Testing in Dart

Setup

# pubspec.yaml
dev_dependencies:
  test: ^1.24.0

# Run tests:
dart test
dart test test/my_test.dart
dart test --coverage=coverage

Unit Tests

import 'package:test/test.dart';

int add(int a, int b) => a + b;
double divide(double a, double b) {
  if (b == 0) throw ArgumentError('Division by zero');
  return a / b;
}

void main() {
  test('add returns sum of two numbers', () {
    expect(add(2, 3), equals(5));
    expect(add(-1, 1), equals(0));
    expect(add(0, 0), equals(0));
  });

  test('divide throws ArgumentError on zero', () {
    expect(() => divide(10, 0), throwsArgumentError);
  });

  test('divide returns correct result', () {
    expect(divide(10, 2), closeTo(5.0, 0.001));
  });
}

Test Groups

class Calculator {
  int add(int a, int b) => a + b;
  int subtract(int a, int b) => a - b;
  int multiply(int a, int b) => a * b;
  double divide(int a, int b) {
    if (b == 0) throw ArgumentError('Cannot divide by zero');
    return a / b;
  }
}

void main() {
  late Calculator calc;

  setUp(() => calc = Calculator());    // runs before each test
  tearDown(() => print('Test done')); // runs after each test

  group('Calculator', () {
    group('add', () {
      test('positive numbers', () => expect(calc.add(2, 3), 5));
      test('negative numbers', () => expect(calc.add(-1, -1), -2));
    });

    group('divide', () {
      test('returns double', () => expect(calc.divide(10, 3), closeTo(3.333, 0.001)));
      test('throws on zero', () => expect(() => calc.divide(5, 0), throwsArgumentError));
    });
  });
}

Matchers

expect(value, equals(42));
expect(value, isNull);
expect(value, isNotNull);
expect(value, isTrue);
expect(value, isFalse);
expect(value, isA<String>());
expect(value, contains('dart'));
expect(value, startsWith('Hello'));
expect(value, hasLength(5));
expect(value, inInclusiveRange(0, 100));
expect(value, closeTo(3.14, 0.01));
expect(list, containsAll([1, 2, 3]));
expect(fn, throwsA(isA<FormatException>()));

Async Tests

Future<String> fetchData() async {
  await Future.delayed(Duration(milliseconds: 100));
  return 'data';
}

void main() {
  test('fetches data asynchronously', () async {
    var result = await fetchData();
    expect(result, equals('data'));
  });

  test('stream emits values', () async {
    var stream = Stream.fromIterable([1, 2, 3]);
    expect(stream, emitsInOrder([1, 2, 3, emitsDone]));
  });
}

🧠 Quiz

Summary

Use the test package for Dart unit testing. Write tests with test(), group them with group(), and use setUp()/tearDown() for initialization. Assert with expect(value, matcher). Test async code with async/await in tests. Run with dart test.