JS, particularly in the era discussed here, only came with a small number of standard objects. The Host was expected to provide a series of Host Objects. In a browser these might be things like Window, Document and XMLHttpRequest. Scanning the linked article on the JWST it seems these might be things like Command and Telemetry, that would be written in C++, and then scripted in JS.
Having worked on an embedded script engine that was JS-based at the time, this is exactly what we were doing. Basically the script host (C++ code) offered up specific sets of available objects to the engine that you could interact with and essentially trigger the C++ functionality from, passing in data, and such.
It wasn't a fantastic experience at the time but luckily for us the stakes were pretty low as we were working on interactive mall map kiosk devices.
