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Documentation Index

Fetch the complete documentation index at: https://extension.js.org/llms.txt

Use this file to discover all available pages before exploring further.

Add Babel only when you need transforms or plugins that SWC (Speedy Web Compiler) does not cover. Keep SWC as the default pipeline, then scope Babel to the files that need it.

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When Babel is a good fit

  • You depend on Babel-only plugins or presets.
  • You are migrating an existing Babel-heavy project incrementally.
  • You want targeted transforms for specific file types (for example, MDX).

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Babel capabilities

CapabilityWhat it gives you
Targeted transformsRun Babel for specific file types (for example, MDX) without replacing the full build pipeline
Plugin ecosystem accessUse Babel-only presets/plugins from existing web projects
Incremental migrationMove legacy Babel extension projects to Extension.js in smaller steps
Rspack (Extension.js bundler) integrationConfigure Babel through extension.config.* config hook

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Example setup

Install Babel dependencies: Create babel.config.json: 
{
  "presets": ["@babel/preset-env"]
}
Wire Babel into extension.config.js for MDX only: 
export default {
  config: (config) => {
    config.module.rules.push({
      test: /\.mdx$/,
      use: ["babel-loader", "@mdx-js/loader"],
    });
    return config;
  },
};

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Replace SWC for JS/TS only if required

Use this pattern only when you need Babel to process JS/TS entry files directly: 
export default {
  config: (config) => {
    config.module.rules = [
      {
        test: /\.[jt]sx?$/,
        exclude: /node_modules/,
        use: [{ loader: "babel-loader" }],
      },
      ...config.module.rules.filter(
        (rule) => !String(rule?.test || "").includes("[jt]sx"),
      ),
    ];
    return config;
  },
};

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Best practices

  • Prefer targeted Babel usage before replacing the default JS/TS pipeline.
  • Keep Babel rules explicit so loader ordering stays predictable.
  • Reuse Babel only where migration or plugin compatibility requires it.

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Next steps