Stockfish.js 19.0.0 (lite, single-threaded WebAssembly build)
Copyright (C) the Stockfish developers and Chess.com, LLC. Licensed under the GNU General Public License v3 (see COPYING.txt).
These two files are the unmodified build published in the npm package 'stockfish' 19.0.0.
Complete corresponding source code: https://github.com/nmrugg/stockfish.js (tag v19.0.0) and https://github.com/official-stockfish/Stockfish
The engine runs as a separate program in a web worker; schackonline.se talks to it only through the UCI text protocol.