fix(types): narrow return type of first and last

src/array/first.ts

@@ -12,10 +12,14 @@
  * ```
  * @version 12.1.0
  */
-export function first<T>(array: readonly T[]): T | undefined
-
-export function first<T, U>(array: readonly T[], defaultValue: U): T | U
-
-export function first(array: readonly unknown[], defaultValue?: unknown) {
+export function first<
+  const TArray extends readonly any[],
+  const TDefault = undefined,
+>(
+  array: TArray,
+  defaultValue?: TDefault,
+): TArray extends readonly [infer TFirst, ...any[]]
+  ? TFirst
+  : TArray[number] | TDefault {
   return array?.length > 0 ? array[0] : defaultValue
 }

src/array/last.ts

@@ -12,10 +12,14 @@
  * ```
  * @version 12.1.0
  */
-export function last<T>(array: readonly T[]): T | undefined
-
-export function last<T, U>(array: readonly T[], defaultValue: U): T | U
-
-export function last(array: readonly unknown[], defaultValue?: unknown) {
+export function last<
+  const TArray extends readonly any[],
+  const TDefault = undefined,
+>(
+  array: TArray,
+  defaultValue?: TDefault,
+): TArray extends readonly [...any[], infer TLast]
+  ? TLast
+  : TArray[number] | TDefault {
   return array?.length > 0 ? array[array.length - 1] : defaultValue
 }

tests/array/first.test-d.ts

@@ -0,0 +1,40 @@
+import * as _ from 'radashi'
+
+describe('first', () => {
+  test('inlined array', () => {
+    expectTypeOf(_.first([])).toEqualTypeOf<undefined>()
+    expectTypeOf(_.first([1, 2, 3])).toEqualTypeOf<1>()
+  })
+
+  test('variable with empty array', () => {
+    const emptyArray = [] as never[]
+
+    expectTypeOf(_.first(emptyArray)).toEqualTypeOf<undefined>()
+  })
+
+  test('variable with mutable array', () => {
+    const array = [1, 2, 3]
+
+    expectTypeOf(_.first(array)).toEqualTypeOf<number | undefined>()
+  })
+
+  test('variable with readonly tuple', () => {
+    const emptyTuple = [] as const
+    const tuple = [1, 2, 3] as const
+
+    expectTypeOf(_.first(emptyTuple)).toEqualTypeOf<undefined>()
+    expectTypeOf(_.first(tuple)).toEqualTypeOf<1>()
+  })
+
+  test('with default value', () => {
+    const emptyArray = [] as never[]
+    const emptyTuple = [] as const
+    const array = [1, 2, 3]
+    const tuple = [1, 2, 3] as const
+
+    expectTypeOf(_.first(emptyArray, false)).toEqualTypeOf<false>()
+    expectTypeOf(_.first(emptyTuple, false)).toEqualTypeOf<false>()
+    expectTypeOf(_.first(array, false)).toEqualTypeOf<number | false>()
+    expectTypeOf(_.first(tuple, false)).toEqualTypeOf<1>()
+  })
+})

tests/array/last.test-d.ts

@@ -0,0 +1,40 @@
+import * as _ from 'radashi'
+
+describe('last', () => {
+  test('inlined array', () => {
+    expectTypeOf(_.last([])).toEqualTypeOf<undefined>()
+    expectTypeOf(_.last([1, 2, 3])).toEqualTypeOf<3>()
+  })
+
+  test('variable with empty array', () => {
+    const emptyArray = [] as never[]
+
+    expectTypeOf(_.last(emptyArray)).toEqualTypeOf<undefined>()
+  })
+
+  test('variable with mutable array', () => {
+    const array = [1, 2, 3]
+
+    expectTypeOf(_.last(array)).toEqualTypeOf<number | undefined>()
+  })
+
+  test('variable with readonly tuple', () => {
+    const emptyTuple = [] as const
+    const tuple = [1, 2, 3] as const
+
+    expectTypeOf(_.last(emptyTuple)).toEqualTypeOf<undefined>()
+    expectTypeOf(_.last(tuple)).toEqualTypeOf<3>()
+  })
+
+  test('with default value', () => {
+    const emptyArray = [] as never[]
+    const emptyTuple = [] as const
+    const array = [1, 2, 3]
+    const tuple = [1, 2, 3] as const
+
+    expectTypeOf(_.last(emptyArray, false)).toEqualTypeOf<false>()
+    expectTypeOf(_.last(emptyTuple, false)).toEqualTypeOf<false>()
+    expectTypeOf(_.last(array, false)).toEqualTypeOf<number | false>()
+    expectTypeOf(_.last(tuple, false)).toEqualTypeOf<3>()
+  })
+})