Merge: Don't turn undefined into optional key (#787)

sindresorhus · Jan 2, 2024 · 0aec247 · 0aec247
1 parent 02a95c5
commit 0aec247
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Showing 2 changed files with 23 additions and 5 deletions.
diff --git a/source/merge.d.ts b/source/merge.d.ts
@@ -1,6 +1,6 @@
 import type {OmitIndexSignature} from './omit-index-signature';
 import type {PickIndexSignature} from './pick-index-signature';
-import type {EnforceOptional} from './enforce-optional';
+import type {Simplify} from './simplify';
 
 // Merges two objects without worrying about index signatures.
 type SimpleMerge<Destination, Source> = {
@@ -41,6 +41,8 @@ export type FooBar = Merge<Foo, Bar>;
 
 @category Object
 */
-export type Merge<Destination, Source> = EnforceOptional<
+export type Merge<Destination, Source> =
+Simplify<
 SimpleMerge<PickIndexSignature<Destination>, PickIndexSignature<Source>>
-& SimpleMerge<OmitIndexSignature<Destination>, OmitIndexSignature<Source>>>;
+& SimpleMerge<OmitIndexSignature<Destination>, OmitIndexSignature<Source>>
+>;
diff --git a/test-d/merge.ts b/test-d/merge.ts
@@ -100,8 +100,8 @@ expectType<{
 	c: undefined;
 	a?: string;
 	d?: string;
-	e?: number;
-	f?: number;
+	e: number | undefined;
+	f: number | undefined;
 	g: undefined;
 }>(fooBarWithOptionalKeys);
 
@@ -148,3 +148,19 @@ expectType<{
 	foo: true;
 	bar?: any;
 }>(sourceWithAny);
+
+// Test for issue https://github.com/sindresorhus/type-fest/issues/601
+type Baz = {
+	t1?: number;
+	t2?: number;
+	t3?: number;
+	t4?: number;
+};
+declare const baz: Merge<Pick<Baz, 't2' | 't4'>, {
+	list: string[];
+}>;
+expectType<{
+	t2?: number;
+	t4?: number;
+	list: string[];
+}>(baz);