fix(partition): Add readonly type to the first argument of partition (#32)

* fix: Add the type readonly array to the partition

* fix: revert the partition documentation

---------

Co-authored-by: Jonghyeon Ko <jonghyeon@toss.im>

src/array/partition.spec.ts

@@ -1,4 +1,4 @@
-import { describe, expect, it } from 'vitest';
+import { describe, expect, it, expectTypeOf } from 'vitest';
 import { partition } from './partition';
 
 describe('partition', () => {
@@ -25,4 +25,15 @@ describe('partition', () => {
       ],
     ]);
   });
+
+  it('should correctly infer the type of a narrow array', () => {
+    const arr = [1, 2, 3, 4, 5] as const;
+    const [evens, odds] = partition(arr, num => num % 2 === 0);
+
+    expect(evens).toEqual([2, 4]);
+    expect(odds).toEqual([1, 3, 5]);
+
+    expectTypeOf(evens).toEqualTypeOf<Array<1 | 2 | 3 | 4 | 5>>();
+    expectTypeOf(odds).toEqualTypeOf<Array<1 | 2 | 3 | 4 | 5>>();
+  });
 });

src/array/partition.ts

@@ -19,7 +19,7 @@
  * const [even, odd] = partition(array, isEven);
  * // even will be [2, 4], and odd will be [1, 3, 5]
  */
-export function partition<T>(arr: T[], isInTruthy: (value: T) => boolean): [truthy: T[], falsy: T[]] {
+export function partition<T>(arr: readonly T[], isInTruthy: (value: T) => boolean): [truthy: T[], falsy: T[]] {
   const truthy: T[] = [];
   const falsy: T[] = [];