Mmm, this entry is hard to judge, since I'm actually a web developer and right now it's 8:45 PM and my brain is yelling "NOOO!! No more JS! Pleaase!". :P
Also I refused some valid MR just because they were making the code confusing without improving it, but I guess this wasn't the right goal.^^'
Nevertheless, this is a cool and unexpected interpretation of the theme! It's interesting how the functions are quite simple, but the modifications tricky enough to give some challenge. In a way it's a great level-design, and I especially levels 2 and 3. I'm curious to see how this game would be appreciated by non-programmers (and people who don't experience MR as work^^).
Maybe the game could benefit from a more user friendly UI? While it's fun to have the GUI syntax, most developpers use tools to look at diff in a more efficient ways, and it could make the puzzle more readable. Also, I'm not sure if the timer is a good idea? It adds some unnecessary stress. Maybe the game could reward quick thinking, but stil let payer analyze the code as long as they want.
Anyway, even if I wasn't in the right mood to enjoy it, it's a pretty cool game just for its premise! :D