WritableKeysOf / ReadonlyKeysOf: Fix behaviour with unions and optional properties

[som-sm]

Fixes #1087

Looks like there's some issue in TS v5.4 and later, because it works fine in v5.3 and below (playground link). Regardless, the updated implementation should work fine across all versions.

Also, fixes behaviour with unions.

The updated implementation also improves behaviour with arrays:

type CurrentBehaviour = WritableKeysOf<[string, number]>;
//   ^? type CurrentBehaviour = 2 | "0" | "1" | (() => ArrayIterator<"0" | "1">) | {
//          [x: number]: boolean | undefined;
//          length?: boolean | undefined;
//          toString?: boolean | undefined;
//          toLocaleString?: boolean | undefined;
//          ... 28 more ...;
//          readonly [Symbol.unscopables]?: boolean | undefined;
//      } | ... 28 more ... | ((searchElement: "0" | "1", fromIndex?: number) => boolean)

type UpdatedBehaviour = WritableKeysOf<[string, number]>;
//   ^? type T = number | typeof Symbol.iterator | "0" | "1" | "length" | "toString" | "toLocaleString" | "pop" | "push" | "concat" | "join" | "reverse" | "shift" | "slice" | "sort" | "splice" | ... 26 more ... | "with"

However, I had to comment out the array cases because looks like there's some issue with readonly arrays in v5.1 and v5.2.

In v5.1 and v5.2, the number key loses it's readonly modifier while mapping, causing WritableKeys<readonly [string]> to incorrectly include number in its result.

type Test<T> = {
	[P in keyof T as number extends P ? P : never]: {[Q in P]: T[P]}
};

type T1 = Test<readonly [string]>;
//   ^? type T1 = {
//          [x: number]: {
//              [x: number]: string;
//          };
//      }

In v5.3 and higher, the number key correctly preserves it's readonly modifier while mapping, so WritableKeys<readonly [string]> no longer has number in it's result.

type Test<T> = {
	[P in keyof T as number extends P ? P : never]: {[Q in P]: T[P]}
};

type T1 = Test<readonly [string]>;
//   ^? type T1 = {
//          readonly [x: number]: {
//              readonly [x: number]: string;
//          };
//      }

We'll probably have to figure out a way in which we can run certain test cases only for certain specific versions.

[som-sm]

We'll probably have to figure out a way in which we can run certain test cases only for certain specific versions.

I had to do a similar thing in ts-essentials as well.

[som-sm]

Also, @sindresorhus in v5, I feel we should add a constraint of object to T for WritableKeysOf and ReadonlyKeysOf, just like we do in OptionalKeysOf and RequiredKeysOf.

type-fest/source/optional-keys-of.d.ts

Line 34 in a6590b9

export type OptionalKeysOf<BaseType extends object> = Exclude<{

[sindresorhus]

I feel we should add a constraint of object to T for WritableKeysOf and ReadonlyKeysOf

Agreed. Added to the list.