Branch coverage underreported when merging Jest shards

[mgoetz-nerdery]

Expected Behavior

I am attempting to run a Jest integration/unit test suite in three parts using the shard flag), then merging the coverage report using nyc merge on the individual shard reports. I expect that the coverage report generated from nyc merge should match the coverage report I get from running the entire test suite at once.

Observed Behavior

The branch coverage generated from nyc merge is slightly underreported.

Example of the branch coverage reported by nyc merge:

Example of the branch coverage reported by running the entire test suite:

The merged coverage report incorrectly shows that the different branches are not covered, even though the individual lines are taken are of.

Troubleshooting steps

• still occurring when I put cache: false in my nyc config

1. Generate the individual shards using this: node --expose-gc --no-compilation-cache $(yarn bin jest) --ci --colors --logHeapUsage --reporters=default --reporters=github-actions --reporters=jest-junit --coverage --coverageReporters=cobertura --coverageReporters=text-summary --coverageReporters=html --coverageReporters=json --shard=X/3
2. Move the coverage-final.json files from each shard into the same folder
3. Run nyc merge shardcoverage nycoutput/output.json and nyc report -t nycoutput --report-dir final-report --reporter=lcov --reporter=text-summary --reporter=cobertura --reporter=html

.nycrc:

{
  "cache": false,
  "check-coverage": true,
  "functions": "85"
}

Environment Information

  System:
    OS: macOS 14.6.1
    CPU: (10) arm64 Apple M1 Max
    Memory: 2.22 GB / 32.00 GB
  Binaries:
    Node: 20.11.1 - /usr/local/bin/node
    Yarn: 3.6.3 - /usr/local/bin/yarn
    npm: 10.2.4 - /usr/local/bin/npm