feat(types): Assign

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

src/types/__tests__/assign.spec-d.ts

@@ -0,0 +1,39 @@
+/**
+ * @file Type Tests - Assign
+ * @module tutils/types/tests/unit-d/Assign
+ */
+
+import type Author from '#fixtures/author.interface'
+import type TestSubject from '../assign'
+import type EmptyArray from '../empty-array'
+import type Merge from '../merge'
+
+describe('unit-d:types/Assign', () => {
+  it('should equal T if U extends EmptyArray', () => {
+    expectTypeOf<TestSubject<Author, EmptyArray>>().toEqualTypeOf<Author>()
+  })
+
+  it('should equal T if U extends EmptyObject', () => {
+    expectTypeOf<TestSubject<Author>>().toEqualTypeOf<Author>()
+  })
+
+  it('should merge U into T if U extends ObjectAny', () => {
+    // Arrange
+    type U = { email: Lowercase<string> }
+
+    // Expect
+    expectTypeOf<TestSubject<Author, U>>().toEqualTypeOf<Merge<Author, U>>()
+  })
+
+  it('should merge U into T if U extends readonly ObjectAny[]', () => {
+    // Arrange
+    type U1 = [{ display_name: string }, { email: Lowercase<string> }]
+    type U2 = { display_name: string; email: Lowercase<string> }[]
+    type E1 = Merge<Merge<Author, U1[0]>, U1[1]>
+    type E2 = Merge<Author, U2[0]>
+
+    // Expect
+    expectTypeOf<TestSubject<Author, U1>>().toEqualTypeOf<E1>()
+    expectTypeOf<TestSubject<Author, U2>>().toEqualTypeOf<E2>()
+  })
+})

src/types/assign.ts

@@ -0,0 +1,36 @@
+/**
+ * @file Type Definitions - Assign
+ * @module tutils/types/Assign
+ */
+
+import type EmptyArray from './empty-array'
+import type EmptyObject from './empty-object'
+import type Head from './head'
+import type Merge from './merge'
+import type ObjectAny from './object-any'
+import type OneOrMany from './one-or-many'
+import type Simplify from './simplify'
+
+/**
+ * Merges one or more source objects into target object `T`.
+ *
+ * Source objects are applied from left to right. Properties of rightmost
+ * sources override those of leftmost sources.
+ *
+ * @template T - Target object
+ * @template U - Source object or source object array
+ */
+type Assign<
+  T extends ObjectAny,
+  U extends OneOrMany<ObjectAny> = EmptyObject
+> = U extends EmptyArray | EmptyObject
+  ? T
+  : U extends ObjectAny
+  ? Merge<T, U>
+  : Simplify<
+      U extends [infer H, ...infer Rest extends readonly ObjectAny[]]
+        ? Assign<H & Omit<T, keyof H>, Rest>
+        : Assign<T, Head<U>>
+    >
+
+export type { Assign as default }

src/types/index.ts

@@ -3,6 +3,7 @@
  * @module tutils/types
  */
 
+export type { default as Assign } from './assign'
 export type { default as At } from './at'
 export type { default as Booleanish } from './booleanish'
 export type { default as BuiltIn } from './built-in'