fix: ensure mapValues and group work together

src/array/group.ts

@@ -6,7 +6,7 @@
 export const group = <T, Key extends string | number | symbol>(
   array: readonly T[],
   getGroupId: (item: T) => Key
-): Partial<Record<Key, T[]>> => {
+): { [K in Key]?: T[] } => {
   return array.reduce((acc, item) => {
     const groupId = getGroupId(item)
     if (!acc[groupId]) acc[groupId] = []

src/array/tests/group.test.ts

@@ -15,4 +15,29 @@ describe('group function', () => {
     expect(groups.c?.length).toBe(1)
     expect(groups.c?.[0].word).toBe('ok')
   })
+  test('works with mapValues', () => {
+    const objects = [
+      { id: 1, group: 'a' },
+      { id: 2, group: 'b' },
+      { id: 3, group: 'a' }
+    ] as const
+
+    // Notice how the types of `groupedObjects` and `groupedIds` are
+    // both partial (in other words, their properties have the `?:`
+    // modifier). At the type level, mapValues is respectful of
+    // preserving the partiality of the input.
+    const groupedObjects = _.group(objects, obj => obj.group)
+    const groupedIds = _.mapValues(groupedObjects, array => {
+      // Importantly, we can map the array without optional chaining,
+      // because of how the overloads of mapValues are defined.
+      // TypeScript knows that when a key is defined inside the result
+      // of a `group(…)` call, its value is never undefined.
+      return array.map(obj => obj.id)
+    })
+
+    expect(groupedIds).toEqual({
+      a: [1, 3],
+      b: [2]
+    })
+  })
 })

src/object/mapValues.ts

@@ -1,17 +1,39 @@
 /**
  * Map over all the keys to create a new object
  */
-export const mapValues = <
+export function mapValues<
   TValue,
   TKey extends string | number | symbol,
   TNewValue
 >(
-  obj: Record<TKey, TValue>,
+  obj: { [K in TKey]: TValue },
   mapFunc: (value: TValue, key: TKey) => TNewValue
-): Record<TKey, TNewValue> => {
+): { [K in TKey]: TNewValue }
+
+// This overload exists to support cases where `obj` is a partial
+// object whose values are never undefined when the key is also
+// defined. For example:
+//   { [key: string]?: number } versus { [key: string]: number | undefined }
+export function mapValues<
+  TValue,
+  TKey extends string | number | symbol,
+  TNewValue
+>(
+  obj: { [K in TKey]?: TValue },
+  mapFunc: (value: TValue, key: TKey) => TNewValue
+): { [K in TKey]?: TNewValue }
+
+export function mapValues<
+  TValue,
+  TKey extends string | number | symbol,
+  TNewValue
+>(
+  obj: { [K in TKey]?: TValue },
+  mapFunc: (value: TValue, key: TKey) => TNewValue
+): Record<TKey, TNewValue> {
   const keys = Object.keys(obj) as TKey[]
   return keys.reduce((acc, key) => {
-    acc[key] = mapFunc(obj[key], key)
+    acc[key] = mapFunc(obj[key]!, key)
     return acc
   }, {} as Record<TKey, TNewValue>)
 }

src/object/tests/mapValues.test.ts

@@ -15,4 +15,15 @@ describe('mapValues function', () => {
       y: 'xbye'
     })
   })
+  test('objects with possibly undefined values', () => {
+    const result = _.mapValues({ x: 'hi  ', y: undefined }, value => {
+      // Importantly, the value is typed as "string | undefined"
+      // here, due to how the overloads of mapValues are defined.
+      return value?.trim()
+    })
+    expect(result).toEqual({
+      x: 'hi',
+      y: undefined
+    })
+  })
 })