Custom Webpack Config

Before continuing to add custom webpack configuration to your application make sure Next.js doesn't already support your use-case:

• CSS imports
• CSS modules
• Sass/SCSS imports
• Sass/SCSS modules
• preact
• Customizing babel configuration

Some commonly asked for features are available as plugins:

• @next/mdx
• @next/bundle-analyzer

In order to extend our usage of webpack, you can define a function that extends its config inside next.config.js, like so:

module.exports = {
  webpack: (config, { buildId, dev, isServer, defaultLoaders, webpack }) => {
    // Important: return the modified config
    return config
  },
}

The webpack function is executed twice, once for the server and once for the client. This allows you to distinguish between client and server configuration using the isServer property.

The second argument to the webpack function is an object with the following properties:

• buildId: String - The build id, used as a unique identifier between builds
• dev: Boolean - Indicates if the compilation will be done in development
• isServer: Boolean - It's true for server-side compilation, and false for client-side compilation
• defaultLoaders: Object - Default loaders used internally by Next.js:
  • babel: Object - Default babel-loader configuration

Example usage of defaultLoaders.babel:

// Example config for adding a loader that depends on babel-loader
// This source was taken from the @next/mdx plugin source:
// https://github.com/vercel/next.js/tree/canary/packages/next-mdx
module.exports = {
  webpack: (config, options) => {
    config.module.rules.push({
      test: /\.mdx/,
      use: [
        options.defaultLoaders.babel,
        {
          loader: '@mdx-js/loader',
          options: pluginOptions.options,
        },
      ],
    })

    return config
  },
}

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