babel-preset-expo

This preset extends the default React Native preset (metro-react-native-babel-preset) and adds support for decorators, tree-shaking web packages, and loading font icons with optional native dependencies if they're installed.

You can use this preset in any React Native project as a drop-in replacement for metro-react-native-babel-preset. If your project isn't using native font loading or web support then this preset will only add support for decorators with @babel/plugin-proposal-decorators - this is mostly used for supporting legacy community libraries.

If you start your web project with @expo/webpack-config or npx expo start and your project doesn't contain a babel.config.js or a .babelrc then it will default to using babel-preset-expo for loading.

If you have problems with the code in this repository, please file issues & bug reports at https://github.com/expo/expo. Thanks!

Expo Bundler Spec Compliance

A bundler must follow these requirements if they are to be considered spec compliant for use with a universal React (Expo) project.

Babel Loader

The babel loading mechanism must include the following properties on its caller.

platform

A platform property denoting the target platform. If the platform is not defined, it will default to using web when the bundler is webpack -- this is temporary and will throw an error in the future.

Value	Description
ios	Runs on iOS devices
android	Runs on Android devices
web	Runs in web browsers

bundler

A bundler property denoting the name of the bundler that is being used to create the JavaScript bundle. If the bundler is not defined, it will default to checking if a babel-loader is used, if so then webpack will be used, otherwise it will default to metro.

Value	Description
metro	Bundling with Metro
webpack	Bundling with Webpack

Options

jsxRuntime

classic | automatic, defaults to automatic

• automatic automatically convert JSX to JS without the need to import React from 'react' in every file. Be sure to follow the rest of the setup guide after enabling this, otherwise ESLint and other tools will throw warnings.
• classic does not automatically import anything, React must imported into every file that uses JSX syntax.

[
  'babel-preset-expo',
  {
    jsxRuntime: 'classic',
  },
];

This property is passed down to @babel/plugin-transform-react-jsx. This flag does nothing when native.useTransformReactJSXExperimental is set to true because @babel/plugin-transform-react-jsx is omitted.

jsxImportSource

string, defaults to react

This option allows specifying a custom import source for importing functions.

[
  'babel-preset-expo',
  {
    jsxRuntime: 'automatic',
    jsxImportSource: 'react',
  },
];

This property is passed down to @babel/plugin-transform-react-jsx. This options does nothing when jsxRuntime is not set to automatic.

lazyImports

Changes Babel's compiled import statements to be lazily evaluated when their imported bindings are used for the first time.

Note: this option has an effect only when the disableImportExportTransform option is set to false. On Android and iOS, disableImportExportTransform defaults to false, and on web it defaults to true to allow for tree shaking.

This can improve the initial load time of your app because evaluating dependencies up front is sometimes entirely un-necessary, particularly when the dependencies have no side effects.

The value of lazyImports has a few possible effects:

• null - metro-react-native-babel-preset will handle it. (Learn more about it here: https://github.com/facebook/metro/commit/23e3503dde5f914f3e642ef214f508d0a699851d)

• false - No lazy initialization of any imported module.

• true - Lazy-init all imported modules except local imports (e.g., ./foo), certain Expo packages that have side effects, and the two cases mentioned here.

• Array<string> - babel-plugin-transform-modules-commonjs will handle it.

• (string) => boolean - babel-plugin-transform-modules-commonjs will handle it.

  If you choose to do this, you can also access the list of Expo packages that have side effects by using const lazyImportsBlacklist = require('babel-preset-expo/lazy-imports-blacklist'); which returns a Set.

default: null

[
    'babel-preset-expo',
    {
        lazyImports: true
    }
],

web.disableImportExportTransform

Enabling this option will allow your project to run with older JavaScript syntax (i.e. module.exports). This option will break tree shaking and increase your bundle size, but will eliminate the following error when module.exports is used:

TypeError: Cannot assign to read only property 'exports' of object '#<Object>'

default: false when using Webpack. true otherwise.

[
    'babel-preset-expo',
    {
        web: { disableImportExportTransform: true }
    }
],