Return type not assignable to Record

[delicate parcel]

NoSpoon#0148

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Preview:```ts
type SomeType = Record<string, string>

function myFunc<
A extends SomeType,
K extends keyof A & string

(arr: A[], keys: readonly K[]): Record<K, string> {
// Do something here

ret
...```

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[modern blade]

Not sure why Typescript is giving me an error even though I've explicitly returned an object with property keys conforming to Record<K, string)

[solar jasper]

keys is not typed to be all elements of K, that's impossible, and that's the important part

fromEntries is giving { [key: K]: string }, while Record<K, string> is [key in K]: string }
the difference is that in the latter, all of K is said to be a member, while not so in the former

[modern blade]

But even if I change the return type to Partial<Record<K, string>>, I will still get an error

Just to make sure I understand the problem correctly, { [key: K]: string } is an object with property keys that are a subset of K?

[solar jasper]

oh it's giving [key: string] not [key: K], misread

[solar jasper]

yes

i suppose fromEntries loses type info like the other Object methods

[modern blade]

So are Partial<Record<K, string>> and { [key: K]: string } the same thing?

[modern blade]

Not sure if that's the case though, if you hover your mouse over keys.map(key, you can see key is typed correctly as K

[solar jasper]

yes, the fromEntries loses that exact type info

K is broadened to string

[modern blade]

Without casting the return value with as Record< K, string>, how else would I do something like Object.fromEntries without losing type info?

[solar jasper]

not sure something like that exists

[modern blade]

So I'm guessing casting it is my only option

[solar jasper]

someone else might know something

[modern blade]

https://dev.to/svehla/typescript-object-fromentries-389c

I suppose one option would be to redeclare the typings for Object.fromEntries

[wanton bloom]

those are mutually assignable

[wanton bloom]

or rather, it's unsound if it retains type info

[wanton bloom]

quite unsafe, but yes that's probably the best option here

(being unsafe is the reason why it's not the default typings)

[modern blade]

Apparently sindresorhus created a utility for this:

https://github.com/sindresorhus/ts-extras/blob/main/source/object-from-entries.ts

[modern blade]

Can you please elaborate on why it's unsafe?

[wanton bloom]

K[] may not have all the keys

const x: (1 | 2 | 3)[] = [];

or rather [K, V][] may not have all the keys

[modern blade]

But it would if you're passing in a literal array?

[wanton bloom]

yeah. but at that point there's no reason to do that instead of having an object in the first place

[modern blade]

True, but in my case it would still be useful and type safe since my keys are inferred from my function arguments

[wanton bloom]

except your parameters aren't literal tuples