Dart (/Flutter): A null safe approach to convert a dynamic List inside a Map into a List of Strings.

dart_null_safe_map_key.dart

void main() {
  /// I got the below working as a null safe approach used to 
  ///   convert a `dynamic` List inside a Map, such as one received
  ///   and json.decoded from an API endpoint, into a List of `String`s.
  ///
  /// If anyone knows if anything is incorrect or can be done better, please comment. 
  /// Always looking to learn proper methodologies and better approaches.

  /// Two scenarios: A key exists in a Map, or it doesn't (is null).
  ///
  /// Map<String, List<dynamic>> _statusObj = {'message': ['Hello!']};
  /// Map<String, List<dynamic>> _statusObj = {};

  /// Test #1: A key exists.
  ///
  Map<String, List<dynamic>> _statusObj = {'message': ['Hello!']};

  List<String> _alertMsgStrA = List<String>.from(_statusObj["message"]?.toList() ?? <String>[]);
  // Works --- [Hello!] 1

  List<String> _alertMsgStrB = List<String>.from(_statusObj["message"]?.toList() ?? <String>['Missing message.']);
  // Works --- [Hello!] 1
  
  List<dynamic> _alertMsgStrC = _statusObj["message"]?.toList() ?? <String>['Missing message.'];
  // Works, but List is still dynamic.

  // List<String> _alertMsgStr = List<String>.from(_statusObj["message"]?.toList() as List<String>? ?? <String>['Missing message.']);
  // List<String> _alertMsgStr = _statusObj["message"]?.toList() as List<String>? ?? <String>['Missing message.'];
  // Uncaught Error: TypeError: Instance of 'JSArray<dynamic>': type 'JSArray<dynamic>' is not a subtype of type 'List<String>?'

  print(_alertMsgStrA); //     // [Hello!]
  print(_alertMsgStrA.length); // 1
  print(_alertMsgStrB); //     // [Hello!]
  print(_alertMsgStrB.length); // 1
  print(_alertMsgStrC); //     // [Hello!]
  print(_alertMsgStrC.length); // 1

  /// Test #2: No key exists.
  ///
  Map<String, List<dynamic>> _statusObj2 = {};

  List<String> _alertMsgStr2A = List<String>.from(_statusObj2["message"]?.toList() ?? <String>['Missing message.']);
  // Works --- [Missing message.] 1

  List<String> _alertMsgStr2B = List<String>.from(_statusObj2["message"]?.toList() ?? <String>[]);
  // Works --- [] 0

  // List<String> _alertMsgStr2C = _statusObj2["message"]?.toList() as List<String>? ?? <String>['Missing message.'];
  // This only works when key is null.

  print(_alertMsgStr2A); //     // [Missing message.]
  print(_alertMsgStr2A.length); // 1
  print(_alertMsgStr2B); //     // []
  print(_alertMsgStr2B.length); // 0
}