test_runner: coverage reports the largest block count as the function call count (FNDA) #67319

Version

v26.13.0, also v28.0.0-nightly20261107a43bfb5bb2

Platform

Linux 6.1.0-54-cloud-amd64 #1 SMP PREEMPT_DYNAMIC Debian 6.1.190-1 (2026-10-16) x86_64 GNU/Linux

Subsystem

test_runner

What steps will reproduce the bug?

// math.js
function sum(n) {
  let s = 0;
  for (let i = 0; i < n; i++) s += i;
  return s;
}
module.exports = { sum };

// math.test.js
const test = require('node:test');
const { sum } = require('./math.js');
test('sum', () => { sum(10); });

node --test --experimental-test-coverage --test-reporter=lcov --test-reporter-destination=stdout math.test.js

How often does it reproduce? Is there a required condition?

Always, when a block inside a function runs more often than the function is called.

What is the expected behavior? Why is that the expected behavior?

sum is called once:

FN:1,sum
FNDA:1,sum

In LCOV, FNDA is the number of times the function was called, and the test:coverage docs describe functions[].count as the number of times the function was called.

What do you see instead?

TN:
SF:math.js
FN:1,sum
FNDA:10,sum
FNF:1
FNH:1
BRDA:1,0,0,1
BRDA:1,1,0,1
BRDA:3,2,0,10

The same value appears in summary.files[].functions[].count of the test:coverage event.

Additional information

In TestCoverage.summary() (lib/internal/test_runner/coverage.js), the function report uses count: maxCountPerFunction, the maximum over all block ranges, instead of ranges[0].count, the function range's own count.

Found with a differential-testing tool; the report was drafted with AI assistance and reviewed and edited by me.

Credit: https://ucsbplse.github.io/