Metro

Also called Metro bundler

Part of: React Native Glossary
Also known as: Metro bundler
Related: JavaScript Bundle Fast Refresh JavaScript Runtime Development Build
Mentioned in: Dev Menu Development Build Fast Refresh JavaScript Bundle JavaScript Runtime Platform-Specific Code
Updated: September 12, 2026

What is Metro in React Native?

Metro is React Native’s JavaScript bundler. It resolves imports, transforms code, serves the bundle to the app in development and powers Fast Refresh.

Metro is the bundler React Native uses, filling the role Webpack or Vite plays on the web. It takes your entry file, follows every import, transforms the code, and produces a single JavaScript bundle the app can execute.

It runs in two modes:

In development it is a server, usually on port 8081. The app fetches the bundle from it on launch and keeps a connection open for Fast Refresh.
At build time it emits the static bundle that gets embedded in the .ipa or .aab you ship.

Metro is mobile-specific in ways that surprise web developers: it resolves platform extensions such as Button.ios.tsx automatically (see platform-specific code), and it handles asset registration for images.

Most React Native error messages a newcomer sees during setup come from Metro — "Unable to resolve module", or a stale cache that npx expo start --clear fixes. See the Metro docs.