feat(types): IsUndefined, IfUndefined

Signed-off-by: Lexus Drumgold <[email protected]>

src/types/__tests__/if-undefined.spec-d.ts

@@ -0,0 +1,19 @@
+/**
+ * @file Type Tests - IfUndefined
+ * @module tutils/types/tests/unit-d/IfUndefined
+ */
+
+import type TestSubject from '../if-undefined'
+
+describe('unit-d:types/IfUndefined', () => {
+  type False = false
+  type True = true
+
+  it('should equal False if IsUndefined<T> extends false', () => {
+    expectTypeOf<TestSubject<undefined[], True, False>>().toEqualTypeOf<False>()
+  })
+
+  it('should equal True if IsUndefined<T> extends true', () => {
+    expectTypeOf<TestSubject<undefined, True, False>>().toEqualTypeOf<True>()
+  })
+})

src/types/__tests__/is-undefined.spec-d.ts

@@ -0,0 +1,16 @@
+/**
+ * @file Type Tests - IsUndefined
+ * @module tutils/types/tests/unit-d/IsUndefined
+ */
+
+import type TestSubject from '../is-undefined'
+
+describe('unit-d:types/IsUndefined', () => {
+  it('should equal false if T does not extend undefined', () => {
+    expectTypeOf<TestSubject<null>>().toEqualTypeOf<false>()
+  })
+
+  it('should equal true if T extends undefined', () => {
+    expectTypeOf<TestSubject<undefined>>().toEqualTypeOf<true>()
+  })
+})

src/types/if-undefined.ts

@@ -0,0 +1,20 @@
+/**
+ * @file Type Definitions - IfUndefined
+ * @module tutils/types/IfUndefined
+ */
+
+import type IsUndefined from './is-undefined'
+
+/**
+ * Conditional type that resolves depending on whether or not `T` extends
+ * `undefined`.
+ *
+ * @see {@linkcode IsUndefined}
+ *
+ * @template T - Type to evaluate
+ * @template True - Type if `T` extends `undefined`
+ * @template False - Type if `T` does not extend `undefined`
+ */
+type IfUndefined<T, True, False> = IsUndefined<T> extends true ? True : False
+
+export type { IfUndefined as default }

src/types/index.ts

@@ -37,6 +37,7 @@ export type { default as IfPrimitive } from './if-primitive'
 export type { default as IfString } from './if-string'
 export type { default as IfSymbol } from './if-symbol'
 export type { default as IfTuple } from './if-tuple'
+export type { default as IfUndefined } from './if-undefined'
 export type { default as IndexSignature } from './index-signature'
 export type { default as IsAny } from './is-any'
 export type { default as IsArray } from './is-array'
@@ -53,6 +54,7 @@ export type { default as IsPrimitive } from './is-primitive'
 export type { default as IsString } from './is-string'
 export type { default as IsSymbol } from './is-symbol'
 export type { default as IsTuple } from './is-tuple'
+export type { default as IsUndefined } from './is-undefined'
 export type { default as Join } from './join'
 export type { default as JsonArray } from './json-array'
 export type { default as JsonObject } from './json-object'

src/types/is-undefined.ts

@@ -0,0 +1,13 @@
+/**
+ * @file Type Definitions - IsUndefined
+ * @module tutils/types/IsUndefined
+ */
+
+/**
+ * Returns a boolean indicating if `T` extends `undefined`.
+ *
+ * @template T - Type to evaluate
+ */
+type IsUndefined<T> = T extends undefined ? true : false
+
+export type { IsUndefined as default }