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Add UnionMember type#1368

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sindresorhus merged 19 commits intosindresorhus:mainfrom
taiyakihitotsu:add/last-of-union-20260204
Mar 18, 2026
Merged

Add UnionMember type#1368
sindresorhus merged 19 commits intosindresorhus:mainfrom
taiyakihitotsu:add/last-of-union-20260204

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@taiyakihitotsu taiyakihitotsu commented Feb 14, 2026 •
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Add UnionMember

  • This is useful to recurse union-types, so I think this should be exported as a global for end-users.
  • We can evade a duplicated definition potentially.

And the current UnionMember, which locally defined in UnionToTuple, returns unknown.

expectType<never>({} as UnionMember<never>);
//=> unknown

This would cause an error if end-users use IsNever for termination check.

Ideally, UnionMember reduces a type, e.g., assuming $A$ and $B$ are type arguments of UnionMember<A | B>, $A \cup B$ -> $B$, so $\emptyset$ -> $\emptyset$ intuitively ($\bot$ -> $\bot$).
This PR's UnionMember handles this.

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@taiyakihitotsu taiyakihitotsu mentioned this pull request Feb 14, 2026
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taiyakihitotsu and others added 3 commits February 15, 2026 09:26
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Update readme.md
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Co-authored-by: Sindre Sorhus <sindresorhus@gmail.com>
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Update test-d/last-of-union.ts
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Co-authored-by: Sindre Sorhus <sindresorhus@gmail.com>
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Update test-d/last-of-union.ts
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Co-authored-by: Sindre Sorhus <sindresorhus@gmail.com>
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taiyakihitotsu commented Feb 15, 2026

@sindresorhus

Thanks for your reviews!

And I believe this suggestion implies that test-d/last-of-union.ts should have a test for recursion without directly importing UnionToTuple to clarify it. Right?

If so, UnionToTuple in test-d/last-of-union.ts fulfills the requirement. Would it be okay to integrate both in test-d/last-of-union.ts?

#1368 (comment)

type UnionToTupleWithExclude<T, L = LastOfUnion<T>> =
	IsNever<T> extends false
		? [...UnionToTupleWithExclude<Exclude<T, L>>, L]
		: [];

expectType<1 | 2 | 3>({} as UnionToTupleWithExclude<1 | 2 | 3>[number]);

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@taiyakihitotsu taiyakihitotsu changed the title LastOfUnion: export internal type, useful to recurse union-types. UnionMember: export internal type, useful to recurse union-types. Feb 15, 2026
@som-sm som-sm requested a review from Copilot February 17, 2026 13:02

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PR mostly LGTM, just few minor comments.

@taiyakihitotsu taiyakihitotsu force-pushed the add/last-of-union-20260204 branch from 4332fc5 to 1c642f4 Compare March 1, 2026 16:34
@taiyakihitotsu taiyakihitotsu requested a review from som-sm March 1, 2026 16:56
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doc: rewrite UnionMember example
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Co-authored-by: Som Shekhar Mukherjee <49264891+som-sm@users.noreply.github.com>
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taiyakihitotsu commented Mar 3, 2026

@som-sm

Thank you!
Picked your comment: #1368 (comment)

I guess that's why you chose not to add an example.

Yes.
Now I think UnionLength defined in the doc is enough to describe how to write a recursion of union type.

@taiyakihitotsu taiyakihitotsu requested a review from som-sm March 3, 2026 02:26
@som-sm som-sm changed the title UnionMember: export internal type, useful to recurse union-types. Add UnionMember type Mar 3, 2026
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som-sm commented Mar 17, 2026 •
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@sindresorhus If this LGTY, this can be merged. Also, #1349.

@sindresorhus sindresorhus merged commit 878b6df into sindresorhus:main Mar 18, 2026
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