Tree shaking and code removal

Learn about how Expo CLI optimizes production JavaScript bundles.

Tree shaking (also referred to as dead code removal) is a technique to remove unused code from the production bundle. Expo CLI employs different techniques, including minification, to improve startup time by removing unused code.

Platform shaking

Expo CLI employs a process known as platform shaking for app bundling, where it creates separate bundles for each platform (Android, iOS, web). It ensures that the code is only used on one platform and is removed from other platforms.

Any code that is used conditionally based on the Platform module from react-native is removed from the other platforms. However, this exclusion specifically applies to instances where Platform.select and Platform.OS are directly imported from react-native in each file. If these are re-exported through a different module, they will not be removed during the bundling process for different platforms.

For example, consider the following transformation input:

import { Platform } from 'react-native';

if (Platform.OS === 'ios') {
  console.log('Hello on iOS');
}

The production bundle will remove the conditional based on the platform:

%%placeholder-start%%Empty on Android %%placeholder-end%%

console.log('Hello on iOS');

This optimization is production only and runs on a per-file basis. If you re-export Platform.OS from a different module, it will not be removed from the production bundle.

The process.env.EXPO_OS can be used to detect the platform that the JavaScript was bundled for (cannot change at runtime). This value does not support platform shaking imports due to how Metro minifies code after dependency resolution.

Remove development-only code

In your project, there might be code designed to help with the development process. It should be excluded from the production bundle. To handle these scenarios, use the process.env.NODE_ENV environment variable or the non-standard __DEV__ global boolean.

if (process.env.NODE_ENV === 'development') {
  console.log('Hello in development');
}

if (__DEV__) {
  console.log('Another development-only conditional...');
}

if ('production' === 'development') {
  console.log('Hello in development');
}

if (false) {
  console.log('Another development-only conditional...');
}

%%placeholder-start%%Empty file %%placeholder-end%%

To improve speed, Expo CLI only performs code elimination in production builds. Conditionals from the above code snippet are kept in development builds.

Custom code removal

EXPO_PUBLIC_ environment variables are inlined before the minification process. This means they can be used to remove code from the production bundle. For example:

EXPO_PUBLIC_DISABLE_FEATURE=true;

if (!process.env.EXPO_PUBLIC_DISABLE_FEATURE) {
  console.log('Hello from the feature!');
}

if (!'true') {
  console.log('Hello from the feature!');
}

// Empty file

• This system does not apply to server code as environment variables are not inlined in server bundles.
• Library authors should not use EXPO_PUBLIC_ environment variables as they only run in application code for security reasons.

Removing server code

It's common to use typeof window === 'undefined' to conditionally enable or disable code for server and client environments.

babel-preset-expo will transform typeof window === 'undefined' to true when bundling for server environments. By default, this check remains unchanged when bundling for web client environments. This transform runs in both development and production but only removes conditional requires in production.

You can configure babel-preset-expo to enable this transform by passing { minifyTypeofWindow: true }. By default, this transform remains disabled even for web environments since web workers won't have a window global.

if (typeof window === 'undefined') {
  console.log('Hello on the server!');
}

if (true) {
  console.log('Hello on the server!');
}

if (typeof window === 'undefined') {
  console.log('Hello on the server!');
}

console.log('Hello on the server!');

if (typeof window === 'undefined') {
  console.log('Hello on the server!');
}
// Empty file

React Native web imports

babel-preset-expo provides a built-in optimization for the react-native-web barrel file. If you import react-native directly using ESM, then the barrel file will be removed from the production bundle.

import { View, Image } from 'react-native';

import View from 'react-native-web/dist/exports/View';
import Image from 'react-native-web/dist/exports/Image';

const { View, Image } = require('react-native');

const { View, Image } = require('react-native-web');

Remove unused imports and exports

Experimentally available in SDK 52 and later.

You can experimentally enable support for automatically removing unused imports and exports across modules. This is useful for speeding up native OTA downloads and optimizing web performance where JavaScript must be parsed and executed using a standard JavaScript engine.

Consider the following example code:

import { ArrowUp } from './icons';

export default function Home() {
  return <ArrowUp />;
}

export function ArrowUp() {
  /* ... */
}

export function ArrowDown() {
  /* ... */
}

export function ArrowRight() {
  /* ... */
}

export function ArrowLeft() {
  /* ... */
}

Since only ArrowUp is used in index.js, the production bundle will remove all other components from icons.js.

export function ArrowUp() {
  /* ... */
}

This system scales up to automatically optimize all import and export syntax in your app, across all platforms. While this results in smaller bundles, processing JS still requires time and computer memory so avoid importing millions of modules.

• Tree-shaking only runs in production bundles and can only run on modules that use import and export syntax. Files that use module.exports and require will not be tree-shaken.
• Avoid adding Babel plugins such as @babel/plugin-transform-modules-commonjs which convert import/export syntax to CJS. This will break tree-shaking across your project.
• Modules that are marked as side-effects will not be removed from the graph.
• export * from "..." will be expanded and optimized unless the export uses module.exports or exports.
• All modules in the Expo SDK are shipped as ESM and can be exhaustively tree-shaken.

Enabling tree shaking

Experimentally available in SDK 52 and later.

const { getDefaultConfig } = require('expo/metro-config');

const config = getDefaultConfig(__dirname);

config.transformer.getTransformOptions = async () => ({
  transform: {
    experimentalImportSupport: true,
  },
});

module.exports = config;

EXPO_UNSTABLE_METRO_OPTIMIZE_GRAPH=1

EXPO_UNSTABLE_TREE_SHAKING=1

This feature is very experimental because it changes the fundamental structure of how Metro bundles code. By default, Metro bundles everything on-demand and lazily to ensure the fastest possible development times. In contrast, tree shaking requires some transformation to be delayed until after the entire bundle has been created. This means less code can be cached, which is generally fine because tree shaking is a production-only feature and production bundles often don't use transform caches.

Barrel files

Experimentally available in SDK 52 and later.

With Expo tree shaking, star exports will automatically be expanded and shaken based on usage. For example, consider the following code snippet:

export * from './icons';

The optimization pass will crawl ./icons and add the exports to the current module. If the exports are unused, they will be removed from the production bundle.

export { ArrowRight, ArrowLeft } from './icons';

This will be shaken according to standard tree shaking rules. If you only import ArrowRight, then ArrowLeft will be removed from the production bundle.

If the star export pulls in ambiguous exports such as module.exports.ArrowUp or exports.ArrowDown, then the optimization pass will not expand the star export and no exports will be removed from the barrel file. You can use Expo Atlas to inspect the expanded exports.

You can use this strategy with libraries like lucide-react to remove all icons that are not used in your app.

Recursive optimizations

Experimentally available in SDK 52 and later.

Expo optimizes a module by recursing through the graph exhaustively to find unused imports. Consider the following code snippet:

export function foo() {
  // Because bar is used here, it cannot be removed.
  bar();
}

export function bar() {}

In this case, bar is used in foo, so it cannot be removed. However, if foo is not used anywhere in the app, then foo will be removed and the module will be scanned again to see if bar can be removed. This process recurses 5 times for a given module before bailing out due to performance reasons.

Side-effects

Expo CLI respects module side-effects according to the Webpack system. Side-effects are generally used for defining global variables (console.log) or modifying prototypes (avoid doing this).

You can mark if your module has side-effects in the package.json:

{
  "name": "library",
  "sideEffects": ["./src/*.js"]
}

Side-effects will prevent the removal of unused modules and disable module inlining to ensure JS code runs in the expected order. Side-effects will be removed if they're empty or contain only comments and directives ("use strict", "use client", and so on).

When Expo tree shaking is enabled, you can safely enable inlineRequires in your metro.config.js for production bundles. This will lazily load modules when they're evaluated, leading to faster startup time. Avoid using this feature without Expo tree shaking as it will move modules around in ways that can change the execution order of side-effects.

const { getDefaultConfig } = require('expo/metro-config');

const config = getDefaultConfig(__dirname);

config.transformer.getTransformOptions = async () => ({
  transform: {
    experimentalImportSupport: true,
    inlineRequires: true,
  },
});

module.exports = config;

Optimizing for tree shaking

Before Expo tree shaking, React Native libraries would remove imports by wrapping them in conditional blocks such as:

if (process.env.NODE_ENV === 'development') {
  require('./dev-only').doSomething();
}

This is problematic because you don't have accurate TypeScript support and it makes the graph ambiguous since you cannot statically analyze the code. With Expo tree shaking enabled, you can restructure this code to use ESM imports:

import { doSomething } from './dev-only';

if (process.env.NODE_ENV === 'development') {
  doSomething();
}

In both cases, the entire module will be empty in production bundles.