JavaScript Bundle

Also called JS bundle, Bundle

Part of
React Native Glossary
Also known as
JS bundle, Bundle
Related
MetroHermesJavaScript RuntimeOver-the-Air Update
Mentioned in
Fast RefreshJavaScript RuntimeMetroOver-the-Air Update
Updated

What is JavaScript Bundle in React Native?

The JavaScript bundle is the single file Metro produces from your source code. It ships inside the app binary and is what the JavaScript engine actually executes.

The JavaScript bundle is the compiled output of your entire application's JavaScript — your code plus every npm dependency — collapsed by Metro into one file.

Where it lives depends on the build:

  • Development — served over HTTP by the Metro dev server and fetched on launch, which is why the app needs Metro running.
  • Release — written into the binary as main.jsbundle (iOS) or index.android.bundle (Android), and precompiled to Hermes bytecode.

Two consequences worth internalising as a newcomer.

Bundle size affects startup, not download size, in the way you might expect — the bundle is a small fraction of an app binary, but everything in it is parsed or loaded at launch, so an unused 2MB dependency still costs you.

And the JS/native split is exactly this file. Changing bundle contents can ship as an over-the-air update; changing anything outside it requires a new store build.