Really impressive piece of software! It also has excellent documentation [1].
"Photopea runs completely in your device, just like Sketch or Photoshop do. It does not upload any of your files to the internet. You can load Photopea.com, disconnect from the internet and keep using it completely offline. None of your files ever leaves your computer."
Also, IIRC, it is fully built on vanilla JS last I read about it.
If someone doubts the potential of JS, VS code and Photopea always tops my mind as examples.