Map deep merge in Dart.

deep_merge_map.dart

Map<String, dynamic> deepMergeMap(Map<String, dynamic> a, Map<String, dynamic> b) {
  b.forEach((k, v) {
    if (!a.containsKey(k)) {
      a[k] = v;
    } else {
      // TODO handle List type
      if (a[k] is Map) {
        deepMergeMap(a[k], b[k]);
      } else {
        a[k] = b[k];
      }
    }
  });

  return a;
}

deep_merge_map_test.dart

import 'package:map_merge/map_merge.dart';
import 'package:test/test.dart';

void main() {
  test('deepMergeMap', () {
    final Map<String, dynamic> a = {
      'key1': {
        'key1-1': 111,
        'key1-2': 222,
      },
      'key2': {
        'key2-1': 'hello',
      },
    };
    final Map<String, dynamic> b = {
      'key1': {
        'key1-2': 123456,
      },
      'key2': {
        'key2-2': 'world',
      },
    };
    final Map<String, dynamic> expected = {
      'key1': {
        'key1-1': 111,
        'key1-2': 123456,
      },
      'key2': {
        'key2-1': 'hello',
        'key2-2': 'world',
      },
    };
    expect(deepMergeMap(a, b), expected);
  });
}

 Map<String, dynamic> deepMergeMap(Map<String, dynamic> a, Map<String, dynamic> b) {
  if(b == null) return a;
  b.forEach((k, v) {
    if (!a.containsKey(k)) {
      a[k] = v;
    } else {
      // TODO handle List type
      if (a[k] is Map) {
        deepMergeMap(a[k], b[k]);
      } else {
        a[k] = b[k];
      }
    }
  });

  return a;
} 

fix null key