PartialDeep: Add allowUndefinedInArrays option

source/partial-deep.d.ts

@@ -10,6 +10,13 @@ export type PartialDeepOptions = {
 	@default false
 	*/
 	readonly recurseIntoArrays?: boolean;
+
+	/**
+	Whether to authorize undefined values in arrays and tuples.
+
+	@default true
+	*/
+	readonly allowUndefinedInArrays?: boolean;
 };
 
 /**
@@ -54,6 +61,21 @@ const partialSettings: PartialDeep<Settings, {recurseIntoArrays: true}> = {
 };
 ```
 
+You can prevent undefined values in recurse arrays and tuples by passing `{recurseIntoArrays: true; allowUndefinedInArrays: false}` as the second type argument:
+
+```
+import type {PartialDeep} from 'type-fest';
+
+interface Settings {
+	languages: string[];
+}
+
+const partialSettings: PartialDeep<Settings, {recurseIntoArrays: true; allowUndefinedInArrays: false}> = {
+	languages: [undefined] 	// Error
+	languages: []			// OK
+};
+```
+
 @category Object
 @category Array
 @category Set
@@ -74,8 +96,8 @@ 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['allowUndefinedInArrays'] extends false ? ItemType : ItemType | undefined, Options>>
+										: Array<PartialDeep<Options['allowUndefinedInArrays'] 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 `{allowUndefinedInArrays: true}` is the default
+const partialDeepAllowUndefinedInArraysFoo: PartialDeep<typeof foo, {recurseIntoArrays: true}> = foo;
+expectType<Array<string | undefined> | undefined>(partialDeepAllowUndefinedInArraysFoo.bar!.array);
+// Check that `{allowUndefinedInArrays: false}` behaves as intended
+const partialDeepDoNotAllowUndefinedInArraysFoo: PartialDeep<typeof foo, {recurseIntoArrays: true; allowUndefinedInArrays: false}> = foo;
+expectType<string[] | undefined>(partialDeepDoNotAllowUndefinedInArraysFoo.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);