Javascript engine

[WarMad]

is there a free engine out there that mainly runs on JS? i am asking because im working on my own engine and i would like to learn how to apply some more js to it. i was talking to someone who said he has a script thats more JS then php. he does not have a free version of it and i cant afford to buy it just for learning purposes.

[Dayo]

By JS do you mean Node.js for running the back end? or just JS to improve the frontend?

[WarMad]

To improve front end 

[WarMad]

I stand corrected I was told BOTH