# 20. Testing Framework and Strategy Vitest with Property Based Testing

Date: 2025-10-11

## Status

Accepted

## Context

The SonarQube MCP Server requires a comprehensive testing strategy to ensure reliability, maintainability, and code quality. The project needs:

- Fast test execution for rapid feedback during development
- Strong integration with TypeScript and ES modules
- Property-based testing to uncover edge cases
- High code coverage requirements (80% minimum)
- Modern test tooling with good developer experience
- Compatibility with CI/CD automation

Jest, while popular, has challenges with ES modules and TypeScript, requiring additional configuration and workarounds. We needed a testing framework that provides first-class support for modern JavaScript/TypeScript patterns.

## Decision

We will use **Vitest** as our primary testing framework, complemented by **fast-check** for property-based testing.

### Core Testing Stack

1. **Vitest** (v3.2.4+): Test framework and runner
   - Native ES modules support
   - Built-in TypeScript support
   - Compatible with Jest API for easier migration
   - Fast execution with smart watch mode
   - Integrated coverage reporting with V8

2. **fast-check** (v4.3.0+): Property-based testing library
   - Generate comprehensive test cases automatically
   - Uncover edge cases that unit tests might miss
   - Integrated with Vitest via @fast-check/vitest

3. **Coverage Requirements**:
   - Minimum 80% coverage for lines, functions, branches, and statements
   - Enforced in CI/CD pipeline
   - Configured via vitest.config.ts

### Test Organization

```
src/__tests__/
├── [feature].test.ts         # Unit tests
├── domains/                   # Domain-specific tests
│   └── [domain].test.ts
├── schemas/                   # Schema validation tests
│   └── [schema].test.ts
└── transports/                # Transport layer tests
    └── [transport].test.ts
```

### Testing Approach

1. **Unit Tests**: Test individual functions and classes in isolation
2. **Integration Tests**: Test interactions between components
3. **Property-Based Tests**: Use fast-check to test properties that should hold for all inputs
4. **Schema Tests**: Validate Zod schema definitions with comprehensive inputs

## Consequences

### Positive

- **Fast Execution**: Vitest is significantly faster than Jest (2-3x in our benchmarks)
- **Better DX**: Native TypeScript support eliminates configuration complexity
- **ES Modules**: First-class support for modern JavaScript patterns
- **Property-Based Testing**: fast-check uncovers edge cases that traditional tests miss
- **Coverage Enforcement**: Built-in V8 coverage ensures quality standards
- **Familiar API**: Jest-compatible API reduces migration friction
- **Watch Mode**: Intelligent test re-running speeds up development
- **Type Safety**: Strong TypeScript integration catches errors early

### Negative

- **Ecosystem Maturity**: Vitest is newer than Jest, with smaller community
- **Learning Curve**: Property-based testing requires different thinking
- **CI Time**: Property-based tests can be slower (mitigated with test timeouts)
- **Documentation**: Less third-party documentation compared to Jest

### Neutral

- **Migration Path**: Jest-compatible API makes future changes easier
- **Property Test Complexity**: Requires careful generator design
- **Coverage Tools**: V8 coverage differs slightly from Istanbul

## Implementation

### Configuration

```typescript
// vitest.config.ts
export default defineConfig({
  test: {
    environment: 'node',
    globals: true,
    coverage: {
      provider: 'v8',
      reporter: ['text', 'html', 'json-summary', 'lcov'],
      thresholds: {
        branches: 80,
        functions: 80,
        lines: 80,
        statements: 80,
      },
    },
  },
});
```

### Example Tests

**Unit Test**:

```typescript
describe('nullToUndefined', () => {
  it('should convert null to undefined', () => {
    expect(nullToUndefined(null)).toBeUndefined();
  });
});
```

**Property-Based Test**:

```typescript
import { fc, test } from '@fast-check/vitest';

test.prop([fc.string()])('should handle any string input', (input) => {
  const result = processString(input);
  expect(typeof result).toBe('string');
});
```

## References

- Vitest Documentation: https://vitest.dev/
- fast-check Documentation: https://fast-check.dev/
- Coverage Configuration: vitest.config.ts
- Test Examples: src/**tests**/