Strict versions of Extract and Exclude

[Vages]

I have been looking for a strict version of Extract<T, U>. For simplicity, let's call it ExtractStrict<T, U>. ExtractStrict would have one additional feature compared to Extract: ExtractStrict would throw an error if some element in U is not part of T.

I know of an implementation by @ChristianIvicevic which has been proposed for inclusion in Typescript itself: microsoft/TypeScript#31474 I've asked him if he'd be willing to submit it here instead.

Edit: I've found a use case in my code for a strict version of Exclude too (ExcludeStrict), so I've added it to the title.

[sindresorhus]

Sounds useful. A pull request would be welcome.

[Vages]

I'll wait until next Thursday (June 24) for an answer from @ChristianIvicevic; he deserves the chance to get some credit for his contribution.

Otherwise, I'll write my own implementation and submit a PR.

[ChristianIvicevic]

Hey folks, I'll be taking a look at it this week. Quick question about tests @sindresorhus: I noticed that the tests are used to assert what resulting types look like. Does the project use anything to handle certain test cases that are expected to yield invalid results in order to test erroneous type arguments?

[sindresorhus]

Yes, there's an expectError assertion: https://github.com/SamVerschueren/tsd#expecterrorfunction

[Vages]

Here's my implementation of ExcludeStrict, @ChristianIvicevic (a derivative of your ExtractStrict): export type ExcludeStrict<T, U extends T> = Exclude<T, U>;.

[younho9]

Here's my implementation of ExcludeStrict, @ChristianIvicevic (a derivative of your ExtractStrict): export type ExcludeStrict<T, U extends T> = Exclude<T, U>;.

Could ExtractStrict be implemented in the same way? export type ExtractStrict<T, U extends T> = Extract<T, U>;

[VeryCrazyDog]

Just curious, can ExtractStrict implemented in this way?

export type ExtractStrict<T, U extends T> = U;

[Vages]

I'll put up a PR for this in a few hours.

[NN---]

You can use ts-essentials package which has these definitions.

https://github.com/ts-essentials/ts-essentials#StrictExtract

[Vages]

Thank you for the tip, @NN--- . I no longer see any point in fixing PR #291 :/