MIDI Mapping Assistant versus JS code

When I use the Mapping Assistant to define a surface control, I can map directly to a parameter within a VST instrument. For example: Arturia Arp 2600, I can click on any of the ADSR controls and map them directly.
However, if I am writing JavaScript, I cannot locate any functions that provide access to map those.
Intuitively, this doesn’t make any sense to me. Obviously, MIDI Remote has enough information available to do the mapping in Assistant. What am I missing?
I tried looking at the json generated by Mapping Assistant, but it was not obvious to me how this is represented, and I cannot locate any documentation that addresses this.