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- // Helper type. Not useful on its own.
- type Without<FirstType, SecondType> = {[KeyType in Exclude<keyof FirstType, keyof SecondType>]?: never};
- /**
- Create a type that has mutually exclusive keys.
- This type was inspired by [this comment](https://github.com/Microsoft/TypeScript/issues/14094#issuecomment-373782604).
- This type works with a helper type, called `Without`. `Without<FirstType, SecondType>` produces a type that has only keys from `FirstType` which are not present on `SecondType` and sets the value type for these keys to `never`. This helper type is then used in `MergeExclusive` to remove keys from either `FirstType` or `SecondType`.
- @example
- ```
- import {MergeExclusive} from 'type-fest';
- interface ExclusiveVariation1 {
- exclusive1: boolean;
- }
- interface ExclusiveVariation2 {
- exclusive2: string;
- }
- type ExclusiveOptions = MergeExclusive<ExclusiveVariation1, ExclusiveVariation2>;
- let exclusiveOptions: ExclusiveOptions;
- exclusiveOptions = {exclusive1: true};
- //=> Works
- exclusiveOptions = {exclusive2: 'hi'};
- //=> Works
- exclusiveOptions = {exclusive1: true, exclusive2: 'hi'};
- //=> Error
- ```
- */
- export type MergeExclusive<FirstType, SecondType> =
- (FirstType | SecondType) extends object ?
- (Without<FirstType, SecondType> & SecondType) | (Without<SecondType, FirstType> & FirstType) :
- FirstType | SecondType;
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