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Learn why Metro is the future of universal bundling in React Native and how it benefits developers.
Metro is the official bundler for Expo and React Native. It's a central build tool in the Expo framework. Bundlers comprise thousands of opinions and trade-offs. This document outlines the key reasons why Expo is developed around Metro and how it benefits developers.
Metro is maintained by Meta, the maintainers of React, React Native, Yoga, and Hermes. It's used for developing some of the world's largest apps, ranging across all categories in the app stores.
Meta engineers actively develop Metro with the express requirement of bundling all their apps, across 400k+ source files, while remaining fast and reliable.
By having first-class Metro support, we ensure Expo developers have continuity across Meta's tools and get instant access to emerging features. This includes:
New and upcoming features that are planned to come to Metro include:
The Expo team collaborates with Meta to develop Metro for Expo Router, adding features like file-based routing, web support, bundle splitting, tree shaking, CSS, DOM components, server components, and API routes.
Nearly every React Native app in the world uses Metro, making it a battle-tested solution optimized for large-scale projects. This makes it suitable for developers of all sizes, from hobbyists to large companies. Metro is designed specifically to handle large-scale Meta apps, which is why it has features such as native file watching with Watchman and shared remote caches.
In development, Metro doesn't perform any platform-specific work until requested. This allows developers to work on large projects without paying a performance cost for the number of platforms they support. In conjunction with aggressive caching and async routes, developers can incrementally bundle only the parts of the app that they're actively working on.
Unlike traditional bundlers, which create multiple instances to bundle server and client code, Metro maximizes resource reuse across platforms and environments (server, client, DOM components). This architecture is ideal for multiplatform and server development.
Metro is incremental and can create cached transform artifacts that can be used across machines. This enables large teams to reuse work from remote builders, a technique used at Meta for all large projects.
While other bundlers are designed around the static specification of web browsers, Metro is optimized for the flexibility of React Native. This enables features like generating the specific set of supported language features required for Hermes bytecode compilation which enables faster app startup in production. This will also extend to Static Hermes, which will compile static type information into machine code for native apps.
Expo leverages Metro's technology to create novel functionality like DOM components. This allows a React component in a native app to be dynamically bundled as an entire website with all the same defaults as the parent app, on-demand.
Unlike traditional bundlers, where the end result is a fully hosted app, Metro's configuration options support exporting bundles to embed as native artifacts in standalone app binaries. This leverages OS-specific optimizations such as xcassets on Apple platforms.
All AST transformation in Metro is performed concurrently across all available threads, maximizing the use of hardware.
While Metro is designed for universal app development, it's often compared to other web-only bundlers. Here are some key differences:
While bundlers like Vite leverage built-in ESM support in the browser, this approach can lead to slower practical development times at medium to large scales due to thousands of cascading network requests. Metro performs bundling in local development, which aligns the development results much closer to the production results and is better suited for React Native's larger module count.
Several bundlers are opting to write their core in Rust for performance reasons, but this comes with some trade-offs, such as more challenging contributions, patches, and development. Metro uses a mix of technologies based on the operation:
This approach aligns with Meta and community tools while allowing easier debugging, profiling, and patching for developers.