rsnext/docs/api-reference/next.config.js/custom-webpack-config.md

# Custom Webpack Config

Some commonly asked for features are available as plugins:

- [@zeit/next-css](https://github.com/zeit/next-plugins/tree/master/packages/next-css)
- [@zeit/next-sass](https://github.com/zeit/next-plugins/tree/master/packages/next-sass)
- [@zeit/next-less](https://github.com/zeit/next-plugins/tree/master/packages/next-less)
- [@zeit/next-preact](https://github.com/zeit/next-plugins/tree/master/packages/next-preact)
- [@next/mdx](https://github.com/zeit/next.js/tree/canary/packages/next-mdx)
- [@next/bundle-analyzer](https://github.com/zeit/next.js/tree/canary/packages/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:

```js
module.exports = {
  webpack: (config, { buildId, dev, isServer, defaultLoaders, webpack }) => {
    // Note: we provide webpack above so you should not `require` it
    // Perform customizations to webpack config
    // Important: return the modified config
    config.plugins.push(new webpack.IgnorePlugin(/\/__tests__\//))
    return config
  },
  webpackDevMiddleware: config => {
    // Perform customizations to webpack dev middleware config
    // 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`:

```js
// Example config for adding a loader that depends on babel-loader
// This source was taken from the @next/mdx plugin source:
// https://github.com/zeit/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
  },
}
```