🐛 Bug: Spec Configuration Option Incorrectly takes Priority over a Spec Command-line Argument

[X-Guardian]

Bug Report Checklist

• I have read and agree to Mocha's Code of Conduct and Contributing Guidelines
• I have searched for related issues and issues with the faq label, but none matched my issue.
• I have 'smoke tested' the code to be tested by running it outside the real test suite to get a better sense of whether the problem is in the code under test, my usage of Mocha, or Mocha itself.
• I want to provide a PR to resolve this

Expected

With a Mocha configuration file that contains the spec option, I would expect that running the Mocha CLI with a spec argument, the CLI argument should take priority.

Actual

The spec option in the configuration file takes priority.

Minimal, Reproducible Example

Create two test files:

• src/test1/test1.spec.js
• src/test2/test2.spec.js

Create the following .mocharc.yml with a spec option:

spec: ["src/**/*.spec.js"]

Run Mocha specifying only the first test file:

yarn mocha src/test1/test1.spec.js

The result is that incorrectly, both test files will run

  test1
    ✔ should be true

  test2
    ✔ should be true


  2 passing (2ms)

Remove the spec option from .mocharc.yml and re-run Mocha specifying only the first test file:

yarn mocha src/test1/test1.spec.js

The results is that correctly, only the first test will run:

  test1
    ✔ should be true


  1 passing (1ms)

Versions

11.7.1

Additional Info

According to https://mochajs.org/#merging, the command line arguments should take priority over the configuration file.

[mark-wiemer]

Hmm, I'm able to reproduce this behavior, but I'm not sure yet that it's a bug. Because spec is an array, the command-line arg is just inserted at the beginning of the array: spec: [ 't1.spec.js', '*.spec.js' ]. Mocha doesn't see the two definitions as conflicting. This does feel like weird behavior, and I would like to change it, but it would definitely be a breaking change. CC'ing @mochajs/maintenance-crew for their thoughts.

To workaround, for now, I encourage you to just comment out the spec line you don't want, or use the --config option

[X-Guardian]

Thanks for the response Mark. Now that I know it is merging the array, that makes sense. I'm already working around this by using multiple configuration files.
Maybe at the minimum it could do with explicit clarification of this in the docs, as I don't believe it is obvious.