Basically Bangle.js puts its REPL on Bluetooth LE - once you can connect and send/receive characters you're sorted. You just send the JS code you want it to execute.
Espruino is a JavaScript interpreter for low-power Microcontrollers. This site is both a support community for Espruino and a place to share what you are working on.
Web Bluetooth (from the browser) is definitely the easiest way to get started. Something like https://www.espruino.com/Bangle.js+Data+Streaming might give you some useful info there.
But to connect from Node.js this is probably most helpful: https://www.espruino.com/Interfacing#bluetooth-le
Basically Bangle.js puts its REPL on Bluetooth LE - once you can connect and send/receive characters you're sorted. You just send the JS code you want it to execute.