Analyzing JavaScript bundles

Learn about improving the production JavaScript bundle size of Expo apps and websites.

Bundle performance varies for different platforms. For example, web browsers don't support precompiled bytecode, so the JavaScript bundle size is important for improving startup time and performance. The smaller the bundle, the faster it can be downloaded and parsed.

Analyzing bundle size with Atlas

Available only for SDK 51 and above.

The libraries used in a project influence the size of the production JavaScript bundle. Starting from Expo SDK 51, you can use Expo Atlas to visualize the production bundle and identify which libraries contribute to the bundle size.

Using Atlas with npx expo start

You can use Expo Atlas with the local development server. This method allows Atlas to update whenever you change any code in your project.

Once your app is running using the local development server on Android, iOS, and/or web, you can open Atlas through the dev tools plugin menu using shift + m.

Changing development mode to production

By default, Expo starts the local development server in development mode. Development mode disables some optimizations that are enabled in production mode. You can also start the local development server in production mode to get a more accurate representation of the production bundle size:

Using Atlas with npx expo export

You can also use Expo Atlas when generating a production bundle for your app or EAS Update. Atlas generates a .expo/atlas.jsonl file during export, which you can share and open without having access to the project.

You can also specify the platforms you want to analyze using the --platform option. Atlas will gather the data for the exported platforms only.

Analyzing transformed modules

Inside Atlas, you can hold ⌘ Cmd and click on a graph node to see the transformed module details. This feature helps you understand how a module is transformed by Babel, which modules it imports, and which modules imported it. This can be used to trace the origin of a module across the dependency graph.

Analyzing bundle size with source-map-explorer

Alternative method for SDK 50 and below.

If you are using SDK 50 or below, you can use the source-map-explorer library to visualize and analyze the production JavaScript bundle.

{
  "scripts": {
    "analyze:web": "source-map-explorer 'dist/_expo/static/js/web/*.js' 'dist/_expo/static/js/web/*.js.map'",
    "analyze:ios": "source-map-explorer 'dist/_expo/static/js/ios/*.js' 'dist/_expo/static/js/ios/*.js.map'",
    "analyze:android": "source-map-explorer 'dist/_expo/static/js/android/*.js' 'dist/_expo/static/js/android/*.js.map'"
  }
}

You must provide the URL of lib/mappings.wasm by calling SourceMapConsumer.initialize({ 'lib/mappings.wasm': ... }) before using SourceMapConsumer

You might encounter a warning such as Unable to map 809/13787 bytes (5.87%). This occurs because source maps often exclude bundler runtime definitions (for example, __d(() => {}, [])). This value is consistent and not a reason for concern.

Lighthouse

Lighthouse is a great way to see how fast, accessible, and performant your website is. You can test your project with the Audit tab in Chrome, or with the Lighthouse CLI.

After creating a production build with npx expo export -p web and serving it (using either npx serve dist, or production deployment, or custom server), run Lighthouse with the URL your site is hosted at.