gatsby-plugin-sass prevents configuration of CSS Loader

Description

When setting the following options to gatsby-config.js:

module.exports = {
  plugins: [
    {
      resolve: `gatsby-plugin-sass`,
      options: {
        cssLoaderOptions: {
          modules: {
            exportLocalsConvention: 'camelCaseOnly'
          }
        }
      }
    }
  ]
}

We expect the exports of css-loader to be camelCaseOnly and not dashesOnly (default).

Steps to reproduce

Very straightforward, just place this in a Gatsby SASS project.

Expected result

The webpack configuration should be modified accordingly and the exported values should be in camel case.

Actual result

Nothing changes. After investigation, it seems that the culprit is in gatsby-plugin-sass/gatsby-node.js where there is the following:

const sassRuleModules = {
    test: sassRuleModulesTest || /\.module\.s(a|c)ss$/,
    use: [!isSSR && loaders.miniCssExtract({
      modules: true
    }), loaders.css({
      importLoaders: 2,
      ...cssLoaderOptions,
      modules: true
    }), loaders.postcss({
      plugins: postCssPlugins
    })].filter(Boolean)
  };

This causes the modules option to be overwritten even if it was provided. There should be a check to see if there is already a modules option to prevent overwriting.

Environment

  System:
    OS: Windows 10 10.0.19041
    CPU: (8) x64 Intel(R) Core(TM) i7-7700HQ CPU @ 2.80GHz
  Binaries:
    Node: 15.3.0 - C:\Program Files\nodejs\node.EXE
    Yarn: 1.22.10 - ~\AppData\Roaming\npm\yarn.CMD
    npm: 6.14.9 - ~\AppData\Roaming\npm\npm.CMD
  Languages:
    Python: 3.6.4
  Browsers:
    Chrome: 88.0.4324.190
    Edge: Spartan (44.19041.423.0), Chromium (88.0.705.81)
  npmPackages:
    gatsby: ^3.0.0 => 3.0.0
    gatsby-plugin-mdx: ^2.0.0 => 2.0.0
    gatsby-plugin-sass: ^4.0.0 => 4.0.0
    gatsby-source-filesystem: ^3.0.0 => 3.0.0
    gatsby-telemetry: ^2.0.0 => 2.0.0
  npmGlobalPackages:
    undefined: 0.1.0

[LekoArts]

Thanks for the info and the note that the README will need an update. We’ll look into both things 😃

[LekoArts]

@juanbiberretta It’s been released in gatsby-plugin-sass@4.0.1 😃

We publish a minor every two weeks and hotfixes ad-hoc.