Test coverage statistics and analysis

Test Coverage Statistics and Analysis

Test coverage is an important metric for measuring the completeness of code testing, reflecting the extent to which test cases cover code logic. High coverage means the code is thoroughly tested, with fewer potential issues. In Koa2 projects, using tools to measure and analyze coverage can effectively improve code quality.

Basic Concepts of Test Coverage

Test coverage typically includes the following types:

1. Line Coverage: How many lines of code were executed by tests
2. Branch Coverage: How many conditional branches were covered by tests
3. Function Coverage: How many functions were called by tests
4. Statement Coverage: How many statements were executed by tests

In Koa2 projects, we primarily focus on coverage for API routes, middleware, and business logic. For example:

// Example route
router.get('/users/:id', async (ctx) => {
  if (!ctx.params.id) {
    ctx.status = 400
    return
  }
  
  const user = await UserService.getById(ctx.params.id)
  ctx.body = user
})

This simple route has multiple points that need coverage: parameter validation, database queries, and response returns.

Configuring Coverage Tools in Koa2

Using Istanbul (nyc) for coverage statistics is a common choice. First, install the dependencies:

npm install --save-dev nyc cross-env

Then configure in package.json:

{
  "scripts": {
    "test": "mocha",
    "coverage": "cross-env NODE_ENV=test nyc --reporter=html --reporter=text mocha"
  }
}

For Koa2 projects, pay special attention to:

1. Ensuring the test environment is separate from the production environment
2. Correctly setting up Babel configuration (if using ES6+)
3. Excluding configuration files and non-test code

Example .nycrc configuration:

{
  "extends": "@istanbuljs/nyc-config-babel",
  "all": true,
  "include": [
    "src/**/*.js"
  ],
  "exclude": [
    "**/*.spec.js",
    "**/config/**",
    "**/migrations/**"
  ]
}

Writing Testable Koa2 Code

The prerequisite for improving coverage is that the code itself is easy to test. In Koa2 applications, note the following:

1. Middleware should be small and focused
2. Separate business logic from framework code
3. Use dependency injection for easier mocking

Bad example:

// Hard-to-test middleware
app.use(async (ctx, next) => {
  const start = Date.now()
  await next()
  const ms = Date.now() - start
  console.log(`${ctx.method} ${ctx.url} - ${ms}ms`)
  ctx.set('X-Response-Time', `${ms}ms`)
})

Improved version:

// Testable middleware
function responseTime() {
  return async (ctx, next) => {
    const start = Date.now()
    await next()
    const ms = Date.now() - start
    return { method: ctx.method, url: ctx.url, ms }
  }
}

// Usage in application
app.use(async (ctx, next) => {
  const timing = await responseTime()(ctx, next)
  ctx.set('X-Response-Time', `${timing.ms}ms`)
})

Testing Strategies and Coverage Improvement

For Koa2 applications, layered testing is recommended:

1. Unit Tests: Cover individual functions and middleware
2. Integration Tests: Test interactions between multiple components
3. API Tests: Test complete request flows

Example unit test:

const { expect } = require('chai')
const sinon = require('sinon')
const userController = require('../../src/controllers/user')

describe('User Controller', () => {
  afterEach(() => {
    sinon.restore()
  })

  it('should return 400 when id is missing', async () => {
    const ctx = { params: {}, status: null }
    await userController.getUser(ctx)
    expect(ctx.status).to.equal(400)
  })
})

Common low-coverage scenarios and solutions:

1. Error handling branches: Add tests to trigger error conditions
2. Boundary conditions: Test empty arrays, extreme values, etc.
3. Asynchronous code: Ensure all async operations are awaited

Coverage Report Analysis

The generated HTML report contains rich information:

1. File list sorted by coverage
2. Source code annotations (covered/uncovered)
3. Coverage percentages by type

Focus on:

1. Red-marked uncovered code
2. Files with coverage below average
3. Coverage of critical business logic

Typical problematic patterns:

// Often overlooked exception handling
try {
  await someAsyncOperation()
} catch (err) {
  logger.error(err) // This line is often not covered
  throw err
}

Coverage in Continuous Integration

Incorporate coverage checks in CI workflows:

1. Set coverage thresholds
2. Upload reports to third-party services
3. Check coverage changes during PRs

Example .travis.yml:

language: node_js
node_js:
  - "12"
script:
  - npm run coverage
after_success:
  - npx nyc report --reporter=text-lcov | npx coveralls

Set minimum requirements in package.json:

{
  "nyc": {
    "check-coverage": true,
    "lines": 80,
    "statements": 80,
    "functions": 75,
    "branches": 70
  }
}

Advanced Coverage Techniques

1. Dynamic import testing: Ensure all routes are loaded
2. Snapshot testing: Combine with coverage to validate UI output
3. Mutation testing: Evaluate test effectiveness

Example dynamic route test:

const fs = require('fs')
const path = require('path')
const Router = require('@koa/router')

describe('Route Coverage', () => {
  it('should test all API routes', async () => {
    const router = new Router()
    const routeFiles = fs.readdirSync(path.join(__dirname, '../src/routes'))
    
    routeFiles.forEach(file => {
      require(`../src/routes/${file}`)(router)
    })
    
    // Verify router.stack contains all expected routes
  })
})

Coverage Pitfalls and Misconceptions

1. Blind pursuit of 100%: Some code isn't worth testing
2. Ignoring test quality: Coverage ≠ test effectiveness
3. False coverage: Code executed but not validated

Bad test example:

// This test improves coverage but is worthless
it('should call the function', () => {
  const stub = sinon.stub(service, 'method')
  controller.method()
  expect(stub.called).to.be.true
})

Good tests should validate behavior and output:

it('should return user data with status 200', async () => {
  const user = { id: 1, name: 'Test' }
  sinon.stub(UserService, 'getById').resolves(user)
  
  const ctx = { params: { id: '1' }, status: null, body: null }
  await userController.getUser(ctx)
  
  expect(ctx.status).to.equal(200)
  expect(ctx.body).to.deep.equal(user)
})

Long-Term Maintenance Strategies

1. Incremental coverage: Higher coverage requirements for new code
2. Regular reviews: Investigate reasons for coverage drops
3. Technical debt tracking: Document legacy code with low coverage

Create TESTING.md in the project root:

## Coverage Standards

- New features: Line coverage ≥85%  
- Bug fixes: Must include regression tests  
- Legacy code: Supplement tests when modified  

## Known Low-Coverage Areas

1. `src/legacy/auth.js` - Historical reasons, planned Q3 refactor  
2. `src/utils/date.js` - Edge cases not covered