type-fest/source/shared-union-fields.d.ts

import type {NonRecursiveType, IsUnion} from './internal';
import type {IsNever} from './is-never';
import type {Simplify} from './simplify';
import type {UnknownArray} from './unknown-array';

/**
Create a type with shared fields from a union of object types.

Use-cases:
- You want a safe object type where each key exists in the union object.
- You want to focus on the common fields of the union type and don't want to have to care about the other fields.

@example
```
import type {SharedUnionFields} from 'type-fest';

type Cat = {
	name: string;
	type: 'cat';
	catType: string;
};

type Dog = {
	name: string;
	type: 'dog';
	dogType: string;
};

function displayPetInfo(petInfo: Cat | Dog) {
	// typeof petInfo =>
	// {
	// 	name: string;
	// 	type: 'cat';
	// 	catType: string; // Needn't care about this field, because it's not a common pet info field.
	// } | {
	// 	name: string;
	// 	type: 'dog';
	// 	dogType: string; // Needn't care about this field, because it's not a common pet info field.
	// }

	// petInfo type is complex and have some needless fields

	console.log('name: ', petInfo.name);
	console.log('type: ', petInfo.type);
}

function displayPetInfo(petInfo: SharedUnionFields<Cat | Dog>) {
	// typeof petInfo =>
	// {
	// 	name: string;
	// 	type: 'cat' | 'dog';
	// }

	// petInfo type is simple and clear

	console.log('name: ', petInfo.name);
	console.log('type: ', petInfo.type);
}
```

@see SharedUnionFieldsDeep

@category Object
@category Union
*/
export type SharedUnionFields<Union> =
Extract<Union, NonRecursiveType | ReadonlyMap<unknown, unknown> | ReadonlySet<unknown> | UnknownArray> extends infer SkippedMembers
	? Exclude<Union, SkippedMembers> extends infer RelevantMembers
		?
		| SkippedMembers
		| (IsNever<RelevantMembers> extends true
			? never
			: Simplify<Pick<RelevantMembers, keyof RelevantMembers>>)
		: never
	: never;