[Bug]: modifying resolve.extensions in Rspack config object doesn't work

[mdrobny]

Version

System:
  OS: macOS 14.6.1
  CPU: (10) arm64 Apple M1 Pro
  Memory: 269.59 MB / 32.00 GB
  Shell: 5.9 - /bin/zsh
Browsers:
  Chrome: 128.0.6613.120
  Safari: 17.6
npmPackages:
  @rsbuild/core: workspace:* => 1.0.1-rc.5 
  @rsbuild/plugin-react: workspace:* => 1.0.1-rc.5

Details

when using Rsbuild, I noticed that modifying resolve.extensionsproperty in Rspack config object doesn't work.
Luckily it works when modifying the Rspack config with a function so I it's possible to workaround the problem currently.
But I expected to achieve it by just modifying the object like the docs

Reproduce link

https://codepen.io/mdrobny/pen/KKjJVwN

Reproduce Steps

1. try to modify resolve.extensions in example rsbuild.config.ts like this

export default defineConfig({
  plugins: [pluginReact()],
  tools: {
    rspack: {
      resolve: {
        extensions: ['.jsx', '.tsx', '.ts', '.js'],
      },
    }
  }
});

2. add App.jsx component in src folder

const App = () =>  <div>App JSX</div>

export default App;

3. run rsbuild dev

4. open website and you can still see content from App.tsx. Rsbuild should use App.jsx file because jsx extension has priority over tsx but it doesn't

   • 

5. Change rsbuild.config.ts to

export default defineConfig({
  plugins: [pluginReact()],
  tools: {
    rspack: config => {
      config.resolve ||= {};
      config.resolve.extensions = ['.jsx', '.tsx', '.ts', '.js'];
    }
  }
});

7. run rsbuild dev
8. open website and now content is App JSX so it worked
   • 

[chenjiahan]

Rsbuild uses webpack-merge to merge Rspack config objects, so the extensions array is merged instead of overwritten.

https://rsbuild.dev/config/tools/rspack

[mdrobny]

@chenjiahan thanks for mentioning this in the docs 💪 it will save people some trouble.

Great job with rsbuild BTW 🚀