It's Sokoban. It's written in ClojureScript. So it has a J in it.
Includes Yoshio Murase's automatically generated levels, © 1996-2000 Yoshio Murase. Also includes Microban by David Skinner, who writes that his sets "may be freely distributed provided they remain properly credited."
The other art is based on tiles from Platformer Art Complete Pack by Kenney, and released under the Creative Commons CC0 Public Domain Dedication 1.0. I had to modify the tiles to suit, and anything that looks crappy is solely my fault.