feat: undefinedOnPartialDeep type

index.d.ts

@@ -24,6 +24,7 @@ export type {PickIndexSignature} from './source/pick-index-signature';
 export type {PartialDeep, PartialDeepOptions} from './source/partial-deep';
 export type {RequiredDeep} from './source/required-deep';
 export type {PartialOnUndefinedDeep, PartialOnUndefinedDeepOptions} from './source/partial-on-undefined-deep';
+export type {UndefinedOnPartialDeep} from './source/undefined-on-partial-deep';
 export type {ReadonlyDeep} from './source/readonly-deep';
 export type {LiteralUnion} from './source/literal-union';
 export type {Promisable} from './source/promisable';

package.json

@@ -15,7 +15,8 @@
 		"node": ">=16"
 	},
 	"scripts": {
-		"test": "xo && tsd && tsc && node script/test/source-files-extension.js"
+		"test": "xo && tsd && tsc && npm run test:undefined-on-partial-deep && node script/test/source-files-extension.js",
+		"test:undefined-on-partial-deep": "cd test-d/undefined-on-partial-deep && tsc --project tsconfig.json"
 	},
 	"files": [
 		"index.d.ts",

readme.md

@@ -129,6 +129,7 @@ Click the type names for complete docs.
 - [`PickIndexSignature`](source/pick-index-signature.d.ts) - Pick only index signatures from the given object type, leaving out all explicitly defined properties.
 - [`PartialDeep`](source/partial-deep.d.ts) - Create a deeply optional version of another type. Use [`Partial<T>`](https://www.typescriptlang.org/docs/handbook/utility-types.html#partialtype) if you only need one level deep.
 - [`PartialOnUndefinedDeep`](source/partial-on-undefined-deep.d.ts) - Create a deep version of another type where all keys accepting `undefined` type are set to optional.
+- [`UndefinedOnPartialDeep`](source/undefined-on-partial-deep.d.ts) - Create a deep version of another type where all optional keys are set to also accept `undefined`.
 - [`ReadonlyDeep`](source/readonly-deep.d.ts) - Create a deeply immutable version of an `object`/`Map`/`Set`/`Array` type. Use [`Readonly<T>`](https://www.typescriptlang.org/docs/handbook/utility-types.html#readonlytype) if you only need one level deep.
 - [`LiteralUnion`](source/literal-union.d.ts) - Create a union type by combining primitive types and literal types without sacrificing auto-completion in IDEs for the literal type part of the union. Workaround for [Microsoft/TypeScript#29729](https://github.com/Microsoft/TypeScript/issues/29729).
 - [`Tagged`](source/opaque.d.ts) - Create a [tagged type](https://medium.com/@KevinBGreene/surviving-the-typescript-ecosystem-branding-and-type-tagging-6cf6e516523d) that can support [multiple tags](https://github.com/sindresorhus/type-fest/issues/665) if needed.

source/undefined-on-partial-deep.d.ts

@@ -0,0 +1,81 @@
+import type {BuiltIns} from './internal';
+import type {Merge} from './merge';
+
+/**
+Create a deep version of another type where all optional keys are set to also accept `undefined`.
+
+Note: This is only needed when the [`exactOptionalPropertyTypes`](https://www.typescriptlang.org/tsconfig#exactOptionalPropertyTypes) TSConfig setting is enabled.
+
+Use-cases:
+- When `exactOptionalPropertyTypes` is enabled, an object like `{a: undefined}` is not assignable to the type `{a?: number}`. You can use `UndefinedOnPartialDeep<{a?: number}>` to make it assignable.
+
+@example
+```
+import type {UndefinedOnPartialDeep} from 'type-fest';
+
+interface Settings {
+	optionA: string;
+	optionB?: number;
+	subOption: {
+		subOptionA: boolean;
+		subOptionB?: boolean;
+	}
+};
+
+const testSettingsA: Settings = {
+	optionA: 'foo',
+	optionB: undefined, // TypeScript error if `exactOptionalPropertyTypes` is true.
+	subOption: {
+		subOptionA: true,
+		subOptionB: undefined, // TypeScript error if `exactOptionalPropertyTypes` is true
+	},
+};
+
+const testSettingsB: UndefinedOnPartialDeep<Settings> = {
+	optionA: 'foo',
+	optionB: undefined, // 👉 `optionB` can be set to undefined now.
+	subOption: {
+		subOptionA: true,
+		subOptionB: undefined, // 👉 `subOptionB` can be set to undefined now.
+	},
+};
+```
+*/
+export type UndefinedOnPartialDeep<T> =
+	// Handle built-in type and function
+	T extends BuiltIns | Function
+		? T
+		// Handle tuple and array
+		: T extends readonly unknown[]
+			? UndefinedOnPartialList<T>
+			// Handle map and readonly map
+			: T extends Map<infer K, infer V>
+				? Map<K, UndefinedOnPartialDeep<V>>
+				: T extends ReadonlyMap<infer K, infer V>
+					? ReadonlyMap<K, UndefinedOnPartialDeep<V>>
+					// Handle set and readonly set
+					: T extends Set<infer K>
+						? Set<UndefinedOnPartialDeep<K>>
+						: T extends ReadonlySet<infer K>
+							? ReadonlySet<UndefinedOnPartialDeep<K>>
+							// Handle object
+							: T extends Record<any, any>
+								? {
+									[KeyType in keyof T]: undefined extends T[KeyType]
+										? UndefinedOnPartialDeep<T[KeyType]> | undefined
+										: UndefinedOnPartialDeep<T[KeyType]>
+								}
+								: T; // If T is not builtins / function / array / map / set / object, return T
+
+// Handle tuples and arrays
+type UndefinedOnPartialList<T extends readonly unknown[]> = T extends []
+	? []
+	: T extends [infer F, ...infer R]
+		? [UndefinedOnPartialDeep<F>, ...UndefinedOnPartialDeep<R>]
+		: T extends readonly [infer F, ...infer R]
+			? readonly [UndefinedOnPartialDeep<F>, ...UndefinedOnPartialDeep<R>]
+			: T extends Array<infer F>
+				? Array<UndefinedOnPartialDeep<F>>
+				: T extends ReadonlyArray<infer F>
+					? ReadonlyArray<UndefinedOnPartialDeep<F>>
+					: never;

test-d/undefined-on-partial-deep/tsconfig.json

@@ -0,0 +1,7 @@
+{
+	"extends": "../../tsconfig",
+	"compilerOptions": {
+		"exactOptionalPropertyTypes": true,
+	},
+	"exclude": []
+}

test-d/undefined-on-partial-deep/undefined-on-partial-deep.ts

@@ -0,0 +1,80 @@
+/**
+@note This file is used for testing by `tsc` but not `tsd`, so we can just test assignable.
+*/
+import {expectAssignable} from 'tsd';
+import type {UndefinedOnPartialDeep} from '../../source/undefined-on-partial-deep';
+
+type TestType1 = UndefinedOnPartialDeep<{required: string; optional?: string; optional2?: number; optional3?: string}>;
+expectAssignable<TestType1>({required: '', optional: undefined, optional2: 1});
+
+type TestType2 = UndefinedOnPartialDeep<{optional?: string | undefined}>;
+expectAssignable<TestType2>({optional: undefined});
+
+type TestType3 = UndefinedOnPartialDeep<{requiredWithUndefind: string | undefined}>;
+expectAssignable<TestType3>({requiredWithUndefind: undefined});
+
+// Test null and undefined
+type NullType = UndefinedOnPartialDeep<{null?: null}>;
+expectAssignable<NullType>({null: undefined});
+type UndefinedType = UndefinedOnPartialDeep<{ud?: undefined}>;
+expectAssignable<UndefinedType>({ud: undefined});
+
+// Test mixed types
+type MixedType = UndefinedOnPartialDeep<{
+	required: string;
+	union?: 'test1' | 'test2';
+	boolean?: boolean;
+	string?: string;
+	symbol?: symbol;
+	date?: Date;
+	regExp?: RegExp;
+	func?: (args0: string, args1: number) => boolean;
+}>;
+expectAssignable<MixedType>({
+	required: '',
+	union: undefined,
+	boolean: undefined,
+	string: undefined,
+	symbol: undefined,
+	date: undefined,
+	regExp: undefined,
+	func: undefined,
+});
+
+// Test object
+type ObjectType = UndefinedOnPartialDeep<{obj?: {key: string}}>;
+expectAssignable<ObjectType>({obj: undefined});
+
+type ObjectDeepType = UndefinedOnPartialDeep<{obj?: {subObj?: {key?: string}}}>;
+expectAssignable<ObjectDeepType>({obj: undefined});
+expectAssignable<ObjectDeepType>({obj: {subObj: undefined}});
+expectAssignable<ObjectDeepType>({obj: {subObj: {key: undefined}}});
+
+// Test map
+type MapType = UndefinedOnPartialDeep<{map?: Map<string, {key?: string}>}>;
+expectAssignable<MapType>({map: undefined});
+expectAssignable<MapType>({map: new Map([['', {key: undefined}]])});
+
+// Test set
+type SetType = UndefinedOnPartialDeep<{set?: Set<{key?: string}>}>;
+expectAssignable<SetType>({set: undefined});
+expectAssignable<SetType>({set: new Set([{key: undefined}])});
+
+// Test array and tuple
+type TupleType = UndefinedOnPartialDeep<{tuple?: [string, number]}>;
+expectAssignable<TupleType>({tuple: undefined});
+type ArrayType = UndefinedOnPartialDeep<{array?: string[]}>;
+expectAssignable<ArrayType>({array: undefined});
+type ArrayDeepType = UndefinedOnPartialDeep<{array?: Array<{subArray?: string[]}>}>;
+expectAssignable<ArrayDeepType>({array: undefined});
+expectAssignable<ArrayDeepType>({array: [{subArray: undefined}]});
+type ObjectListType = UndefinedOnPartialDeep<{array?: Array<{key?: string}>}>;
+expectAssignable<ObjectListType>({array: undefined});
+expectAssignable<ObjectListType>({array: [{key: undefined}]});
+
+// Test readonly array
+type ReadonlyType = UndefinedOnPartialDeep<{readonly?: readonly string[]}>;
+expectAssignable<ReadonlyType>({readonly: undefined});
+// eslint-disable-next-line @typescript-eslint/array-type
+type ReadonlyArrayTest = UndefinedOnPartialDeep<{readonly?: ReadonlyArray<string>}>;
+expectAssignable<ReadonlyArrayTest>({readonly: undefined});