树摇动和代码删除

了解 Expo CLI 如何优化生产 JavaScript 包。

Tree Shaking（也称为死代码删除）是一种从生产包中删除未使用代码的技术。Expo CLI 采用不同的技术（包括 minification），通过删除未使用的代码来缩短启动时间。

¥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.

平台晃动

¥Platform shaking

Expo CLI 采用称为平台抖动的过程进行应用打包，为每个平台（Android、iOS、Web）创建单独的打包包。它确保代码仅在一个平台上使用，并从其他平台上删除。

¥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.

任何基于 react-native 中的 Platform 模块有条件使用的代码都将从其他平台中删除。但是，此排除特别适用于每个文件中直接从 React-Native 导入 Platform.select 和 Platform.OS 的实例。如果通过不同的模块重新导出它们，则它们在不同平台的打包过程中不会被删除。

¥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.

例如，考虑以下转换输入：

¥For example, consider the following transformation input:

import { Platform } from 'react-native';

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

生产包将根据平台删除条件：

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

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

console.log('Hello on iOS');

此优化仅适用于生产，并且在每个文件的基础上运行。如果你从不同的模块重新导出 Platform.OS，它不会从生产包中删除。

¥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.

从 SDK 51 开始，process.env.EXPO_OS 可用于检测打包 JavaScript 的平台（无法在运行时更改）。由于 Metro 在依赖解析后缩小代码的方式，该值不支持平台抖动导入。

¥Starting in SDK 51, 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.

删除仅供开发的代码

¥Remove development-only code

在你的项目中，可能有一些旨在帮助开发过程的代码。它应该被排除在生产包之外。要处理这些场景，请使用 process.env.NODE_ENV 环境变量或非标准 __DEV__ 全局布尔值。

¥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

SDK 51 and greater

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

babel-preset-expo automatically transforms typeof window === 'undefined' to true when bundling for server environments and false when bundling for websites. The check remains unchanged when bundling for native client environments to support apps that polyfill window. This transform runs in both development and production but only removes conditional requires in production.

You can configure babel-preset-expo to skip the transform by passing { preserveTypeofWindow: false }.

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

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

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

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

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 above.

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 above.

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 above.

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

实验性地在 SDK 52 及以上版本中可用。

¥Experimentally available in SDK 52 and above.

Expo 通过详尽地递归图表来查找未使用的导入，从而优化模块。考虑以下代码片段：

¥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() {}

在这种情况下，bar 用于 foo，因此无法删除。但是，如果应用中的任何地方都没有使用 foo，那么 foo 将被删除，并且将再次扫描模块以查看是否可以删除 bar。由于性能原因，此过程在退出之前针对给定模块递归 5 次。

¥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.

副作用

¥Side-effects

Expo CLI 根据 Webpack 系统 尊重模块副作用。副作用通常用于定义全局变量（console.log）或修改原型（避免这样做）。

¥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).

你可以在 package.json 中标记你的模块是否有副作用：

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

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

副作用将阻止删除未使用的模块并禁用模块内联以确保 JS 代码按预期顺序运行。如果副作用为空或仅包含注释和指令（"use strict"、"use client" 等），则会被删除。

¥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).

启用 Expo 树摇动后，你可以安全地在 metro.config.js 中为生产包启用 inlineRequires。这将在评估模块时延迟加载模块，从而缩短启动时间。避免在没有 Expo tree shake 的情况下使用此功能，因为它会以可能改变副作用执行顺序的方式移动模块。

¥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;

针对摇树进行优化

¥Optimizing for tree shaking

在 Expo tree shake 之前，React Native 库会通过将导入封装在条件块中来删除导入，例如：

¥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();
}

这是有问题的，因为你没有准确的 TypeScript 支持，并且由于你无法静态分析代码，因此会使图表变得模糊。启用 Expo tree shake 后，你可以重构此代码以使用 ESM 导入：

¥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();
}

在这两种情况下，整个模块在生产打包包中都是空的。

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