Dart Lists

dartlists.md

For the Dart language when we pass a List to a function as a parameter we pass its reference by default, which means we are passing and editing the original list.

This may cause confusion because it doesn't follow the same pattern in all languages, the Solitity language is an example of how this could be the oposite as default.

In the first test of the following example we can see how this happens and in the second test we see how to avoid it by simply calling .toList() at the end of the original List, creating a new reference by doing this.

To open this example on dartpad, follow dartpad link

The same applies to Maps, here are the examples: dartpad link

list_reference_test.dart

import 'package:flutter_test/flutter_test.dart';

void main() {
  test('should edit the original list as default', () {
    final mockList = ['string1', 'string2', 'string3'];
    expect(mockList, ['string1', 'string2', 'string3']);
    removeFirstItemFromOriginalList(mockList);
    expect(mockList, ['string2', 'string3']);
  });
  test('should create a new reference and shouldn\'t edit the original list', () {
    final mockList = ['string1', 'string2', 'string3'];
    expect(mockList, ['string1', 'string2', 'string3']);
    dontRemoveFirstItemFromOriginalList(mockList);
    expect(mockList, ['string1', 'string2', 'string3']);
  });
}

void removeFirstItemFromOriginalList(List<String> listParam) {
  listParam.removeAt(0);
}
void dontRemoveFirstItemFromOriginalList(List<String> listParam) {
  final newListReference = listParam.toList();
  newListReference.removeAt(0);
}