Hermes + Node API: A Match Made in Heaven

Node-API is a C-based interface for native modules that works across JavaScript engines. Its stable ABI lets modules compiled against an earlier Node-API version keep working in newer runtimes without recompilation. The API covers more than 100 functions, and its current version is 10.

Vladimir describes how Node-API functions work with an environment, JavaScript values, and status codes, with functions for errors and exceptions. Values live on a stack that is cleared after a native call, so modules use explicitly deleted global references to preserve them across calls. Modules can be written in C or languages that interoperate with C, including C++, Rust, and Go.

React Native for Windows adopted Node-API to support multiple JavaScript engines and native languages such as C and C++. The team began a JSI runtime implementation in 2021, added an implementation to its Hermes fork in 2022, and later began upstreaming it to Hermes. They ported Node.js tests and run them using a Node-like process with Hermes.

The Windows implementation currently wraps Node-API through JSI, with a goal of supporting both interfaces side by side. The team maps JSI versions to React Native versions because releases expose different JSI functions. This lets one Hermes DLL and JSI wrapper serve multiple React Native versions, including older apps.

The demos show a Node-API module running in Node.js and Hermes, with the same result, and demonstrate code in C and C++. Hosany says precompiled modules can work across React Native versions, and developers can use languages that bind to C. Reusing Node.js modules in React Native still needs more work, while using the latest Hermes is already supported in React Native for Windows.