WritableKeysOf / ReadonlyKeysOf: Fix behavior with unions and optional properties (#1088)

Author: som-sm

source/readonly-keys-of.d.ts

@@ -1,4 +1,5 @@
-import type {IsEqual} from './is-equal';
+import type {KeysOfUnion} from './keys-of-union';
+import type {WritableKeysOf} from './writable-keys-of';
 
 /**
 Extract all readonly keys from the given type.
@@ -24,6 +25,4 @@ const update1: UpdateResponse<User> = {
 
 @category Utilities
 */
-export type ReadonlyKeysOf<T> = NonNullable<{
-	[P in keyof T]: IsEqual<{[Q in P]: T[P]}, {readonly [Q in P]: T[P]}> extends true ? P : never
-}[keyof T]>;
+export type ReadonlyKeysOf<T> = Exclude<KeysOfUnion<T>, WritableKeysOf<T>>;

source/writable-keys-of.d.ts

@@ -1,4 +1,5 @@
 import type {IsEqual} from './is-equal';
+import type {KeysOfUnion} from './keys-of-union';
 
 /**
 Extract all writable keys from the given type.
@@ -25,6 +26,6 @@ const update1: UpdateRequest<User> = {
 
 @category Utilities
 */
-export type WritableKeysOf<T> = NonNullable<{
-	[P in keyof T]: IsEqual<{[Q in P]: T[P]}, {readonly [Q in P]: T[P]}> extends false ? P : never
-}[keyof T]>;
+export type WritableKeysOf<T> = KeysOfUnion<{
+	[P in keyof T as IsEqual<{[Q in P]: T[P]}, {readonly [Q in P]: T[P]}> extends false ? P : never]: never
+}>;

test-d/readonly-keys-of.ts

@@ -27,3 +27,16 @@ declare const test3: ReadonlyKeysOf3;
 expectType<'b'>(test1);
 expectType<'a' | 'b'>(test2);
 expectType<never>(test3);
+
+expectType<'a' | 'c'>({} as ReadonlyKeysOf<{readonly a?: string; b: number; readonly c: boolean}>);
+expectType<'c'>({} as ReadonlyKeysOf<{a?: string; b: number; readonly c: boolean}>);
+
+// Unions
+expectType<'b' | 'c'>({} as ReadonlyKeysOf<{a: string; readonly b: number} | {readonly c?: string; d?: number}>);
+
+// TODO: Uncomment when targeting TypeScript 5.3 or later.
+// Arrays
+// expectType<never>({} as Extract<ReadonlyKeysOf<[string, number, boolean]>, number | `${number}`>);
+// expectType<number | '0' | '1' | '2'>({} as Extract<ReadonlyKeysOf<readonly [string, number, boolean]>, number | `${number}`>);
+// expectType<never>({} as Extract<ReadonlyKeysOf<string[]>, number | `${number}`>);
+// expectType<number>({} as Extract<ReadonlyKeysOf<readonly string[]>, number | `${number}`>);

test-d/writable-keys-of.ts

@@ -27,3 +27,16 @@ declare const test3: WritableKeysOf3;
 expectType<'b'>(test1);
 expectType<'a' | 'b'>(test2);
 expectType<never>(test3);
+
+expectType<'a' | 'c'>({} as WritableKeysOf<{a?: string; readonly b: number; c: boolean}>);
+expectType<'c'>({} as WritableKeysOf<{readonly a?: string; readonly b: number; c: boolean}>);
+
+// Unions
+expectType<'b' | 'c'>({} as WritableKeysOf<{readonly a: string; b: number} | {c?: string; readonly d?: number}>);
+
+// TODO: Uncomment when targeting TypeScript 5.3 or later.
+// Arrays
+// expectType<number | '0' | '1' | '2'>({} as Extract<WritableKeysOf<[string, number, boolean]>, number | `${number}`>);
+// expectType<never>({} as Extract<WritableKeysOf<readonly [string, number, boolean]>, number | `${number}`>);
+// expectType<number>({} as Extract<WritableKeysOf<string[]>, number | `${number}`>);
+// expectType<never>({} as Extract<WritableKeysOf<readonly string[]>, number | `${number}`>);