The fastest way to build and run universal React Native apps for Android, iOS, and the web

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The @expo/cli package is a CLI binary that should be used via the expo package, like npx expo start (or npx expo for short).

npx expo

> ⭐️ Be sure to star the Expo GitHub repo if you enjoy using the project!

Design

This CLI has the following purposes:

• Be a minimal interface for starting a local development server that emulates a production EAS Updates server. The development server is the proxy between a native runtime (Expo Go, Dev Client) and a JS Bundler (Metro, Webpack).
  • To accomplish secure manifest signing (think https/TSL/SSL for web (required for sandboxing AsyncStorage, Permissions, etc.)) we need an authenticated Expo user account. This is the only reason we include the authentication commands login, logout, whoami, register. Standard web CLIs don't have authentication commands because they either don't set up https or they use emulation via packages like devcert.
• Orchestrating various native tools like Xcode, Simulator.app, Android Studio, ADB, etc. to make native builds as painless as possible. run:ios, run:android commands.
• Implementing a versioned prebuild command that can reliably work with a project for long periods of time. Prebuild is like a bundler for native code, it generates the ios, android folders based on the project Expo config (app.json).
  • npx expo config is auxiliary to npx expo prebuild and used for debugging/introspection.
• Installing versioned libraries with npx expo install this is a minimal utility born out of pure necessity since versioning in React Native is hard to get right.

Contributing

To develop the CLI run (defaults to watch mode):

yarn build

We highly recommend setting up an alias for the Expo CLI so you can try it in projects all around your computer. Open your .zshrc or other config file and add:

alias nexpo="/path/to/expo/packages/@expo/cli/build/bin/cli"

Then use it with nexpo like nexpo config. You can also set up a debug version:

alias expo-inspect="node --inspect /path/to/expo/packages/@expo/cli/build/bin/cli"

Then you can run it and visit chrome://inspect/#devices in Chrome, and press "Open dedicated DevTools for Node" to get a debugger attached to your process. When debugging the CLI, you'll want to disable workers whenever possible, this will make all code run on the same thread, this is mostly applicable to the start command, i.e. expo-inspect start --max-workers 0.

Format

• Be sure to update the CHANGELOG.md with changes for every PR. You only need to add the message, our GitHub bot will automatically suggest adding your name and PR number to the diff.
• End async functions with Async like runAsync. This is just how we format functions at Expo.
• When throwing errors, always opt for CommandError instead of Error -- this helps with debugging and making the experience feel more coherent.
• Utilize the unified Log module instead of console.log.
• When logging with variables, utilize the following format Something happened (foo: bar, baz: foz).
  • Avoid other formats like Something happened: bar, foz or Something happened: foo=bar, baz=foz.
• Main UI components like command names (expo start), arguments (--port), and --help messages should be modified internally, by the Expo team to ensure the developer experience is unified across Expo tooling. External contributions modifying these core aspects may be rejected.
• Use the profile utility method with the EXPO_PROFILE=1 environment variable to measure execution time.
• Avoid globals and singletons as these make testing harder and less predictable. The only global we have (at the time of writing this) is the isOffline boolean.

Environment

• Always be cautious of the transitive size of dependencies. packagephobia is a great resource for determining if a package is lean. Try to minimize adding dependencies to the CLI.
• We build the CLI using taskr + swc, this is partially inspired by Next.js' local CLI.
• The build pipeline will inline the CLI version as an environment variable that is accessible anywhere in the CLI codebase. You can access it via process.env.__EXPO_VERSION instead of reading the local package.json at runtime.
• Unlike the legacy global Expo CLI, this CLI is shipped with expo meaning the SDK Version is always present.
  • Reduce SDK specific tasks since only one SDK should be accounted for in a single version of @expo/cli.
  • The @expo/config method getConfig does not need the skipSDKVersionRequirement in any case since expo should always be installed. Ex: getConfig('...', { skipSDKVersionRequirement: true }); shouldn't be used.
• Also unlike the global Expo CLI we can assume that node modules are always installed since this CLI should be used via a project's local node_modules folder.
  • This means we can't perform operations that upgrade the expo package as these may kill the running process. Features that need this pattern (like expo upgrade) should live in standalone global tools.

Testing

There are two testing scripts:

• yarn test: Controlled unit and integration tests.
• yarn test:e2e: End to end testing for CLI commands. This requires the files to be built with yarn build

• You can target a specific set of tests with the --watch flag. Example: yarn test --watch config.
• We use backticks for it blocks. Example it(works).
• If a pull request is fully self-contained to the packages/@expo/cli/ folder (i.e. no yarn.lock modifications, etc.) then most native CI tests will be skipped, making CI pass faster in PRs.

Unit Testing Guidelines

• Use nock for network requests.
• No top level describe blocks that wrap all the tests in a file.
• When testing a function, pass the function to the describe block instead of a stringified function name:
  • describe(foobar, () => {}) instead of describe('foobar', () => {})
• Use virtual fs via memfs whenever possible.
• We have a lot of global module mocks already in place, consider them when writing tests.
• GitHub Copilot can make writing tests a little less tedious.

E2E Testing Guidelines

• E2E tests should be resilient and reliable, be sure to give them plenty of time for network requests.
• When testing locally you should attempt to reuse node modules for faster results. In the npx expo prebuild and npx expo start commands for instance, we utilize a helper method that will default to reusing a project + node_modules when run locally. This can be toggled off to bootstrap a fresh project every time.
• When bootstrapping test projects, utilize the temporary folder os.tmpdir() as this folder is automatically cleaned up when the computer restarts.

Coming from Expo CLI

> TL;DR: expo-cli was 'make it work', whereas @expo/cli is 'make it right, make it fast'.

The legacy global expo-cli package was deprecated in favor of this versioned @expo/cli package for the following reasons:

• expo-cli was too big and took way too long to install. This made CI frustrating to set up since you needed to also target global node modules for caching.
• expo-cli worked for almost all versions of the expo package, meaning it was getting more complex with every release.
• expo-cli combined service commands (like the legacy build, submit, publish) with project-level commands like expo start. We've since divided services into eas-cli and project commands into npx expo (@expo/cli). This structure is more optimal/faster for developers since they can install/update commands when they need them.
• This CLI utilizes more Node.js standard features like $EDITOR instead of the custom $EXPO_EDITOR environment variable. Also transitioning away from $EXPO_DEBUG and more towards $DEBUG=expo:*. These types of changes make Expo CLI play nicer with existing tooling.
• The DevTools UI has been deprecated to reduce the net install size, minimize complexity, and make room for future debugging UIs (Hermes/v8 Chrome debugger).
• The expo start:web and expo web commands have been rolled into npx expo start as we now lazily load platforms until the device requests them.
• Other missing or beta features from expo-cli may still be getting migrated over to this new CLI. For a more comprehensive breakdown see the start command PR.