Testing with package:test

A test is code that checks your other code works. Once written, it runs in a fraction of a second every time you change something, catching mistakes before your users do.

Dart's standard testing library is package:test (Flutter uses flutter_test, which builds on it). dart create includes it.

The code under test

// lib/cart.dart
class Cart {
  final _items = <(String, int)>[];

  void add(String name, int price) {
    if (price <= 0) throw ArgumentError.value(price, 'price', 'must be positive');
    _items.add((name, price));
  }

  int get total => _items.fold(0, (sum, item) => sum + item.$2);
  bool get isEmpty => _items.isEmpty;
}

The tests

// test/cart_test.dart
import 'package:expense_tracker/cart.dart';
import 'package:test/test.dart';

void main() {
  group('Cart', () {
    late Cart cart;

    setUp(() {
      cart = Cart();
    });

    test('starts empty', () {
      expect(cart.isEmpty, isTrue);
      expect(cart.total, 0);
    });

    test('total adds up prices', () {
      cart.add('book', 12);
      cart.add('pen', 3);
      expect(cart.total, equals(15));
    });

    test('rejects non-positive prices', () {
      expect(() => cart.add('free', 0), throwsArgumentError);
    });
  });
}

$ dart test
00:01 +3: All tests passed!

• test('description', () { ... }) defines a test; group organizes related tests.
• setUp runs before each test, so every test gets a fresh cart.
• expect(actual, matcher) checks a result. Matchers include equals, isTrue, isNull, contains, hasLength, throwsA, and throwsArgumentError.
• To check that code throws, pass a function — () => cart.add(...) — not the call itself.

When a test fails, the output shows Expected: <15> Actual: <12> and the line number. Editors show a clickable ▶ next to each test.

Async tests

Test bodies can be async: test('loads user', () async { ... }), and matchers like completion(...) and throwsA(...) work with futures.

Common mistakes

• Writing expect(cart.add('free', 0), throwsArgumentError) — the exception is thrown before expect runs. Wrap it in () => ....
• Tests that depend on the network or on each other. Keep each test self-contained; use fakes for outside services.