Add SharedUnionFields type

[Emiyaaaaa]

Simple version of SharedUnionFieldsDeep

[Emiyaaaaa]

Hi @sindresorhus, would it be possible to add me to the funding.yml? Thanks a lot!

[sindresorhus]

Of course: ca449dc

[som-sm]

@Emiyaaaaa Was going through this PR, and I have shared some thoughts. Let me know what you think.

[som-sm]

Currently, there's no test case that verifies this conditional [Union] extends [NonRecursiveType | ReadonlyMap<unknown, unknown> | ReadonlySet<unknown> | UnknownArray]. All tests continue to pass even if the conditional is removed.

A test like the following is needed to verify the above conditional:

expectType<Map<string, string> | Set<string>>({} as SharedUnionFields<Map<string, string> | Set<string>>);

[som-sm]

Also, is this the intended behaviour?

type T = SharedUnionFields<RegExp | {test: 1} | {test: 2}>;

T is currently of the following type:

{ test: 1 | 2 | ((string: string) => boolean); }

Feels like one of these behaviours might be better than the existing behaviour in scenarios like these:

1. We simply return back the input type, without any manipulation, like we'd do for RegExp | Date.

   RegExp | {test: 1} | {test: 2}

2. Or, we can leave RegExp as is, and resolve the remaining members

   RegExp | {test: 1 | 2}

@Emiyaaaaa WDYT?

[Emiyaaaaa]

2. Or, we can leave RegExp as is, and resolve the remaining members

   RegExp | {test: 1 | 2}

This is better

[som-sm]

The implementation of this type could be refactored to simply this:

export type SharedUnionFields<Union> = [Union] extends [
	NonRecursiveType | ReadonlyMap<unknown, unknown> | ReadonlySet<unknown> | UnknownArray,
]
	? Union
	: Pick<Union, keyof Union>;

This seems to handle all the scenarios.