Add IsUndefined type

Fixes #1212

Author: sindresorhus

index.d.ts

@@ -145,6 +145,7 @@ export type {SharedUnionFields} from './source/shared-union-fields.d.ts';
 export type {SharedUnionFieldsDeep} from './source/shared-union-fields-deep.d.ts';
 export type {IsNull} from './source/is-null.d.ts';
 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 {AllExtend} from './source/all-extend.d.ts';

readme.md

@@ -199,6 +199,7 @@ Click the type names for complete docs.
 - [`IsUnknown`](source/is-unknown.d.ts) - Returns a boolean for whether the given type is `unknown`.
 - [`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`.
+- [`IsUndefined`](source/is-undefined.d.ts) - Returns a boolean for whether the given type is `undefined`.
 - [`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.
 - [`IsLowercase`](source/is-lowercase.d.ts) - Returns a boolean for whether the given string literal is lowercase.

source/is-undefined.d.ts

@@ -0,0 +1,20 @@
+/**
+Returns a boolean for whether the given type is `undefined`.
+
+@example
+```
+import type {IsUndefined} from 'type-fest';
+
+type UndefinedFallback<T, Fallback> = IsUndefined<T> extends true ? Fallback : T;
+
+type Example1 = UndefinedFallback<undefined, string>;
+//=> string
+
+type Example2 = UndefinedFallback<number, string>;
+//=> number
+```
+
+@category Type Guard
+@category Utilities
+*/
+export type IsUndefined<T> = [T] extends [undefined] ? true : false;

test-d/is-undefined.ts

@@ -0,0 +1,10 @@
+import {expectType} from 'tsd';
+import type {IsUndefined} from '../source/is-undefined.d.ts';
+
+expectType<IsUndefined<undefined>>(true);
+expectType<IsUndefined<any>>(true);
+expectType<IsUndefined<never>>(true);
+expectType<IsUndefined<null>>(false); // Depends on `strictNullChecks`
+expectType<IsUndefined<unknown>>(false);
+expectType<IsUndefined<void>>(false);
+expectType<IsUndefined<{}>>(false);