feat(types): ArrayFallback

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

src/types/__tests__/fallback-array.spec-d.ts

@@ -0,0 +1,48 @@
+/**
+ * @file Type Tests - ArrayFallback
+ * @module tutils/types/tests/unit-d/ArrayFallback
+ */
+
+import type Vehicle from '#fixtures/types/vehicle'
+import type EmptyArray from '../empty-array'
+import type TestSubject from '../fallback-array'
+import type Nilable from '../nilable'
+import type Partial from '../partial'
+
+describe('unit-d:types/ArrayFallback', () => {
+  it('should equal F if T extends NIL', () => {
+    // Arrange
+    type F = [Partial<Vehicle>]
+
+    // Expect
+    expectTypeOf<TestSubject<null, F>>().toEqualTypeOf<F>()
+    expectTypeOf<TestSubject<undefined, F>>().toEqualTypeOf<F>()
+  })
+
+  it('should equal T if IsAny<T> extends true', () => {
+    // Arrange
+    type T = any
+
+    // Expect
+    expectTypeOf<TestSubject<T>>().toEqualTypeOf<T>()
+  })
+
+  it('should equal T if T extends {}', () => {
+    // Arrange
+    type T = Vehicle
+
+    // Expect
+    expectTypeOf<TestSubject<T>>().toEqualTypeOf<T>()
+  })
+
+  describe('unions', () => {
+    it('should distribute over unions', () => {
+      // Arrange
+      type T = Nilable<readonly [Vehicle, Vehicle]>
+      type Expect = EmptyArray | NonNullable<T>
+
+      // Expect
+      expectTypeOf<TestSubject<T>>().toEqualTypeOf<Expect>()
+    })
+  })
+})

src/types/fallback-array.ts

@@ -0,0 +1,24 @@
+/**
+ * @file Type Definitions - ArrayFallback
+ * @module tutils/types/ArrayFallback
+ */
+
+import type EmptyArray from './empty-array'
+import type Fallback from './fallback'
+import type NIL from './nil'
+
+/**
+ * Fallback to an array type if `T` extends {@linkcode NIL}.
+ *
+ * @todo examples
+ *
+ * @template T - Type to evaluate
+ * @template [F=EmptyArray] - Fallback array type
+ */
+type ArrayFallback<T, F extends readonly unknown[] = EmptyArray> = Fallback<
+  T,
+  F,
+  NIL
+>
+
+export type { ArrayFallback as default }

src/types/index.ts

@@ -32,6 +32,7 @@ export type { default as EmptyValue } from './empty-value'
 export type { default as Entries } from './entries'
 export type { default as ExactOptionalPropertyTypes } from './exact-optional-property-types'
 export type { default as Fallback } from './fallback'
+export type { default as ArrayFallback } from './fallback-array'
 export type { default as Falsy } from './falsy'
 export type { default as FIXME } from './fixme'
 export type { default as Flat } from './flat'