fix(groupBy): correction to return type

src/array/groupBy.ts

@@ -9,8 +9,9 @@
  * @template K - The type of keys.
  * @param {T[]} arr - The array to group.
  * @param {(item: T) => K} getKeyFromItem - A function that generates a key from an element.
- * @returns {Record<K, T[]>} An object where each key is associated with an array of elements that
- * share that key.
+ * @returns {Partial<Record<K, T[]>>} An object where each key is associated with an array of elements that
+ * share that key. If a specific key exists in K but at runtime no array element groups to that specific key,
+ * then that key maps to undefined rather than T[].
  *
  * @example
  * const array = [
@@ -30,8 +31,11 @@
  * //   ]
  * // }
  */
-export function groupBy<T, K extends PropertyKey>(arr: readonly T[], getKeyFromItem: (item: T) => K): Record<K, T[]> {
-  const result = {} as Record<K, T[]>;
+export function groupBy<T, K extends PropertyKey>(
+  arr: readonly T[],
+  getKeyFromItem: (item: T) => K
+): Partial<Record<K, T[]>> {
+  const result = {} as Partial<Record<K, T[]>>;
 
   for (let i = 0; i < arr.length; i++) {
     const item = arr[i];
@@ -41,7 +45,7 @@ export function groupBy<T, K extends PropertyKey>(arr: readonly T[], getKeyFromI
       result[key] = [];
     }
 
-    result[key].push(item);
+    result[key]!.push(item);
   }
 
   return result;

src/compat/array/groupBy.ts

@@ -41,7 +41,7 @@ export function groupBy<T>(
 export function groupBy<T extends object>(
   collection: T | null | undefined,
   iteratee?: ValueIteratee<T[keyof T]>
-): Record<string, Array<T[keyof T]>>;
+): Partial<Record<string, Array<T[keyof T]>>>;
 
 /**
  * Groups the elements of an array or object based on a provided key-generating function.
@@ -58,8 +58,9 @@ export function groupBy<T extends object>(
  *   - If a property name (string) is provided, that property of each element is used as the key.
  *   - If a property-value pair (array) is provided, elements with matching property values are used.
  *   - If a partial object is provided, elements with matching properties are used.
- * @returns {Record<K, T>} An object where each key is associated with an array of elements that
- * share that key.
+ * @returns {Partial<Record<K, T>>} An object where each key is associated with an array of elements that
+ * share that key. If a specific key exists in K but at runtime no array element groups to that specific key,
+ * then that key maps to undefined rather than T[].
  *
  * @example
  * // Using an array
@@ -81,9 +82,9 @@ export function groupBy<T, K extends PropertyKey>(
     | [keyof T, unknown]
     | PropertyKey
     | null
-): Record<K, T[]> {
+): Partial<Record<K, T[]>> {
   if (source == null) {
-    return {} as Record<K, T[]>;
+    return {} as Partial<Record<K, T[]>>;
   }
 
   const items = isArrayLike(source) ? Array.from(source) : Object.values(source);