Learn how Expo CLI generates native code of your project before compiling it.
Before a native app can be compiled, the native source code must be generated. Expo CLI provides a unique and powerful system called prebuild, which generates the native code for your project based on four factors:
npx expo prebuildcommand.
expothat's installed in the project and its corresponding prebuild template.
Prebuild can be used by running:
npx expo prebuild
This creates the android and ios directories for running your React code. If you modify the generated directories manually then you risk losing your changes the next time you run
npx expo prebuild --clean. Instead, you should create config plugins — functions that perform modifications on native projects during prebuild.
You can configure EAS Build to run
npx expo prebuild before building by using the managed preset.
You can perform a native build locally by running:
# Build your native Android project
npx expo run:android
# Build your native iOS project
npx expo run:ios
If native build directories are not present
npx expo prebuild will be run one time for the specific platform you wish to run. On subsequent uses of the run commands, you will need to manually run
npx expo prebuild to ensure the native code is freshly synchronized with your local configuration.
Prebuild currently supports Android and iOS. Web support is not required because there is no native project to generate for the web — the app it runs in is always your web browser. You can build for individual platforms by using the
npx expo prebuild --platform ios
The first step in prebuild is to initialize new native projects from a template. There is a template for each Expo SDK version, and each Expo SDK version corresponds to a specific version of React and React Native. The React and React Native versions in your project will be updated to the versions used in the
dependencies field in the template package.json, only, if they are not already the same.
You can skip changing npm package versions with the
npx expo prebuild --skip-dependency-update react-native,react
When the dependencies are changed, prebuild will reinstall packages using the package manager that is currently used in the project (this is inferred from the lockfile). You can force a specific package manager by providing one of:
All installations can be skipped by passing the
--no-install command, which is useful for testing generation quickly.
--clean flag will delete any existing native directories before generating.
When you re-run
npx expo prebuild without the
--clean flag, it layers the changes on top of your existing files. This is faster than re-generating from scratch, but it may not produce the same results in some cases. For example, not all config plugins are idempotent — when your project utilizes a lot of "dangerous modifiers" for performing changes like adding regex changes to application code, this can sometimes lead to unexpected behavior. This is why using the
--clean flag is the safest way to use the prebuild command and it is generally recommended in most cases.
Due to the destructive nature of the flag, you'll be warned to have a clean git status when the
--clean flag is used. This prompt is optional and will be skipped when encountered in CI.
If you'd like, you can disable the check by enabling the environment variable
The purpose of the prompt is to encourage managed workflow users to add the android and ios directories to the project's .gitignore, ensuring that the project is always managed. However, this can make custom config plugins harder to develop so we haven't introduced any mechanism to enforce this behavior.
There are cases where developers may want to swap between workflows often. For example, you may want to build custom functionality natively in Xcode and Android Studio, and then move that functionality into local config plugins.
You can customize how the native folders are generated while remaining in the managed workflow by building config plugins. Many config plugins already exist for lots of modifications. You can see a list of some popular plugins for more information.
Prebuild generates template files before modifying them with config plugins. The template files are based on the Expo SDK version and come from the npm package
expo-template-bare-minimum. You can change the template used by passing
--template /path/to/template.tgz to the
npx expo prebuild command. This is not generally recommended because the base modifiers in
@expo/prebuild-config make some undocumented assumptions about the template files, so it may be tricky to maintain your custom template.
As of SDK 46,
npx expo prebuild performs several side effects outside of generating the android and ios directories. Work is in progress to eliminate these side effects — ideally, running
npx expo prebuild would generate the Android and iOS projects and leave the rest of the project untouched.
In addition to generating the native folders, prebuild also makes the following modifications:
scriptsfield in the package.json.
dependenciesfield in the package.json.
mainfield in the package.json.
The convenience change to the
scripts field is the only side effect that alters how a developer works on their app before/after prebuild. All other changes can be left in place and committed to git to minimize the diff when running prebuild.
Prebuilding is completely optional and it works great with all tools and services offered by Expo. We created the designation bare workflow to refer to projects that do not use
npx expo prebuild — projects where developers make direct changes to their native projects, rather than continuously generating them on demand as with prebuild.
Everything offered by Expo including EAS, Expo CLI, and the libraries in the Expo SDK are built to fully support the bare workflow as this is a minimum requirement for supporting projects using
A single native project on its own is complicated to maintain, scale, and grow. In a cross-platform app, you have multiple native projects that you must maintain and keep up to date with the latest operating system releases to avoid falling too far behind in any third-party dependencies. We created the optional Expo Prebuild system to streamline this process. Below are a few issues we've identified with native development in the context of React Native and some corresponding reasons we believe Expo Prebuild solves these issues.
Prebuild can be used in any React Native project. For more information, see adopting prebuild.
Building native code requires a basic familiarity with that native platform's default tooling leading to a fairly difficult learning curve. In cross-platform, this curve is multiplied by the number of platforms you wish to develop for. Cross-platform tooling doesn't solve this issue if you need to drop down and implement many features individually in platform-specific native code.
When you bootstrap a native app, there is a bunch of code and configuration that you don't need to understand to get started. But you now own it. Eventually, you will want to upgrade your native application and now you'll need to be familiar with how all of the initial code works to safely upgrade it. This is extremely challenging and users often either upgrade their app incorrectly, missing crucial changes, or they'll bootstrap a new app and copy all of their source code into the new application.
Cross-platform configurations such as the app icon, name, splash screen, and so on must be implemented manually in native code. These are often implemented very differently across platforms.
With Prebuild cross-platform configurations are handled at the config plugin level, and the developer only needs to set a single value like
"icon": "./icon.png" to have all icon generation taken care of.
Many complex native packages require additional setup beyond installing and autolinking. For example, a camera library requires permission settings to be added to AndroidManifest.xml for Android and Info.plist for iOS. This additional setup can be considered a configuration side effect of a package. Pasting the required side effect code into your project's native files can lead to difficult native compilation errors, and it's also code that you now own and maintain.
With Prebuild library authors, who know how to configure their library better than anyone, can create a testable and versioned script called a config plugin, to automate adding the required configuration side effects for their library. This means library side effects can be more expressive, powerful, and stable. For native code side effects, we also provide AppDelegate Subscribers and Android Lifecycle Listeners which come standard in the default prebuild template.
When you uninstall a package you have to be certain you removed all of the side effects required to make that package work. If you miss anything, it leads to orphaned code that you cannot trace back to any particular package, this code builds up and makes your project harder to understand and maintain.
With Prebuild the only side effect is the config plugin in a project's Expo config (app.json), which will throw an error when the corresponding node module has been uninstalled, meaning a lot less orphaned configuration.
Here are some reasons Expo Prebuilding might not be the right fit for a particular project.
npx expo prebuild generates native code based on the version of
expo a project has installed, so a project with SDK 46 (
firstname.lastname@example.org) would generate a
Expo releases a new version approximately every quarter, and
react-native does not follow a calendar-based release schedule. This means there are times when you cannot use
npx expo prebuild with the latest release of React Native. This could potentially be circumvented by using a custom prebuild template if you are willing to experiment. You can also mitigate this by cherry-picking any changes you need from the latest version of React Native into a fork and using that in your project.
Prebuild can only be used for native platforms that are supported by the Expo SDK. This means Android and iOS for the time being. Except for web, which doesn't require
npx expo prebuild since it uses the browser instead of a custom native runtime.
In the future, we would like to support additional platforms, such as
react-native-windows, but this is not currently a priority.
If you are targeting additional platforms, you can still use prebuild for your android, and ios projects — any extra platforms will have to be configured manually in the meantime.
All native changes must be added with native modules (using React Native's built-in Native Module APIs or the Expo Modules API) and config plugins. This means if you want to quickly add a native file to your project to experiment, then you may be better off running prebuild and adding the file manually, then working your way back into the system with a monorepo. We plan to speed this process up by adding functionality to Expo Autolinking that finds project native files outside of the native folders and links them before building.
If you want to modify the configuration, such as the gradle.properties file, you'll have to write a plugin (example). This can be easily automated with helper plugin libraries, however, it is a bit slower if you need to do it often.
Not all packages support Expo Prebuilding yet. If you find a library that requires extra setup after installation and doesn't yet have a config plugin, we recommend opening a pull request or an issue so that the maintainer is aware of the feature request.
Other packages, such as
react-native-ble-plx, do require additional setup and therefore require a config plugin to be used with
npx expo prebuild (in this case there's an external plugin called
Alternatively, we also have a repo for out-of-tree config plugins which provides plugins for popular packages that haven't adopted the system yet. Think of this like DefinitelyTyped for TypeScript. We prefer packages ship their own config plugin, but if they haven't adopted the system yet, the community can use the packages listed in the repo.