UnionToIntersection: Allow indexing by the resulting type (#682)

sindresorhus · Oct 7, 2023 · 61f2ff9 · 61f2ff9
1 parent 113400b
commit 61f2ff9
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Showing 2 changed files with 8 additions and 2 deletions.
diff --git a/source/union-to-intersection.d.ts b/source/union-to-intersection.d.ts
@@ -56,5 +56,6 @@ export type UnionToIntersection<Union> = (
 		// Infer the `Intersection` type since TypeScript represents the positional
 		// arguments of unions of functions as an intersection of the union.
 ) extends ((mergedIntersection: infer Intersection) => void)
-	? Intersection
+	// The `& Union` is to allow indexing by the resulting type
+	? Intersection & Union
 	: never;
diff --git a/test-d/union-to-intersection.ts b/test-d/union-to-intersection.ts
@@ -1,4 +1,4 @@
-import {expectAssignable} from 'tsd';
+import {expectAssignable, expectType} from 'tsd';
 import type {UnionToIntersection} from '../index';
 
 declare const intersection1: UnionToIntersection<{a: string} | {b: number}>;
@@ -7,3 +7,8 @@ expectAssignable<{a: string; b: number}>(intersection1);
 // Creates a union of matching properties.
 declare const intersection2: UnionToIntersection<{a: string} | {b: number} | {a: () => void}>;
 expectAssignable<{a: string | (() => void); b: number}>(intersection2);
+
+// It's possible to index by the resulting type.
+type ObjectsUnion = {a: string; z: string} | {b: string; z: string} | {c: string; z: string};
+declare const value: ObjectsUnion[UnionToIntersection<keyof ObjectsUnion>];
+expectType<string>(value);