Why Are My CSS Files in Chrome DevTools Displaying JavaScript and Duplicated?

Below is an explanation of what’s likely happening and some steps to help debug:

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Why Do CSS Files Contain JavaScript in DevTools?

When using tools like Webpack with the "style-loader", the CSS isn’t loaded as a standalone CSS file in the browser. Instead, it gets bundled into JavaScript that dynamically creates and injects <style> tags into the DOM. In the Sources tab, you end up seeing that JavaScript module instead of your original CSS file.

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Why Are the CSS Files Duplicated with More JavaScript?

There are a couple of potential reasons for duplication:

1. Multiple Injections:
   If both your source code and some build tool configuration (like HMR from the dev server) are causing the CSS to be injected multiple times, you may see duplicated entries in the sources.

2. Source Map Transformations:
   With the inclusion of loaders such as source-map-loader, the generated modules (including those from style-loader) can appear as if the original CSS file appears in multiple forms. Essentially, the JavaScript that takes care of injecting the CSS is layered with additional source mapping code, making it look "polluted" with JavaScript.

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Could It Be Caused by the Build System?

Yes, it very likely is. In your setup using Webpack (or similarly Vite if used) with the style-loader and css-loader, the behavior you’re observing is an artifact of this system. The CSS is transformed into JavaScript at build time, and when debugging in Chrome DevTools, you see the resulting injected code rather than the isolated CSS content.

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Debugging & Suggestions

• Switch to CSS Extraction in Dev:
  Instead of using style-loader, try using a plugin like MiniCssExtractPlugin. This will extract your CSS into separate files so that the browser loads them as standalone CSS. This can also help you see “clean” CSS in DevTools.

  JAVASCRIPTCopied
  // webpack.config.js modifications:
  import MiniCssExtractPlugin from 'mini-css-extract-plugin';
  
  export default {
      // ... other config ...
      module: {
          rules: [
              // For CSS
              {
                  test: /\.css$/i,
                  use: [MiniCssExtractPlugin.loader, "css-loader"]
              },
              // ... other rules ...
          ]
      },
      plugins: [
          new MiniCssExtractPlugin({
              filename: "[name].[contenthash].css",
          }),
          // ... other plugins ...
      ]
  };

• Check Source Map Settings:
  Ensure that your source map settings are not over-processing your styles. Sometimes unintended transformations occur if loaders are applied to files already processed by other loaders. You may experiment by temporarily disabling the source-map-loader for CSS files to see if that changes the behavior.

• Review DevServer HMR Settings:
  Hot Module Replacement (HMR) can sometimes inject multiple versions of the CSS during live reloads. If duplicates are only appearing during HMR, try disabling HMR for styles as a test.

• Inspect the Network Tab:
  If you inspect the CSS file request in the Network tab, see if the content there is as expected for pure CSS. This will confirm that the transformation is really happening during module injection rather than during file serving.

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Summary

1. CSS containing JavaScript is due to the use of loaders like style-loader that inject CSS at runtime.
2. Duplicated files with extra JavaScript likely result from multiple injections or source map transformations layered over the injected CSS.
3. The build system is the prime suspect. Reviewing how Webpack (or Vite) handles CSS processing and considering the extraction of CSS into separate files might resolve the issue.

Following these steps should help isolate and resolve the confusion in Chrome DevTools.