refactor: improve groupBy

src/internal/group-by.ts

@@ -4,21 +4,50 @@
  * @internal
  *
  * @param values The values to group.
- * @param keyFunction The function to get the key from the value.
+ * @param keyMapper The function to get the key from the value.
  */
 export function groupBy<TValue>(
   values: ReadonlyArray<TValue>,
-  keyFunction: (value: TValue) => string | number
-): Record<string, TValue[]> {
-  const result: Record<string, TValue[]> = {};
+  keyMapper: (value: TValue) => string | number
+): Record<string, TValue[]>;
+/**
+ * Groups the values by the key function and maps the values.
+ *
+ * @internal
+ *
+ * @param values The values to group.
+ * @param keyMapper The function to get the key from the value.
+ * @param valueMapper The function to get the value from the value.
+ */
+export function groupBy<TOriginalValue, TMappedValue>(
+  values: ReadonlyArray<TOriginalValue>,
+  keyMapper: (value: TOriginalValue) => string | number,
+  valueMapper: (value: TOriginalValue) => TMappedValue
+): Record<string, TMappedValue[]>;
+/**
+ * Groups the values by the key function and maps the values.
+ *
+ * @internal
+ *
+ * @param values The values to group.
+ * @param keyMapper The function to get the key from the value.
+ * @param valueMapper The function to map the value.
+ */
+export function groupBy<TOriginalValue, TMappedValue>(
+  values: ReadonlyArray<TOriginalValue>,
+  keyMapper: (value: TOriginalValue) => string | number,
+  valueMapper: (value: TOriginalValue) => TMappedValue = (value) =>
+    value as unknown as TMappedValue
+): Record<string, TMappedValue[]> {
+  const result: Record<string, TMappedValue[]> = {};
 
   for (const value of values) {
-    const key = keyFunction(value);
+    const key = keyMapper(value);
     if (result[key] === undefined) {
       result[key] = [];
     }
 
-    result[key].push(value);
+    result[key].push(valueMapper(value));
   }
 
   return result;