thedixitjain 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
- 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.
- JavaScript:
- 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).
- 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.
- Analyze uncovered branches specifically:
- Find
if/elseblocks where only one branch is tested. - Identify
switch/casestatements with missing case coverage. - Locate error handling paths (
catch,except) never exercised. - Check
||and&&short-circuit conditions.
- Find
- 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.
- 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.
- 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:
{
"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
Source: jeremylongshore/claude-code-plugins-plus-skills → skills/.curated/analyzing-test-coverage/SKILL.md
Also appears in: jeremylongshore/claude-code-plugins-plus-skills/plugins/testing/test-coverage-analyzer/skills/analyzing-test-coverage/SKILL.md