Function type inference

[schusovskoy]

TypeScript Version: 3.9.2

Search Terms:
function type

Code

type GetKeys = <T>(obj: T) => (keyof T)[]
const keys: GetKeys = Object.keys

Expected behavior:
The last line should throw an error, cause string[] is not assignable to (keyof T)[]

Actual behavior:
No error

Playground Link: https://www.typescriptlang.org/play/index.html?ssl=1&ssc=1&pln=2&pc=34#code/C4TwDgpgBA4hwGkIgM5QLxQDwBUB8AFAPYBGAVgFxQ4CUGeUBA1skQGbU0DaAugFABjIgDsUwKC1RU4iZGkwB5chAHAAdJJRA

Related Issues:
Maybe that #38879

[MartinJohns]

This is correct. Object.keys returns string[], not (keyof T)[].

See #37495 (and many many many more).

[RyanCavanaugh]

https://stackoverflow.com/questions/55012174/why-doesnt-object-keys-return-a-keyof-type-in-typescript

[schusovskoy]

My question was not about why Object.keys has (obj: object) => string[] type, but rather why (obj: object) => string[] is assignable to <T>(obj: T) => (keyof T)[]

[RyanCavanaugh]

Assignability to generic types substitutes any for unconstrained type parameters

[schusovskoy]

@RyanCavanaugh thanks for the explanation, but why then does the code below raise an error?

https://www.typescriptlang.org/play/index.html#code/CYUwxgNghgTiAEYD2A7AzgF3kgRgK3AwGkQBPNALngApc8q7CBKeAXgD55MYBLFAcwDaAXQBQojKQAOCAOIhiZNG3gAeACrwQADwwgUwZYzAZ2tfFXUsONANZkkAM3hWRo5Oiz3yARirzFchUAeXxCADpvNHdUTHgogCZ-BRIg1mwwk1S0IA

[RyanCavanaugh]

@schusovskoy the definition you have there doesn't match Object.keys's definition

[schusovskoy]

If I understand correctly there are next object types:
Object – is any object including primitives
{} – is the same as Object
object – any non primitive object

Object.keys has type

type Keys = {
  (o: object): string[]
  (o: {}): string[]
}

I guess it's the same as

type Keys = ((o: object) => string[]) & ((o: {}) => string[])

I thought it means that Object.keys is the function which can accept any non primitive object and returns array of string and also it's the function which can accept any object including primitives and returns array of string. So it looks like the second case covers the first one.
I thought that (o: {}) => string[] is equivalent to ((o: object) => string[]) & ((o: {}) => string[]). But it's also not assignable to <T>(obj: T) => (keyof T)[]. So only intersection of the types is assignable.

I really want to understand how and why it's work like this. So can you help me with that please.

[schusovskoy]

@MartinJohns @RyanCavanaugh Guys excuse me for my impatience but I can’t find any information, and it seems that you’re the only people who can help me.