In fact it is probably all Javascript. The formula editor at the top is an Ace editor, the spreadsheet itself is based on https://github.com/openfin/fin-hypergrid, which is an incredibly enourmous piece of software I had never heard of and which I don't understand at all what it is trying to accomplish, but I can see it renders to canvas.
Yes, the computations are all done server-side, which lets us parallelize big computations. It also keeps our thorny evaluation logic crisp and maintainable, because Haskell. =)