JSI (JavaScript Interface) & JSC (JavaScript Core) Discussion

[kelset]

Intro

With this issue I'd like to try and create a "one stop" for all the information available around the JavaScript Interface, the unified lightweight general purpose API for (theoretically) any JavaScript virtual machine.

Currently, the default Javascript Virtual Machine used by React Native is JavaScriptCore (JSC) - it is used in WebKit.

Terminology

• Fabric: just the UI layer re-architecture, to take full advantage of concurrent React architecture. (dedicated issue)
• TurboModules: re-architecture of NativeModules, also using JSI. (dedicated issue)
• CodeGen: a tool to "glue" JS and native side: it will allow for supercharge static types (TS, Flow) for compatibilty with native binaries, among other things. (dedicated issue)

TL;DR

From @axe-fb's blogpost, here's a temporary description of the JSI (please consider that this is not yet finalized, it may change in the future)

Instead of using the bridge for queuing messages, the new architecture allows us to directly "invoke" (think RPC) Java/ObjC methods.
An analogy would be how we call DOM methods from JavaScript in the browser. For example, in the statement var el = document.createElement('div'); the variable el holds a reference not to a JavaScript object, but to an object that was possibly instantiated in C++. When JavaScript calls el.setAttribute('width', 100), we end up synchronously invoking the setWidth method in C++ that changes the actual width of that element.
In React Native, we can similarly use the JavaScript interface to invoke methods on UI Views and Native Modules that are implemented in Java/ObjC.

Available Materials

At ReactConf 2018 @axe-fb did a talk about React Native's New Architecture, which also explains the 3 concepts above: JSI, Fabric, TurboModule.

Recently the JSC was upgraded for Android, you can check the commit here and the related issue in the main repo.

On twitter, @karanjthakkar did a thread detailing how he integrated the new JSC in the Skyscanner app.

Q&A

This is also a place for questions related to this effort and its direction.