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Customizing Turbopack

Customizing Turbopack

Turbopack can be customized to transform different files and change how modules are resolved. It supports a subset of webpack's loader API and offers similar configuration aliasing module resolution.

webpack loaders for Next.js

Turbopack for Next.js does not require loaders nor loader configuration for built-in functionality, just as they aren't required for Next.js. Turbopack has built-in support for css and compiling modern JavaScript, so there's no need for css-loader, postcss-loader, or babel-loader if you're just using @babel/preset-env.

If you need loader support beyond what's built in, many webpack loaders already work with Turbopack. There are currently some limitations:

  • Only a core subset of the webpack loader API is implemented. This is enough for some popular loaders, and we'll expand our support for this API in the future.
  • Only loaders that return JavaScript code are supported. Loaders that transform files like stylesheets or images are not currently supported.
  • Options passed to webpack loaders must be plain JavaScript primitives, objects, and arrays. For example, it's not possible to pass require()d plugin modules as option values.

As of Next 13.2, configuring webpack loaders is possible for Next.js apps through an experimental option in next.config.js. turbo.rules can be set as a mapping of file globs to a list of package names or {loader, options} pairs:

module.exports = {
  experimental: {
    turbo: {
      rules: {
        // Option format
        '*.md': [
          {
            loader: '@mdx-js/loader',
            options: {
              format: 'md',
            },
          },
        ],
        // Option-less format
        '*.mdx': ['@mdx-js/loader'],
      },
    },
  },
}

Note: Prior to Next.js version 13.4.4, experimental.turbo.rules was named experimental.turbo.loaders and only accepted file extensions like .mdx instead of *.mdx.

If you need to pass something like the result of importing an external package as a loader option, it's possible to wrap the webpack loader with your own, specifying options there. This is an interim solution and should not be necessary in the future. This loader wraps @mdx-js/loader and configures the rehypePrism rehype plugin:

const mdxLoader = require('@mdx-js/loader');
const rehypePrism = require('@mapbox/rehype-prism');
 
module.exports = function (code) {
	const prevGetOptions = this.getOptions.bind(this);
	this.getOptions = function getOptions(...args) {
		return {
			...prevGetOptions(...args),
			rehypePlugins: [rehypePrism]
		}
	}
 
	mdxLoader.call(this, code);
}

Then, configure Next.js to load the wrapper loader:

module.exports = {
  experimental: {
    turbo: {
      rules: {
        '*.mdx': ['./my-mdx-loader'],
      },
    },
  },
}

Supported loaders

The following loaders have been tested to work with Turbopack's webpack loader implementation:

Resolve aliases

Turbopack can be configured to modify module resolution through aliases, similar to webpack's resolve.alias (opens in a new tab) configuration:

module.exports = {
  experimental: {
    turbo: {
      resolveAlias: {
        underscore: 'lodash',
        mocha: { browser: 'mocha/browser-entry.js' },
      },
    },
  },
}

This aliases imports of the underscore package to the lodash package. In other words, import underscore from 'underscore' will load the lodash module instead of underscore.

Resolve Extensions

Through next.config.js, Turbopack can be configured to resolve modules with custom extensions, similar to webpack's resolve.extensions (opens in a new tab) configuration.

To configure resolve extension, use the resolveExtensions field in next.config.js:

module.exports = {
  experimental: {
    turbo: {
      resolveExtensions: [
        '.mdx',
        '.tsx',
        '.ts',
        '.jsx',
        '.js',
        '.mjs',
        '.json',
      ],
    },
  },
}

This overwrites the original resolve extensions with the provided list. Make sure to include the default extensions listed in the above config.

Turbopack also supports conditional aliasing through this field, similar to Node.js's conditional exports (opens in a new tab). At the moment only the browser condition is supported. In the case above, imports of the mocha module will be aliased to mocha/browser-entry.js when Turbopack targets browser environments.