Deep Copy Map

main.dart

/*
 * Deep copy of Map in Dart.
 * https://gist.github.com/PlugFox/d651661f465199195edd1c08834b04e8
 * https://dartpad.dev?id=d651661f465199195edd1c08834b04e8
 * Mike Matiunin <[email protected]>, 16 October 2025
 */

import 'dart:convert' show JsonEncoder;

extension type const CopyMap<K, V>._(Map<K, V> _source) implements Map<K, V> {
  CopyMap(Map<K, V> source, {Object? Function(Object object)? copy})
    : _source = source.isEmpty ? <K, V>{} : _deepCopyMap(source, copy);

  static Map<K, V> _deepCopyMap<K, V>(Map<K, V> original, Object? Function(Object object)? copy) => <K, V>{
    for (final MapEntry<K, V>(:key, :value) in original.entries) key: _deepCopyValue(value, copy),
  };

  static List<V> _deepCopyList<V>(List<V> original, Object? Function(Object object)? copy) => <V>[
    for (final item in original) _deepCopyValue(item, copy),
  ];

  static V _deepCopyValue<V>(V value, Object? Function(Object object)? copy) => switch (value) {
    Map() => _deepCopyMap(value, copy) as V,
    List() => _deepCopyList(value, copy) as V,
    num() when value.isFinite => value,
    String() || bool() || Null() => value,
    Object() when copy != null => copy(value) as V,
    _ => value,
  };

  String toJsonString() => const JsonEncoder.withIndent('  ').convert(_source);
}

void main() {
  final map = <String, Object?>{
    'map': {'a': 1, 'b': 2},
    'empty': <String, Object?>{},
    'list': [1, 2, 3],
    'nested': [
      {'x': 10, 'y': 20},
      [null, 100, 200, 300],
    ],
    'text': 'hello',
    'num': 42,
    'bool': true,
    'infinity': double.infinity,
    'null': null,
    'timestamp': DateTime.now(),
    'custom': Object(),
  };

  final copy = CopyMap(
    map,
    copy:
        (object) => switch (object) {
          num() when object.isInfinite => '<Infinity>',
          DateTime() => object.toUtc().toIso8601String(),
          Object obj => '${obj.runtimeType.toString()}{}',
        },
  );
  print(copy.toJsonString()); // ignore: avoid_print
}