string means it has proper variance behavior.

Leaving a breadcrumb for future readers: even though a single Record will produce only the desired behavior, if you combine two Record<string, unknown> objects using object spread then the resulting key type will be string | number.

I assume this is #56985.

const foo: Record<string, unknown> = {foo: 'foobar' };
const bar: Record<string, unknown> = {bar: 'foobar' };
const foobar = {...foo, ...bar };

const fooKey: keyof typeof foo = "whatever";
//    ^? const fooKey: string;
const barKey: keyof typeof bar = "whatever";
//    ^? const barKey: string;
const foobarKey: keyof typeof foobar = "whatever";
//    ^? const foobarKey: string | number;

https://www.typescriptlang.org/play/?#code/MYewdgzgLgBAZiEAuGAlApqATgEwDzRYCWYA5gDQwCuYA1mCAO5gB8MAvDAN4LIwDkvAEYBDLPxgBfANwAoUJFiisKDNnyESFanQbM2nLspSDEyiTPnho8M2I7cAdM96Vnj5VLlXFtkAGl0AE8UWmCQOBgoIIAHdAi-BwAiRgALESh0ADd0LCS5AHoCmBKYAD0Afh8bZUCQmDCghOi4hM9OFPTMnLzC4tLK6thhMTrQ8MiW+MiRrGS0jOzc-Nki0vKqoA