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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](
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`.
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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