Add `UnionMember` type #1368

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Merged

sindresorhus merged 19 commits into sindresorhus:main from taiyakihitotsu:add/last-of-union-20260204

Mar 18, 2026

+107 −16

index.d.ts

-Original file line number
+Diff line change
@@ Expand Up / @@ -169,6 +169,7 @@ export type {IsNullable} from './source/is-nullable.d.ts'; @@
     export type {TupleOf} from './source/tuple-of.d.ts';
     export type {ExclusifyUnion} from './source/exclusify-union.d.ts';
     export type {ArrayReverse} from './source/array-reverse.d.ts';
+    export type {UnionMember} from './source/union-member.d.ts';
     // Template literal types
     export type {CamelCase, CamelCaseOptions} from './source/camel-case.d.ts';
@@ Expand Down @@

readme.md

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+Diff line change
@@ Expand Up / @@ -192,6 +192,7 @@ Click the type names for complete docs. @@
     - [`ConditionalSimplifyDeep`](source/conditional-simplify-deep.d.ts) - Recursively simplifies a type while including and/or excluding certain types from being simplified.
     - [`ExclusifyUnion`](source/exclusify-union.d.ts) - Ensure mutual exclusivity in object unions by adding other members’ keys as `?: never`.
     - [`Optional`](source/optional.d.ts) - Create a type that represents either the value or `undefined`, while stripping `null` from the type.
+    - [`UnionMember`](source/union-member.d.ts) - Returns an arbitrary member of a union type.
     ### Type Guard
@@ Expand Down Expand Up / @@ -358,6 +359,8 @@ Click the type names for complete docs. @@
     - `Maybe`, `Option` - See [`Optional`](source/optional.d.ts)
     - `MaybePromise` - See [`Promisable`](source/promisable.d.ts)
     - `ReadonlyTuple` - See [`TupleOf`](source/tuple-of.d.ts)
+    - `LastOfUnion` - See [`UnionMember`](source/union-member.d.ts)
+    - `FirstOfUnion` - See [`UnionMember`](source/union-member.d.ts)
     ## Tips
@@ Expand Down @@

source/union-member.d.ts

-Original file line number
+Diff line change
@@ -0,0 +1,65 @@
+    import type {UnionToIntersection} from './union-to-intersection.d.ts';
+    import type {IsNever} from './is-never.d.ts';
+    /**
+    Returns an arbitrary member of a union type.
+    Use-cases:
+    - Implementing recursive type functions that accept a union type.
+    @example
+    ```
+    import type {UnionMember, IsNever} from 'type-fest';
+    type UnionLength<T, Acc extends any[] = []> =
+    	UnionMember<T> extends infer Member
+    		? IsNever<Member> extends false
+    			? UnionLength<Exclude<T, Member>, [...Acc, Member]>
+    			: Acc['length']
+    		: never;
+    type T1 = UnionLength<'foo' | 'bar' | 'baz'>;
+    //=> 3
+    type T2 = UnionLength<{a: string}>;
+    //=> 1
+    ```
+    - Picking an arbitrary member from a union
+    @example
+    ```
+    import type {UnionMember, Primitive, LiteralToPrimitive} from 'type-fest';
+    type IsHomogenous<T extends Primitive> = [T] extends [LiteralToPrimitive<UnionMember<T>>] ? true : false;
+    type T1 = IsHomogenous<1 | 2 | 3 | 4>;
+    //=> true
+    type T2 = IsHomogenous<'foo' | 'bar'>;
+    //=> true
+    type T3 = IsHomogenous<'foo' | 'bar' | 1>;
+    //=> false
+    ```
+    Returns `never` when the input is `never`.
+    @example
+    ```
+    import type {UnionMember} from 'type-fest';
+    type LastNever = UnionMember<never>;
+    //=> never
+    ```
+    @category Type
+    */
+    export type UnionMember<T> =
+    	IsNever<T> extends true
+    		? never
+    		: UnionToIntersection<T extends any ? () => T : never> extends () => (infer R)
+    			? R
+    			: never;
+    export {};

source/union-to-tuple.d.ts

-Original file line number
+Diff line change
@@ -1,19 +1,5 @@
     import type {IsNever} from './is-never.d.ts';
-    import type {UnionToIntersection} from './union-to-intersection.d.ts';
-    /**
-    Returns the last element of a union type.
-    @example
-    ```
-    type Last = LastOfUnion<1 | 2 | 3>;
-    //=> 3
-    ```
-    */
-    type LastOfUnion<T> =
-    UnionToIntersection<T extends any ? () => T : never> extends () => (infer R)
-    	? R
-    	: never;
+    import type {UnionMember} from './union-member.d.ts';
     /**
     Convert a union type into an unordered tuple type of its elements.
@@ Expand Down Expand Up / @@ -50,7 +36,7 @@ const petList = Object.keys(pets) as UnionToTuple<Pet>; @@
     @category Array
     */
-    export type UnionToTuple<T, L = LastOfUnion<T>> =
+    export type UnionToTuple<T, L = UnionMember<T>> =
     IsNever<T> extends false
     	? [...UnionToTuple<Exclude<T, L>>, L]
     	: [];
@@ Expand Down @@

test-d/union-member.ts

-Original file line number
+Diff line change
@@ -0,0 +1,36 @@
+    import {expectType} from 'tsd';
+    import type {UnionMember, IsNever, IsEqual} from '../index.d.ts';
+    expectType<false>({} as boolean extends UnionMember<boolean> ? true : false);
+    expectType<false>({} as (1 | 'foo' | 'bar') extends UnionMember<1 | 'foo' | 'bar'> ? true : false);
+    expectType<false>({} as ({foo: string} | {bar: number}) extends UnionMember<{foo: string} | {bar: number}> ? true : false);
+    expectType<number>({} as UnionMember<number>);
+    expectType<string>({} as UnionMember<string>);
+    expectType<bigint>({} as UnionMember<bigint>);
+    expectType<true>({} as UnionMember<true>);
+    expectType<false>({} as UnionMember<false>);
+    expectType<null>({} as any as UnionMember<null>);
+    expectType<undefined>({} as any as UnionMember<undefined>);
+    expectType<never>({} as UnionMember<never>);
+    expectType<unknown>({} as UnionMember<unknown>);
+    expectType<any>({} as UnionMember<any>);
+    // `WrapMemberInTuple` ensures `UnionMember` selects exactly one member at a time.
+    type WrapMemberInTuple<T, L = UnionMember<T>> =
+    	IsNever<T> extends false
+    		? WrapMemberInTuple<Exclude<T, L>> | [L]
+    		: never;
+    expectType<[1] | [2] | [3]>({} as WrapMemberInTuple<1 | 2 | 3>);
+    expectType<['foo'] | ['bar'] | ['baz']>({} as WrapMemberInTuple<'foo' | 'bar' | 'baz'>);
+    expectType<[1] | ['foo'] | [true] | [100n] | [null] | [undefined]>(
+    	{} as WrapMemberInTuple<1 | 'foo' | true | 100n | null | undefined>,
+    );
+    expectType<[{a: string}] | [{b: number}]>({} as WrapMemberInTuple<{a: string} | {b: number}>);
+    type UnionType = {a: 0} | {readonly a: 0};
+    type PickedUnionMember = UnionMember<UnionType>;
+    // We can't use `UnionType extends PickedUnionMember ? true : false` for testing here,
+    // because that would always be `true` as `UnionType` extends both of its members individually.
+    expectType<false>({} as IsEqual<UnionType, PickedUnionMember>);

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