Dart Matcher Best Practices

When to use this skill

Use this skill when:

• Writing assertions using expect and package:matcher.
• Migrating legacy manual checks to cleaner matchers.
• Debugging confusing test failures.

When NOT to use (Abstention Guardrails)

Do NOT apply this skill or refactor assertions when:

• Non-Collection Properties: The property happens to be named .length but belongs to a non-collection class (e.g. geometric scalar magnitude on a Vector, duration on a buffer, or line segment distance). hasLength(n) expects an int on an Iterable, Map, or String.
• Custom Domain Booleans: The boolean check tests an arbitrary domain property (e.g. expect(switch.isEnabled, isTrue)). Do not invent non-existent matchers.
• Alternative Assertion Frameworks: The package has adopted package:checks (check(x)...). Do not revert checks back to package:matcher.
• Already Idiomatic Assertions: Tests already use declarative matchers (hasLength, isEmpty, isNotEmpty, contains, containsPair, isA, throwsA). Abstain and make zero modifications.

Discovery

To find candidates for improving matcher usage, search for suboptimal patterns:

Suboptimal Length Checks

Search for length checks that should use hasLength:

• Regex: expect\([^,]+.length,\s*

Suboptimal Boolean Checks

Search for checks on boolean properties that have specific matchers:

• Regex: expect\([^,]+.isEmpty,\s*(true|equals\(true\))
• Regex: expect\([^,]+.isNotEmpty,\s*(true|equals\(true\))
• Regex: expect\([^,]+.contains\(.*\),\s*(true|equals\(true\))

Suboptimal Map Lookups

Search for manual map lookups instead of containsPair:

• Regex: expect\([^,]+\[.*\],\s*

Core Matchers

1. Collections (hasLength, contains, isEmpty, unorderedEquals, containsPair)

• hasLength(n):
  • Prefer expect(list, hasLength(n)) over expect(list.length, n).
  • Gives better error messages on failure (shows actual list content).
• isEmpty / isNotEmpty:
  • Prefer expect(list, isEmpty) over expect(list.isEmpty, true).
  • Prefer expect(list, isNotEmpty) over expect(list.isNotEmpty, true).
• contains(item):
  • Verify existence without manual iteration.
  • Prefer over expect(list.contains(item), true).
• unorderedEquals(items):
  • Verify contents regardless of order.
  • Prefer over expect(list, containsAll(items)).
• containsPair(key, value):
  • Verify a map contains a specific key-value pair.
  • Prefer over checking expect(map[key], value) or expect(map.containsKey(key), true).

2. Type Checks (isA<T> and TypeMatcher<T>)

• isA<T>():
  • Prefer for inline assertions: expect(obj, isA<Type>()).
  • More concise and readable than TypeMatcher<Type>().
  • Allows chaining constraints using .having().
• TypeMatcher<T>:
  • Prefer when defining top-level reusable matchers.
  • Use const: const isMyType = TypeMatcher<MyType>();
  • Chaining .having() works here too, but the resulting matcher is not const.

3. Object Properties (having)

Use .having() on isA<T>() or other TypeMatchers to check properties.

• Descriptive Names: Use meaningful parameter names in the closure (e.g., (e) => e.message) instead of generic ones like p0 to improve readability.

expect(person, isA<Person>()
    .having((p) => p.name, 'name', 'Alice')
    .having((p) => p.age, 'age', greaterThan(18)));

This provides detailed failure messages indicating exactly which property failed.

4. Async Assertions

• completion(matcher):
  • Wait for a future to complete and check its value.
  • Prefer await expectLater(...) to ensure the future completes before the test continues.
  • await expectLater(future, completion(equals(42))).
• throwsA(matcher):
  • Check that a future or function throws an exception.
  • await expectLater(future, throwsA(isA<StateError>())).
  • expect(() => function(), throwsA(isA<ArgumentError>())) (synchronous function throwing is fine with expect).

5. Exception Testing (Avoid try/catch with fail())

• Prefer throwsA over imperative try/catch:
  • Legacy tests often use try { fn(); fail('should throw'); } catch (e) { expect(e, isA<T>()); }.
  • Replace with declarative matchers:

    // Synchronous:
    expect(() => parseData('invalid'), throwsA(isA<FormatException>()));
    
    // Asynchronous:
    await expectLater(client.fetch('bad-url'), throwsA(isA<HttpException>()));

  • Common built-in exception matchers include throwsArgumentError, throwsStateError, throwsRangeError, throwsFormatException, throwsNoSuchMethodError, and throwsUnsupportedError.

6. Using expectLater

Use await expectLater(...) when testing async behavior to ensure proper sequencing.

// GOOD: Waits for future to complete before checking side effects
await expectLater(future, completion(equals(42)));
expect(sideEffectState, equals('done'));

// BAD: Side effect check might run before future completes
expect(future, completion(equals(42)));
expect(sideEffectState, equals('done')); // Race condition!

Principles

1. Readable Failures: Choose matchers that produce clear error messages.
2. Avoid Manual Logic: Don't use if statements or for loops for assertions; let matchers handle it.
3. Specific Matchers: Use the most specific matcher available (e.g., containsPair for maps instead of checking keys manually).

Related Skills

• dart-test-fundamentals: Core concepts for structuring tests, lifecycles, and configuration.