Yup. The read.md on the first page shows you how.
It is Client Server and the server is written in java. Basically the server interfaces with simconnect and the web page interfaces with the server.
From the readme.md :
Quick start and examples
Start FSX and load a flight
Start FSX-saas -
[java -jar path/to/fsx-saas-x.y.z.jar]
Open your browser and point it to
http://localhost:8080/ to see all the available API endpoints
See all the aircrafts:
http://localhost:8080/aircrafts
etc.
Works for me. Should give you a start of how to interface with FSX and javascript.
Also I did a search in github for "simconnect" and it found one link to javascript. So check it out as well:
https://github.com/gabrielgouv/watt