vscode-copilot-chat vitest-unit-tests.instructions.md

Please follow these guidelines when writing unit tests using Vitest. These tests are *.spec.ts

Best Practices

• Prefer explicit Test/Mock classes over mutating real instances or creating adhoc one-off mocks.

  • Never use as any to override private methods or assign properties on real objects.
  • Mock versions of services are typically named Mock* or Test*, you can search to find whether one already exists.
  • Some examples: MockFileSystemService, MockChatResponseStream, TestTasksService.

• If there is no preexisting implementation of a service that is appropriate to reuse in the test, then you can create a simple mock or stub implementation in a file under a test/ folder near the interface definition.

  • A mock class should be configurable so that it can be shared and set up for different test scenarios.

• The helper createExtensionUnitTestingServices returns a TestingServiceCollection preconfigured with some common mock services, use IInstantiationService to create instances with those mocks. Here's an example of using it properly

const serviceCollection = store.add(createExtensionUnitTestingServices());
instantiationService = serviceCollection.createTestingAccessor().get(IInstantiationService);
const mockFs = accessor.get(IFileSystemService) as MockFileSystemService;
const testService = instantiationService.createInstance(SomeServiceToTest);

• When asked to write new tests, add tests to cover the behavior of the code under test, especially things that are interesting, unexpected, or edge cases.

  • Avoid adding tests that simply repeat existing tests or cover trivial code paths.

• If available, prefer the runTests tool to run tests over a terminal command.

• Keep tests deterministic and fast.

  • Avoid starting real servers or performing network I/O.

• Avoid excessive repetition in tests, use beforeEach to set up common state.

  • Use helper functions to encapsulate common test logic.