JSI
Also called JavaScript Interface
- Part of
- React Native Glossary
- Also known as
- JavaScript Interface
- Updated
What is JSI in React Native?
JSI (JavaScript Interface) is a lightweight C++ API that lets JavaScript hold direct references to native objects and call them synchronously, replacing the bridge.
JSI — the JavaScript Interface — is a small C++ API that lets the JavaScript engine and native code talk to each other directly. It is the foundation the whole New Architecture is built on.
Under the legacy bridge, every call was converted to a JSON string, queued, and delivered asynchronously. With JSI, native code can expose a HostObject that JavaScript holds a real reference to and invokes like an ordinary object — no serialisation, no queue, and a synchronous return value when one is wanted.
Three things this unlocks:
- TurboModules — native modules loaded lazily and called directly.
- Fabric — a renderer that can read layout synchronously.
- Engine independence — JSI is an abstraction over the engine, so React Native can run on Hermes or JavaScriptCore behind the same interface.
Most app developers never write JSI by hand; they benefit from libraries built on it. See the New Architecture docs.