Imported from jeremylongshore/tons-of-skills-marketplace (skills/.curated/analyzing-test-coverage/SKILL.md). Install upstream with npx skills add jeremylongshore/tons-of-skills-marketplace --skill analyzing-test-coverage. Copyright stays with the author (MIT).

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

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