My eaglercraft won’t work

Im playing on a Chromebook's and got eaglercraft through GitHub im on 1.8 the version where you can do single player as well there's a built in mod thing so I took a .jar file and renamed it .js thinking it would load with didnt do anything to the code of the mod at all though and it brought up an error box that I can't get rid of help please (once I find the files and screenshots I'll put them here)