where-clause-on-map-in-dart.dart

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import 'dart:developer' as devtools show log;

extension Log on Object {
  void log() => devtools.log(toString());
}

extension DetailedWhere<K, V> on Map<K, V> {
  Map<K, V> where(bool Function(K key, V value) f) => Map<K, V>.fromEntries(
        entries.where((entry) => f(entry.key, entry.value)),
      );

  Map<K, V> whereKey(bool Function(K key) f) =>
      {...where((key, value) => f(key))};
  Map<K, V> whereValue(bool Function(V value) f) =>
      {...where((key, value) => f(value))};
}

typedef Age = int;
typedef Name = String;

const Map<Name, Age> people = {'John': 20, 'Mary': 21, 'Peter': 22};

void testIt() {
  // {Peter: 22}
  people.where((key, value) => key.length > 4 && value > 20).log();
  // {John: 20, Mary: 21}
  people.whereKey((key) => key.length < 5).log();
  // {John: 20, Peter: 22}
  people.whereValue((value) => value.isEven).log();
}