analyzing-test-coverage

What this skill does

# Test Coverage Analyzer

## Current State

!`ls package.json pyproject.toml Cargo.toml go.mod 2>/dev/null || echo 'No project manifest found'`
!`node -v 2>/dev/null || python3 --version 2>/dev/null || echo 'No runtime detected'`

## Overview

Analyze code coverage metrics to identify untested code paths, dead code, and coverage gaps across line, branch, function, and statement dimensions. Supports Istanbul/nyc (JavaScript/TypeScript), coverage.py (Python), JaCoCo (Java), and Go coverage tools.

## Prerequisites

- Coverage tool installed and configured (Istanbul/nyc, c8, coverage.py, JaCoCo, or `go test -cover`)
- Test suite that can run with coverage instrumentation enabled
- Coverage threshold targets defined (recommended: 80% lines, 70% branches)
- Coverage output format set to JSON or LCOV for programmatic analysis
- Git history available for coverage trend comparison

## Instructions

1. Run the test suite with coverage instrumentation enabled:
   - JavaScript: `npx jest --coverage --coverageReporters=json-summary,lcov`
   - Python: `pytest --cov=src --cov-report=json --cov-report=term-missing`
   - Go: `go test -coverprofile=coverage.out ./...`
   - Java: Configure JaCoCo Maven/Gradle plugin with XML report output.
2. Parse the coverage report and extract per-file metrics:
   - Line coverage percentage per file.
   - Branch coverage percentage per file.
   - Function coverage percentage per file.
   - Uncovered line ranges (specific line numbers).
3. Identify critical coverage gaps by prioritizing:
   - Files with coverage below the threshold (sort ascending by coverage %).
   - Files with high complexity but low coverage (use cyclomatic complexity if available).
   - Recently modified files with decreasing coverage trends.
   - Public API functions and exported modules lacking any tests.
4. Analyze uncovered branches specifically:
   - Find `if/else` blocks where only one branch is tested.
   - Identify `switch/case` statements with missing case coverage.
   - Locate error handling paths (`catch`, `except`) never exercised.
   - Check `||` and `&&` short-circuit conditions.
5. Generate a prioritized action plan:
   - List top 10 files needing coverage improvement with specific line ranges.
   - Suggest test scenarios for each uncovered branch.
   - Estimate effort (small/medium/large) for each coverage improvement.
6. Compare current coverage against the previous commit or baseline:
   - Calculate coverage delta per file.
   - Flag files where coverage decreased.
   - Verify new code added since baseline has adequate coverage.
7. Write coverage enforcement configuration (coverage thresholds in Jest config, `.coveragerc`, or CI checks).

## Output

- Coverage summary report with overall and per-file metrics
- Prioritized list of uncovered code paths with file:line references
- Coverage delta report comparing against baseline
- Suggested test cases for top coverage gaps
- Coverage threshold configuration file for CI enforcement

## Error Handling

| Error | Cause | Solution |
|-------|-------|---------|
| Coverage report shows 0% | Tests ran but coverage instrumentation was not enabled | Verify `--coverage` flag is passed; check that source files are not excluded by config |
| Coverage includes `node_modules` | Coverage collection scope too broad | Add `collectCoverageFrom` in Jest config; set `--cov=src` in pytest; exclude vendor dirs |
| Branch coverage much lower than line coverage | Conditional logic is only partially tested | Write tests for both truthy and falsy conditions on each branch; focus on edge cases |
| Coverage drops after refactor | New code added without corresponding tests | Set `coverageThreshold` with `global` minimums; add `--changedSince` for incremental checks |
| Flaky coverage numbers between runs | Non-deterministic test execution skipping code paths | Sort tests deterministically; run coverage with `--runInBand` for consistent results |

## Examples

**Jest coverage configuration with thresholds:**

```json
{
  "jest": {
    "coverageThreshold": {
      "global": {
        "branches": 70,
        "functions": 80,
        "lines": 80,
        "statements": 80
      },
      "src/utils/": {
        "branches": 90,
        "lines": 95
      }
    },
    "collectCoverageFrom": [
      "src/**/*.{ts,tsx}",
      "!src/**/*.d.ts",
      "!src/**/index.ts"
    ]
  }
}
```

**Coverage gap analysis output:**

```
Coverage Gaps (sorted by impact):
1. src/auth/oauth.ts        Lines: 45%  Branches: 30%  [Lines 42-67, 89-103 uncovered]
   Suggestion: Add tests for token refresh failure and expired session handling
2. src/api/middleware.ts     Lines: 62%  Branches: 41%  [Lines 28-35, 71-80 uncovered]
   Suggestion: Test rate limiting edge cases and malformed header handling
3. src/utils/parser.ts       Lines: 71%  Branches: 55%  [Lines 112-130 uncovered]
   Suggestion: Test malformed input, empty string, and encoding edge cases
```

## Resources

- Istanbul/nyc: https://istanbul.js.org/
- c8 (Node.js native coverage): https://github.com/bcoe/c8
- coverage.py: https://coverage.readthedocs.io/
- JaCoCo: https://www.jacoco.org/jacoco/
- Go test coverage: https://go.dev/blog/cover
- Code coverage best practices: https://testing.googleblog.com/2020/08/code-coverage-best-practices.html