groupByKey<T, K> function

Map<K, List<T>> groupByKey<T, K>(List<T> items, K keySelector(T))

Groups the elements of items using the provided keySelector function which extracts a key of type K from each element of type T.

Returns a Map where each key corresponds to a group of items that share that key.

Example:

groupByKey(['apple', 'banana', 'apricot'], (word) => word[0].toUpperCase());
// => {'A': ['apple', 'apricot'], 'B': ['banana']}

Implementation

Map<K, List<T>> groupByKey<T, K>(List<T> items, K Function(T) keySelector) {
  final Map<K, List<T>> grouped = {};

  for (final item in items) {
    final key = keySelector(item);
    grouped.putIfAbsent(key, () => []).add(item);
  }

  return grouped;
}