Add RequireExactlyOne type

[kainiedziela]

Fixes #54

Create a type that requires exactly one of the given properties and disallows more. The remaining properties are kept as is.

Use cases

• Creating interfaces for components that only need one of the properties to display properly.
• Declaring generic properties in a single place for a single use case that get narrowed down via RequireExactlyOne.

The caveat with RequireExactlyOne is that TypeScript doesn't always know at compile time every property that will exist at runtime. Therefore RequireExactlyOne can't do anything to prevent extra properties it doesn't know about.

Example

import {RequireExactlyOne} from 'type-fest';

type Responder = {
	text: () => string;
	json: () => string;
	secure: boolean;
};

const responder: RequireExactlyOne<Responder, 'text' | 'json'> = {
	json: () => '{"message": "ok"}',
	secure: true
};

[SamVerschueren]

This is cool, I need it :).

[BendingBender]

LGTM

[kainiedziela]

@sindresorhus @BendingBender guys?

[SamVerschueren]

Nice! Thanks for working on this!