PartialDeep: Add allowUndefinedInArrays option (#1019)

Co-authored-by: Som Shekhar Mukherjee <49264891+som-sm@users.noreply.github.com>
Co-authored-by: Sindre Sorhus <sindresorhus@gmail.com>

source/partial-deep.d.ts

@@ -1,7 +1,7 @@
 import type {BuiltIns} from './internal';
 
 /**
-@see PartialDeep
+@see {@link PartialDeep}
 */
 export type PartialDeepOptions = {
 	/**
@@ -10,6 +10,32 @@ export type PartialDeepOptions = {
 	@default false
 	*/
 	readonly recurseIntoArrays?: boolean;
+
+	/**
+	Allows `undefined` values in non-tuple arrays.
+
+	- When set to `true`, elements of non-tuple arrays can be `undefined`.
+	- When set to `false`, only explicitly defined elements are allowed in non-tuple arrays, ensuring stricter type checking.
+
+	@default true
+
+	@example
+	You can prevent `undefined` values in non-tuple arrays by passing `{recurseIntoArrays: true; allowUndefinedInNonTupleArrays: false}` as the second type argument:
+
+	```
+	import type {PartialDeep} from 'type-fest';
+
+	type Settings = {
+		languages: string[];
+	};
+
+	declare const partialSettings: PartialDeep<Settings, {recurseIntoArrays: true; allowUndefinedInNonTupleArrays: false}>;
+
+	partialSettings.languages = [undefined]; // Error
+	partialSettings.languages = []; // Ok
+	```
+	*/
+	readonly allowUndefinedInNonTupleArrays?: boolean;
 };
 
 /**
@@ -25,12 +51,12 @@ import type {PartialDeep} from 'type-fest';
 
 const settings: Settings = {
 	textEditor: {
-		fontSize: 14;
-		fontColor: '#000000';
-		fontWeight: 400;
-	}
-	autocomplete: false;
-	autosave: true;
+		fontSize: 14,
+		fontColor: '#000000',
+		fontWeight: 400
+	},
+	autocomplete: false,
+	autosave: true
 };
 
 const applySavedSettings = (savedSettings: PartialDeep<Settings>) => {
@@ -45,7 +71,7 @@ By default, this does not affect elements in array and tuple types. You can chan
 ```
 import type {PartialDeep} from 'type-fest';
 
-interface Settings {
+type Settings = {
 	languages: string[];
 }
 
@@ -54,6 +80,8 @@ const partialSettings: PartialDeep<Settings, {recurseIntoArrays: true}> = {
 };
 ```
 
+@see {@link PartialDeepOptions}
+
 @category Object
 @category Array
 @category Set
@@ -74,8 +102,8 @@ export type PartialDeep<T, Options extends PartialDeepOptions = {}> = T extends
 							? Options['recurseIntoArrays'] extends true
 								? ItemType[] extends T // Test for arrays (non-tuples) specifically
 									? readonly ItemType[] extends T // Differentiate readonly and mutable arrays
-										? ReadonlyArray<PartialDeep<ItemType | undefined, Options>>
-										: Array<PartialDeep<ItemType | undefined, Options>>
+										? ReadonlyArray<PartialDeep<Options['allowUndefinedInNonTupleArrays'] extends false ? ItemType : ItemType | undefined, Options>>
+										: Array<PartialDeep<Options['allowUndefinedInNonTupleArrays'] extends false ? ItemType : ItemType | undefined, Options>>
 									: PartialObjectDeep<T, Options> // Tuples behave properly
 								: T // If they don't opt into array testing, just use the original type
 							: PartialObjectDeep<T, Options>

test-d/partial-deep.ts

@@ -79,6 +79,14 @@ expectAssignable<PartialDeep<RecurseObject>>(recurseObject);
 const partialDeepNoRecurseIntoArraysFoo: PartialDeep<typeof foo> = foo;
 // Check that `{recurseIntoArrays: true}` behaves as intended
 expectType<PartialDeep<typeof foo, {recurseIntoArrays: true}>>(partialDeepFoo);
+
+// Check that `{allowUndefinedInNonTupleArrays: true}` is the default
+const partialDeepAllowUndefinedInNonTupleArraysFoo: PartialDeep<typeof foo, {recurseIntoArrays: true}> = foo;
+expectType<Array<string | undefined> | undefined>(partialDeepAllowUndefinedInNonTupleArraysFoo.bar!.array);
+// Check that `{allowUndefinedInNonTupleArrays: false}` behaves as intended
+const partialDeepDoNotAllowUndefinedInNonTupleArraysFoo: PartialDeep<typeof foo, {recurseIntoArrays: true; allowUndefinedInNonTupleArrays: false}> = foo;
+expectType<string[] | undefined>(partialDeepDoNotAllowUndefinedInNonTupleArraysFoo.bar!.array);
+
 // These are mostly the same checks as before, but the array/tuple types are different.
 // @ts-expect-error
 expectType<Partial<typeof foo>>(partialDeepNoRecurseIntoArraysFoo);