Add Xor type (#1254)

Co-authored-by: Som Shekhar Mukherjee <49264891+som-sm@users.noreply.github.com>

index.d.ts

@@ -148,6 +148,7 @@ export type {IfNull} from './source/if-null.d.ts';
 export type {IsUndefined} from './source/is-undefined.d.ts';
 export type {And} from './source/and.d.ts';
 export type {Or} from './source/or.d.ts';
+export type {Xor} from './source/xor.d.ts';
 export type {AllExtend, AllExtendOptions} from './source/all-extend.d.ts';
 export type {NonEmptyTuple} from './source/non-empty-tuple.d.ts';
 export type {FindGlobalInstanceType, FindGlobalType} from './source/find-global-type.d.ts';

readme.md

@@ -180,6 +180,7 @@ Click the type names for complete docs.
 - [`DistributedPick`](source/distributed-pick.d.ts) - Picks keys from a type, distributing the operation over a union.
 - [`And`](source/and.d.ts) - Returns a boolean for whether two given types are both true.
 - [`Or`](source/or.d.ts) - Returns a boolean for whether either of two given types are true.
+- [`Xor`](source/xor.d.ts) - Returns a boolean for whether only one of two given types is true.
 - [`AllExtend`](source/all-extend.d.ts) - Returns a boolean for whether every element in an array type extends another type.
 - [`NonEmptyTuple`](source/non-empty-tuple.d.ts) - Matches any non-empty tuple.
 - [`NonEmptyString`](source/non-empty-string.d.ts) - Matches any non-empty string.

source/xor.d.ts

@@ -0,0 +1,83 @@
+import type {Not} from './internal/type.d.ts';
+import type {And} from './and.d.ts';
+import type {Or} from './or.d.ts';
+
+/**
+Returns a boolean for whether only one of two given types is true.
+
+Use-case: Constructing complex conditional types where one single condition must be satisfied.
+
+@example
+```
+import type {Xor} from 'type-fest';
+
+type TT = Xor<true, true>;
+//=> false
+
+type TF = Xor<true, false>;
+//=> true
+
+type FT = Xor<false, true>;
+//=> true
+
+type FF = Xor<false, false>;
+//=> false
+```
+
+Note: When `boolean` is passed as an argument, it is distributed into separate cases, and the final result is a union of those cases.
+For example, `Xor<false, boolean>` expands to `Xor<false, true> | Xor<false, false>`, which simplifies to `true | false` (i.e., `boolean`).
+
+@example
+```
+import type {Xor} from 'type-fest';
+
+type A = Xor<false, boolean>;
+//=> boolean
+
+type B = Xor<boolean, false>;
+//=> boolean
+
+type C = Xor<true, boolean>;
+//=> boolean
+
+type D = Xor<boolean, true>;
+//=> boolean
+
+type E = Xor<boolean, boolean>;
+//=> boolean
+```
+
+Note: If `never` is passed as an argument, it is treated as `false` and the result is computed accordingly.
+
+@example
+```
+import type {Xor} from 'type-fest';
+
+type A = Xor<true, never>;
+//=> true
+
+type B = Xor<never, true>;
+//=> true
+
+type C = Xor<false, never>;
+//=> false
+
+type D = Xor<never, false>;
+//=> false
+
+type E = Xor<boolean, never>;
+//=> boolean
+
+type F = Xor<never, boolean>;
+//=> boolean
+
+type G = Xor<never, never>;
+//=> false
+```
+
+@see {@link And}
+@see {@link Or}
+*/
+export type Xor<A extends boolean, B extends boolean> = And<Or<A, B>, Not<And<A, B>>>;
+
+export {};

test-d/xor.ts

@@ -0,0 +1,32 @@
+import {expectType} from 'tsd';
+import type {Xor} from '../source/xor.d.ts';
+
+declare const boolean: boolean;
+
+expectType<Xor<true, true>>(false);
+expectType<Xor<true, false>>(true);
+expectType<Xor<false, true>>(true);
+expectType<Xor<false, false>>(false);
+
+expectType<Xor<true, boolean>>(boolean);
+expectType<Xor<boolean, true>>(boolean);
+expectType<Xor<false, boolean>>(boolean);
+expectType<Xor<boolean, false>>(boolean);
+expectType<Xor<boolean, boolean>>(boolean);
+
+// Boundary cases
+expectType<Xor<true, any>>(boolean);
+expectType<Xor<any, true>>(boolean);
+expectType<Xor<false, any>>(boolean);
+expectType<Xor<any, false>>(boolean);
+expectType<Xor<boolean, any>>(boolean);
+expectType<Xor<any, boolean>>(boolean);
+expectType<Xor<any, any>>(boolean);
+
+expectType<Xor<true, never>>(true);
+expectType<Xor<never, true>>(true);
+expectType<Xor<false, never>>(false);
+expectType<Xor<never, false>>(false);
+expectType<Xor<boolean, never>>(boolean);
+expectType<Xor<never, boolean>>(boolean);
+expectType<Xor<never, never>>(false);