Expose IsUnion type (#1137)

Author: sindresorhus

index.d.ts

@@ -144,6 +144,7 @@ export type {Or} from './source/or.d.ts';
 export type {NonEmptyTuple} from './source/non-empty-tuple.d.ts';
 export type {FindGlobalInstanceType, FindGlobalType} from './source/find-global-type.d.ts';
 export type {If} from './source/if.d.ts';
+export type {IsUnion} from './source/is-union.d.ts';
 
 // Template literal types
 export type {CamelCase} from './source/camel-case.d.ts';

readme.md

@@ -223,6 +223,7 @@ Click the type names for complete docs.
 - [`IsEmptyObject`](source/empty-object.d.ts) - Returns a boolean for whether the type is strictly equal to an empty plain object, the `{}` value.
 - [`IsNull`](source/is-null.d.ts) - Returns a boolean for whether the given type is `null`.
 - [`IsTuple`](source/is-tuple.d.ts) - Returns a boolean for whether the given array is a tuple.
+- [`IsUnion`](source/is-union.d.ts) - Returns a boolean for whether the given type is a union.
 
 ### JSON
 

source/internal/type.d.ts

@@ -79,41 +79,6 @@ export type Not<A extends boolean> = A extends true
 		? true
 		: never;
 
-/**
-Returns a boolean for whether the given type is a union type.
-
-@example
-```
-type A = IsUnion<string | number>;
-//=> true
-
-type B = IsUnion<string>;
-//=> false
-```
-*/
-export type IsUnion<T> = InternalIsUnion<T>;
-
-/**
-The actual implementation of `IsUnion`.
-*/
-type InternalIsUnion<T, U = T> =
-(
-	// @link https://ghaiklor.github.io/type-challenges-solutions/en/medium-isunion.html
-	IsNever<T> extends true
-		? false
-		: T extends any
-			? [U] extends [T]
-				? false
-				: true
-			: never
-) extends infer Result
-	// In some cases `Result` will return `false | true` which is `boolean`,
-	// that means `T` has at least two types and it's a union type,
-	// so we will return `true` instead of `boolean`.
-	? boolean extends Result ? true
-		: Result
-	: never; // Should never happen
-
 /**
 An if-else-like type that resolves depending on whether the given type is `any` or `never`.
 

source/is-union.d.ts

@@ -0,0 +1,37 @@
+import type {IsNever} from './is-never.d.ts';
+
+/**
+Returns a boolean for whether the given type is a union.
+
+@example
+```
+import type {IsUnion} from 'type-fest';
+
+type A = IsUnion<string | number>;
+//=> true
+
+type B = IsUnion<string>;
+//=> false
+```
+*/
+export type IsUnion<T> = InternalIsUnion<T>;
+
+/**
+The actual implementation of `IsUnion`.
+*/
+type InternalIsUnion<T, U = T> =
+(
+	IsNever<T> extends true
+		? false
+		: T extends any
+			? [U] extends [T]
+				? false
+				: true
+			: never
+) extends infer Result
+	// In some cases `Result` will return `false | true` which is `boolean`,
+	// that means `T` has at least two types and it's a union type,
+	// so we will return `true` instead of `boolean`.
+	? boolean extends Result ? true
+		: Result
+	: never; // Should never happen

source/shared-union-fields.d.ts

@@ -1,5 +1,6 @@
-import type {NonRecursiveType, IsUnion} from './internal/index.d.ts';
+import type {NonRecursiveType} from './internal/index.d.ts';
 import type {IsNever} from './is-never.d.ts';
+import type {IsUnion} from './is-union.d.ts';
 import type {Simplify} from './simplify.d.ts';
 import type {UnknownArray} from './unknown-array.d.ts';
 

source/single-key-object.d.ts

@@ -1,6 +1,6 @@
 import type {IsEmptyObject} from './empty-object.js';
 import type {If} from './if.js';
-import type {IsUnion} from './internal/index.d.ts';
+import type {IsUnion} from './is-union.d.ts';
 
 /**
 Create a type that only accepts an object with a single key.

test-d/internal/is-union.ts test-d/is-union.tstest-d/internal/is-union.ts renamed to test-d/is-union.ts

@@ -1,5 +1,5 @@
 import {expectType} from 'tsd';
-import type {IsUnion} from '../../source/internal/index.d.ts';
+import type {IsUnion} from '../index.d.ts';
 
 expectType<IsUnion<1>>(false);
 expectType<IsUnion<true>>(false);