Add RequireOnlyOne type

[izayl]

No description provided.

[CvX]

I've just noticed you've changed never to undefined before. What was the issue with never?

[izayl]

@CvX
there is a test to prove that never is not suitable for some case.
15dc6dd

[CvX]

Hmm… With that change though, it also incorrectly accepts this:

const system = {macos: 'hey', x: 1} || {linux: 'sup', y: 'wat'};
test({default: 'hello', ...system});

[izayl]

Hmm… With that change though, it also incorrectly accepts this:

const system = {macos: 'hey', x: 1} || {linux: 'sup', y: 'wat'};
test({default: 'hello', ...system});

type SystemMessages = {
	default: string;

	macos?: string;
	linux: string;
	windows: string;

	optional?: string;
};
const test = (_: ValidMessages): void => {};

@CvX Because the property x and y not in the type ValidMessages.

[CvX]

That's my point. 🙂 With the current RequireOnlyOne implementation and the …({} || {}) construct you can pass pretty much anything and it doesn't raise an error.

[izayl]

you are right, current implementation RequireOnlyOne not support this Object deconstruction scene.

the case is correct, my editor highlight some error confused me, it works after restart my editor.
😢

[CvX]

my editor highlight some error confused me, it works after restart my editor

Same here 😅 Turns out there isn't really any difference between never and undefined in this case?

See the commented-out test I've just pushed. It doesn't raise an error… 🤔

[izayl]

my editor highlight some error confused me, it works after restart my editor

Same here 😅 Turns out there isn't really any difference between never and undefined in this case?

See the commented-out test I've just pushed. It doesn't raise an error… 🤔

I think it's a bug cause by ts type checker.
The type { x?: never } says either the property does not exist, or it exists with type never. It's mean that x does not exist, though the type should really be isomorphic to never.

[sindresorhus]

I think it's a bug cause by ts type checker.

Can you look for an issue about this in the TS issue tracker and if not, open an issue there?

[sindresorhus]

Can you mention in text some real-world use-cases?

Write about some real-world use-cases where it can be useful. (It can be hard sometimes for users to see where they would use something) - https://github.com/sindresorhus/type-fest/blob/master/.github/contributing.md

[sindresorhus]

Suggested change

	Create a type that requires at only one of the given properties. The remaining properties are kept as is.
	Create a type that requires only one of the given properties, but not more. The remaining properties are kept as is.

?

[sindresorhus]

"Require only one" makes me think that it requires only one, but doesn't make it clear that it disallows more than one. Maybe we should simply name it RequireOne?

[sindresorhus]

Can you fix the merge conflict? (Note that I have moved types to separate files in master).

[sindresorhus]

@izayl Ping :)

[WORMSS]

I just wanted to say that I simplified this concept a little so I could use it to build up a type rather than modify an existing type.

type OnlyOne<T> = {
    [K in keyof T]: (
        Required<Pick<T, K>> & Partial<Record<Exclude<keyof T, K>, never>>
    )
}[keyof T];

Tested with the below code, when wrapped in Partial, it allows the group to also be optional or still only allow one.

type Test = OnlyOne<{
    a: boolean;
    b: boolean;
}> & Partial<OnlyOne<{
    c: boolean;
    d: boolean;
}>>;

const t1: Test = {
    a: true,
};
const t2: Test = {
    b: true,
};
const t3: Test = { // Should fail, a or b must be supplied
    
}
const t4: Test = { // Should fail a and b cannot be supplied together
    a: true,
    b: true,
}
const t5: Test = {
    a: true,
    c: true,
}
const t6: Test = { // Should fail c and d cannot be supplied together
    a: true,
    c: true,
    d: true,
}
const t7: Test = { // Should fail, as even though c is supplied, you still need a or b
    c: true,
}

I hope this helps anyone else in the future.

[sindresorhus]

Closing for lack of response.

#54