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Known Issues

mocha pinned to ^11.3.0

mocha is intentionally kept below 11.7.0. Do not run npm audit fix without reading this first.

What breaks

mocha 11.7.0 introduced a change (#5366) that uses Node's native require(esm) to load test files when running on Node 22.12+, where process.features.require_module === true. This causes each .ts source file to be loaded twice:

  1. Via require(), intercepted by tsx's CJS transform (produces a CJS bundle)
  2. Via the --import=tsx ESM hook (produces native ESM)

V8 emits two separate coverage entries for the same source file with mismatched byte offsets. When c8 applies the tsx source map, it generates phantom uncovered functions/lines at wrong line numbers, dropping coverage from 100% to ~97%.

The loss is not real — all code is covered. It is a false negative in the coverage report.

What was tried

  • Upgrading tsx (already at latest, did not help)
  • Upgrading c8 to v11 (did not help)
  • Loading setup.ts via --import instead of require (broke chai plugin setup)
  • Renaming setup.ts to setup.mts (mocha only bypasses .mjs, not .mts)
  • Renaming all .ts to .mts (mocha still routes .mts through require())

Upstream status

No issue has been filed upstream for this specific tsx + c8 + mocha combination. The relevant mocha PRs are:

  • #5366 — introduced the regression (11.7.0)
  • #5384 — partial fix, does not help with tsx (11.7.1)
  • #5429 — fixes .mjs only, not .ts (11.7.2)

Resolution

Keep mocha pinned to ^11.3.0 in package.json until a mocha release properly handles .ts files in the requireModule path (i.e. routes them through import() instead of require() when an ESM loader like tsx is registered).