Inconsistent coverage levels between runs with "all"

Link to bug demonstration repository

• npm install
• npm run-script oneOff https://github.com/jameswilddev/nyc-repro

I’ve seen a few other issues reporting similar symptoms, but their fixes are things I’m already doing or are not applicable (different tools): https://github.com/istanbuljs/nyc/issues/981

Expected Behavior

Repeated runs produce the same result (which is expected to be 100% here).

Observed Behavior

Repeated runs produce 92% or 100% coverage.

Troubleshooting steps

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

Environment Information

System:
    OS: Linux 4.18 Ubuntu 18.04.2 LTS (Bionic Beaver)
    CPU: (4) x64 Intel(R) Core(TM) i5-6200U CPU @ 2.30GHz
    Memory: 557.58 MB / 7.64 GB
  Binaries:
    Node: 11.15.0 - /usr/bin/node
    npm: 6.7.0 - /usr/bin/npm
  npmPackages:
    nyc: ^14.1.1 => 14.1.1 
    ts-node: ^8.1.0 => 8.1.0 
    typescript: ^3.4.5 => 3.4.5 

[bcoe]

@elliot-nelson great digging. I’ve seen this issue for several years, and you’re correct it seems to be a race condition between which JSON file is loaded from .nyc_output first.

I would love to better document how to avoid this problem, as it’s a frequent complaint for TypeScript users. If TypeScript is being built before running tests, It might also be worth looking at c8 which I’ve been finding works a bit better for my TypeScript libraries (the disclaimer being it doesn’t work if TypeScript is being dynamically built in tests).

[jameswilddev]

That makes sense, and explains why all would trigger it if it’s actively looking for files to include and picking up ones it shouldn’t. Going to have to play and see if I can fix it my end but doubtful.