Node.js Coverage BRDA Bug Reproduction

This repository demonstrates a bug/limitation in Node.js built-in test coverage where /* node:coverage ignore next */ comments exclude lines from DA (line coverage) but leave BRDA (branch coverage) entries intact.

The Problem

When using Node.js native test coverage with ignore comments:

• DA (Data/Line Coverage): Lines are correctly excluded from coverage reporting
• BRDA (Branch Coverage): Branch entries remain in the lcov output, showing as uncovered

This causes misleading coverage reports where ignored code paths still negatively impact branch coverage percentages.

Reproduction

Prerequisites

• Node.js v18+ (tested on v24.13.0)

Steps

1. Clone this repository
2. Install dependencies:

npm install

3. Run the tests with coverage:

npm run test

4. Compare the generated coverage files:
   • coverage/lcov-native.info - Node.js native (has the bug)
   • coverage/lcov.info - c8 (works correctly)

Expected Behavior

The /* node:coverage ignore next */ comment on line 5 of src/example.js should exclude both:

• The DA entry for line 6 (return 'falsy';)
• The BRDA entry for the else branch

Actual Behavior

• DA for line 6 is excluded (not present in lcov-native.info)
• BRDA entry remains: BRDA:4,2,0,0 (branch 2 at line 4, taken 0 times)

File Structure

.
├── src/
│   ├── example.js          # Node.js native (node:coverage ignore)
│   └── example-c8.js       # c8 version (c8 ignore)
├── test/
│   ├── example.test.js     # Test for Node.js native
│   └── example-c8.test.js  # Test for c8
├── coverage/
│   ├── lcov-native.info    # Node.js native output (has bug)
│   └── lcov.info           # c8 output (correct)
├── package.json
├── .gitignore
└── README.md

Source Code

src/example.js (Node.js native):

function getValue(condition) {
  if (condition) {
    return 'truthy';
  }
  /* node:coverage ignore next */
  return 'falsy';
}

module.exports = { getValue };

src/example-c8.js (c8):

function getValue(condition) {
  if (condition) {
    return 'truthy';
  }
  /* c8 ignore next */
  return 'falsy';
}

module.exports = { getValue };

Comparison: Node.js Native vs c8

Test Commands

# Run both (recommended)
npm run test

# Node.js native only
npm run test:native

# c8 only
npm run test:c8

Results Comparison

Aspect	Node.js Native	c8
Ignore comment	/* node:coverage ignore next */	/* c8 ignore next */
Output file	coverage/lcov-native.info	coverage/lcov.info
DA (line) handling	Excluded from report	Marked as covered
BRDA (branch) handling	Remains as uncovered	Marked as covered
Branch coverage	66.66% (2/3)	100% (3/3)

Node.js Native lcov-native.info (bug)

BRDA:1,0,0,1
BRDA:1,1,0,1
BRDA:4,2,0,0    <-- Branch remains as UNCOVERED
BRF:3
BRH:2           <-- Only 2 of 3 branches hit
DA:1,1
DA:2,1
DA:3,1
DA:4,1
DA:5,0
DA:7,1          <-- DA:6 is missing (correctly excluded)
...

c8 lcov.info (correct behavior)

DA:1,1
...
DA:5,1          <-- Line marked as covered (ignored)
DA:6,1          <-- Line marked as covered (ignored)
...
BRDA:1,0,0,1
BRDA:1,1,0,1
BRDA:5,2,0,1    <-- Branch marked as covered (ignored)
BRF:3
BRH:3           <-- All 3 branches "hit"

Impact

This bug affects projects that:

• Use Node.js native test runner with --experimental-test-coverage
• Use lcov format for coverage reporting
• Have intentionally ignored code paths (error handlers, edge cases, etc.)
• Enforce branch coverage thresholds in CI/CD pipelines

Workaround

Recommended: Use c8 instead of Node.js native coverage.

npm install --save-dev c8
npx c8 node --test test/*.test.js

Other alternatives:

• Adjust branch coverage thresholds to account for this limitation
• Avoid using ignore comments and test all branches

Environment

• Node.js: v24.13.0
• OS: macOS (Darwin 24.6.0)
• Coverage flags: --experimental-test-coverage --test-reporter=lcov

Related

• Node.js Test Runner Documentation
• Node.js Coverage Ignore Comments

License

ISC