As of today, there is an alternative to JXcore: Node.js for Mobile Apps, and it's actively maintained.
At its core, the project provides a native library for embedding Node.js into native Android and iOS applications; but it also comes with plugins for React Native and Cordova.
Pre-built binaries for the library are available for Android armeabi-v7a, x86, arm64-v8a, x86_64, and for iOS 64-bit.
The core library is a fork of nodejs/node-chakracore, which in turn is fork of nodejs/node. The Android version is pretty much regular Node.js built as a library, with a few portability fixes. The iOS version uses the ChakraCore engine instead of V8 (replacing V8 with ChakraCore is possible thanks to the changes in the nodejs/node-chakracore fork).
The React Native and Cordova plugins make it easier to add Node.js to applications built using those frameworks. The Node.js code runs in a separate engine and thread than the framework's (React Native / Cordova). Communication between the two JavaScript worlds is achieved via a messaging bridge provided by the plugins.
More information, including some documentation, is available on the project website.
(Full disclosure: I work for the company that develops Node.js for Mobile Apps.)