Merge: Fix optional keys and type override (#456)

source/enforce-optional.d.ts

@@ -0,0 +1,47 @@
+import type {Simplify} from './simplify';
+
+// Returns `never` if the key is optional otherwise return the key type.
+type RequiredFilter<Type, Key extends keyof Type> = undefined extends Type[Key]
+	? Type[Key] extends undefined
+		? Key
+		: never
+	: Key;
+
+// Returns `never` if the key is required otherwise return the key type.
+type OptionalFilter<Type, Key extends keyof Type> = undefined extends Type[Key]
+	? Type[Key] extends undefined
+		? never
+		: Key
+	: never;
+
+/**
+Enforce optional keys (by adding the `?` operator) for keys that have a union with `undefined`.
+
+@example
+```
+import type {Merge} from 'type-fest';
+
+type Foo = {
+	a: string;
+	b?: string;
+	c: undefined;
+	d: number | undefined;
+};
+
+type FooBar = EnforceOptional<Foo>;
+// => {
+// 	a: string;
+// 	b?: string;
+// 	c: undefined;
+// 	d?: number;
+// }
+```
+
+@internal
+@category Object
+*/
+export type EnforceOptional<ObjectType> = Simplify<{
+	[Key in keyof ObjectType as RequiredFilter<ObjectType, Key>]: ObjectType[Key]
+} & {
+	[Key in keyof ObjectType as OptionalFilter<ObjectType, Key>]?: Exclude<ObjectType[Key], undefined>
+}>;

source/merge.d.ts

@@ -1,6 +1,6 @@
 import type {OmitIndexSignature} from './omit-index-signature';
 import type {PickIndexSignature} from './pick-index-signature';
-import type {Simplify} from './simplify';
+import type {EnforceOptional} from './enforce-optional';
 
 /**
 Merge two types into a new type. Keys of the second type overrides keys of the first type.
@@ -36,10 +36,10 @@ export type FooBar = Merge<Foo, Bar>;
 
 @category Object
 */
-export type Merge<Destination, Source> = Simplify<{
-	[Key in keyof OmitIndexSignature<Destination & Source>]: Key extends keyof Source
+export type Merge<Destination, Source> = EnforceOptional<{
+	[Key in keyof OmitIndexSignature<Destination> | keyof OmitIndexSignature<Source>]: Key extends keyof Source
 		? Source[Key]
 		: Key extends keyof Destination
 			? Destination[Key]
 			: never;
-} & PickIndexSignature<Destination & Source>>;
+} & PickIndexSignature<Destination> & PickIndexSignature<Source>>;

test-d/enforce-optional.ts

@@ -0,0 +1,20 @@
+import {expectType} from 'tsd';
+import type {EnforceOptional} from '../source/enforce-optional';
+
+type Foo = {
+	a: string;
+	b?: string;
+	c: undefined;
+	d: number | undefined;
+};
+
+type EnforcedOptionalFoo = EnforceOptional<Foo>;
+
+declare const enforcedOptionalFoo: EnforcedOptionalFoo;
+
+expectType<{
+	a: string;
+	b?: string;
+	c: undefined;
+	d?: number;
+}>(enforcedOptionalFoo);

test-d/merge.ts

@@ -1,4 +1,4 @@
-import {expectAssignable, expectError} from 'tsd';
+import {expectError, expectType} from 'tsd';
 import type {Merge} from '../index';
 
 type Foo = {
@@ -11,7 +11,7 @@ type Bar = {
 };
 
 const ab: Merge<Foo, Bar> = {a: 1, b: 2};
-expectAssignable<{a: number; b: number}>(ab);
+expectType<{a: number; b: number}>(ab);
 
 // eslint-disable-next-line @typescript-eslint/consistent-type-definitions
 interface FooInterface {
@@ -39,7 +39,7 @@ const fooBar: FooBar = {
 	baz: true,
 };
 
-expectAssignable<{
+expectType<{
 	[x: string]: unknown;
 	[x: number]: number;
 	[x: symbol]: boolean;
@@ -56,3 +56,51 @@ expectError(setFooBar({
 	bar: new Date(),
 	baz: true,
 }));
+
+// Checks that a property can be replaced by another property that is not of the same type. This issue was encountered in `MergeDeep' with the default options.
+type FooDefaultOptions = {
+	stripUndefinedValues: false;
+};
+
+type FooOptions = Merge<FooDefaultOptions, {stripUndefinedValues: true}>;
+
+expectType<FooOptions>({stripUndefinedValues: true});
+
+// Test that optional keys are enforced.
+type FooWithOptionaKeys = {
+	[x: string]: unknown;
+	[x: number]: unknown;
+	a: string;
+	b?: string;
+	c: undefined;
+	d: string;
+	e: number | undefined;
+};
+
+type BarWithOptionaKeys = {
+	[x: number]: number;
+	[x: symbol]: boolean;
+	a?: string;
+	b: string;
+	d?: string;
+	f: number | undefined;
+	g: undefined;
+};
+
+type FooBarWithOptionalKeys = Merge<FooWithOptionaKeys, BarWithOptionaKeys>;
+
+declare const fooBarWithOptionalKeys: FooBarWithOptionalKeys;
+
+// Note that `c` and `g` is not marked as optional and this is deliberate, as this is the behaviour expected by the older version of Merge. This may change in a later version.
+expectType<{
+	[x: number]: number;
+	[x: symbol]: boolean;
+	[x: string]: unknown;
+	b: string;
+	c: undefined;
+	a?: string;
+	d?: string;
+	e?: number;
+	f?: number;
+	g: undefined;
+}>(fooBarWithOptionalKeys);