test-coverage

Test Coverage Improvement

When to Use

• Coverage reports show critical modules below target thresholds
• New code was merged without sufficient test coverage
• Preparing for a release and need confidence in code correctness
• Refactoring requires a safety net of comprehensive tests
• Team has set a coverage target that needs to be met

Prerequisites

• Test framework configured with coverage reporting
• Baseline coverage report available or generatable
• Understanding of which code is critical vs. low-risk

Workflow

1. Generate a Coverage Report

# Node.js / Jest
npm test -- --coverage 2>&1 | Select-Object -Last 30

# With specific thresholds
npm test -- --coverage --coverageReporters="text" 2>&1

# Python / pytest
pytest --cov=src --cov-report=term-missing 2>&1

# Go
go test -coverprofile=coverage.out ./... && go tool cover -func=coverage.out

2. Identify Uncovered Code

# Find files with lowest coverage (Jest text output)
npm test -- --coverage --coverageReporters="text" 2>&1 | Select-String "\d+\.\d+\s*\|" | Sort-Object { [double]($_ -split '\|')[1].Trim().TrimEnd('%') }

# Generate detailed HTML report for visual inspection
npm test -- --coverage --coverageReporters="html"
# Report is in coverage/lcov-report/index.html

3. Prioritize What to Test

Focus testing effort where it matters most:

Priority	What to Cover	Why
🔴 High	Business logic, calculations	Bugs here cost money
🔴 High	Authentication, authorization	Security-critical
🟡 Medium	Data transformations, parsers	Common source of bugs
🟡 Medium	Error handling paths	Often untested, often broken
🟢 Low	Simple getters/setters	Low risk
🟢 Low	UI layout components	Better tested with E2E

4. Write Targeted Tests

For each uncovered function or branch:

# View the uncovered code
# Coverage report shows line numbers — use view tool to see the code

# Find existing test file (or create one)
glob pattern="**/*moduleName*.test.*"