compiler.md

description
Learn about the Next.js Compiler, written in Rust, which transforms and minifies your Next.js application.

Next.js Compiler

Version History

Version	Changes
v12.0.0	Next.js Compiler introduced.

The Next.js Compiler, written in Rust using SWC, allows Next.js to transform and minify your JavaScript code for production. This replaces Babel for individual files and Terser for minifying output bundles.

Compilation using the Next.js Compiler is 17x faster than Babel and enabled by default since Next.js version 12. If you have an existing Babel configuration or are using unsupported features, your application will opt-out of the Next.js Compiler and continue using Babel.

Why SWC?

SWC is an extensible Rust-based platform for the next generation of fast developer tools.

SWC can be used for compilation, minification, bundling, and more – and is designed to be extended. It's something you can call to perform code transformations (either built-in or custom). Running those transformations happens through higher-level tools like Next.js.

We chose to build on SWC for a few reasons:

• Extensibility: SWC can be used as a Crate inside Next.js, without having to fork the library or workaround design constraints.
• Performance: We were able to achieve ~3x faster Fast Refresh and ~5x faster builds in Next.js by switching to SWC, with more room for optimization still in progress.
• WebAssembly: Rust's support for WASM is essential for supporting all possible platforms and taking Next.js development everywhere.
• Community: The Rust community and ecosystem are amazing and still growing.

Experimental Features

Minification

You can opt-in to using the Next.js compiler for minification. This is 7x faster than Terser.

// next.config.js

module.exports = {
  swcMinify: true,
}

If you have feedback about swcMinify, please share it on the feedback discussion.

Styled Components

We're working to port babel-plugin-styled-components to the Next.js Compiler.

First, update to the latest version of Next.js: npm install next@latest. Then, update your next.config.js file:

// next.config.js

module.exports = {
  experimental: {
    // ssr and displayName are configured by default
    styledComponents: true,
  },
}

Currently, only the ssr and displayName transforms have been implemented. These two transforms are the main requirement for using styled-components in Next.js.

Jest

Jest support not only includes the transformation previously provided by Babel, but also simplifies configuring Jest together with Next.js including:

• Auto mocking of .css, .module.css (and their .scss variants), and image imports
• Automatically sets up transform using SWC
• Loading .env (and all variants) into process.env
• Ignores node_modules from test resolving and transforms
• Ignoring .next from test resolving
• Loads next.config.js for flags that enable experimental SWC transforms

First, update to the latest version of Next.js: npm install next@latest. Then, update your jest.config.js file:

// jest.config.js
const nextJest = require('next/jest')

// Providing the path to your Next.js app which will enable loading next.config.js and .env files
const createJestConfig = nextJest({ dir })

// Any custom config you want to pass to Jest
const customJestConfig = {
  setupFilesAfterEnv: ['<rootDir>/jest.setup.js'],
}

// createJestConfig is exported in this way to ensure that next/jest can load the Next.js configuration, which is async
module.exports = createJestConfig(customJestConfig)

Relay

To enable Relay support:

// next.config.js
module.exports = {
  experimental: {
    relay: {
      // This should match relay.config.js
      src: './',
      artifactDirectory: './__generated__'
      language: 'typescript',
    },
  },
}

NOTE: In Next.js all JavaScripts files in pages directory are considered routes. So, for relay-compiler you'll need to specify artifactDirectory configuration settings outside of the pages, otherwise relay-compiler will generate files next to the source file in the __generated__ directory, and this file will be considered a route, which will break production build.

Remove React Properties

Allows to remove JSX properties. This is often used for testing. Similar to babel-plugin-react-remove-properties.

To remove properties matching the default regex ^data-test:

// next.config.js
module.exports = {
  experimental: {
    reactRemoveProperties: true,
  },
}

To remove custom properties:

// next.config.js
module.exports = {
  experimental: {
    // The regexes defined here are processed in Rust so the syntax is different from
    // JavaScript `RegExp`s. See https://docs.rs/regex.
    reactRemoveProperties: { properties: ['^data-custom$'] },
  },
}

Legacy Decorators

Next.js will automatically detect experimentalDecorators in jsconfig.json or tsconfig.json and apply that. This is commonly used with older versions of libraries like mobx.

This flag is only supported for compatibility with existing applications. We do not recommend using legacy decorators in new applications.

First, update to the latest version of Next.js: npm install next@latest. Then, update your jsconfig.json or tsconfig.json file:

{
  "compilerOptions": {
    "experimentalDecorators": true
  }
}

Remove Console

This transform allows for removing all console.* calls in application code (not node_modules). Similar to babel-plugin-transform-remove-console.

Remove all console.* calls:

// next.config.js
module.exports = {
  experimental: {
    removeConsole: true,
  },
}

Remove console.* output except console.error:

// next.config.js
module.exports = {
  experimental: {
    removeConsole: {
      exclude: ['error'],
    },
  },
}

importSource

Next.js will automatically detect jsxImportSource in jsconfig.json or tsconfig.json and apply that. This is commonly used with libraries like Theme UI.

First, update to the latest version of Next.js: npm install next@latest. Then, update your jsconfig.json or tsconfig.json file:

{
  "compilerOptions": {
    "jsxImportSource": true
  }
}

Unsupported Features

When your application has a .babelrc file, Next.js will automatically fall back to using Babel for transforming individual files. This ensures backwards compatibility with existing applications that leverage custom Babel plugins.

If you're using a custom Babel setup, please share your configuration. We're working to port as many commonly used Babel transformations as possible, as well as supporting plugins in the future.