add array groupBy function

arrays/groupBy.test.ts

@@ -0,0 +1,27 @@
+import { describe, expect, test } from 'vitest'
+import { groupBy } from './groupBy.ts'
+
+describe('array groupBy', () => {
+
+  test('empty array', () => {
+    expect(groupBy([], () => 'p')).toEqual({})
+  })
+
+  test('selecting to same property', () => {
+    expect(groupBy([1, 2, 3], () => 'p')).toEqual({
+      p: [1, 2, 3],
+    })
+  })
+
+  test('group array of objects by property key', () => {
+    expect(
+      groupBy(
+        [{ id: '1', value: 1 }, { id: '2', value: 2 }, { id: '1', value: 3 }],
+        (item) => item.id
+      )
+    ).toEqual({
+      1: [{ id: '1', value: 1 }, { id: '1', value: 3 }],
+      2: [{ id: '2', value: 2 }],
+    })
+  })
+})

arrays/groupBy.ts

@@ -0,0 +1,23 @@
+import { NonEmptyArray } from '../types/index.js'
+
+export type Grouping<T> = Record<PropertyKey, NonEmptyArray<T>>
+
+/**
+ * Groups an array of items by a property-key
+ * Example:
+ *  groupBy(
+ *    [{ id: "1", value: 1 },{ id: "2", value: 2 },{ id: "1", value: 3 }],
+ *    (item) => item.id
+ *  ) === {
+ *    1: [{ id: "1", value: 1 },{ id: "1", value: 3 }],
+ *    2: [{ id: "2", value: 2 }]
+ *  }
+ */
+export const groupBy = <T>(array: readonly T[], keySelector: (item: T) => PropertyKey): Grouping<T> =>
+  array.reduce<Grouping<T>>((group, item) => {
+    const key = keySelector(item)
+    return {
+      ...group,
+      [key]: group[key] == null ? <NonEmptyArray<T>>[item] : <NonEmptyArray<T>>[...group[key], item],
+    }
+  }, {})

eslint.config.mjs

@@ -32,6 +32,7 @@ export default tseslint.config(
         'exports': 'only-multiline',
         'functions': 'never',
       }],
+      'space-before-blocks': ['warn'],
       'space-unary-ops': ['warn'],
       'consistent-return': ['warn'],
       'eol-last': ['warn'],
@@ -43,16 +44,11 @@ export default tseslint.config(
 
       /* typescript */
       '@typescript-eslint/consistent-type-definitions': 'off',
-      '@typescript-eslint/no-unused-vars': 'off',
+      '@typescript-eslint/consistent-type-assertions': 'off',
       '@typescript-eslint/no-confusing-void-expression': ['error', { ignoreArrowShorthand: true }],
+      '@typescript-eslint/no-unused-vars': 'off', // enforced by tsconfig
       '@typescript-eslint/prefer-readonly': ['warn'],
-      '@typescript-eslint/space-before-blocks': ['error'],
       '@typescript-eslint/switch-exhaustiveness-check': ['warn'],
-      '@typescript-eslint/type-annotation-spacing': ['error', {
-        before: false,
-        after: true,
-        overrides: { arrow: { before: true, after: true } },
-      }],
     },
   },
   {

index.ts

@@ -1,3 +1,4 @@
+export * from './arrays/index.ts'
 export * from './events/index.ts'
 export * from './json/index.ts'
 export * from './lazy/index.ts'

package.json

@@ -19,15 +19,15 @@
   "engines": {
     "npm": "please-use-pnpm",
     "yarn": "please-use-pnpm",
-    "pnpm": ">=9.5.0"
+    "pnpm": ">=9.12.0"
   },
   "devDependencies": {
-    "@eslint/js": "9.7.0",
-    "@types/node": "20.14.11",
-    "eslint": "9.7.0",
-    "typescript": "5.5.3",
-    "typescript-eslint": "7.16.1",
-    "vitest": "2.0.3",
+    "@eslint/js": "9.12.0",
+    "@types/node": "22.7.6",
+    "eslint": "9.12.0",
+    "typescript": "5.6.3",
+    "typescript-eslint": "8.9.0",
+    "vitest": "2.1.3",
     "zod": "3.23.8"
   }
 }