Hix is still a little experimental, but it already provides an easy way to get a nix shell for building ghcjs-base without the need for a lot of nix code in this repo.

Install hix:

nix-env -iA hix -f https://github.com/input-output-hk/haskell.nix/tarball/hkm/hix

Start a nix shell using hix:

git clone https://github.com/ghcjs/ghcjs-base.git
cd ghcjs-base
hix-shell

In the shell build the library run:

js-unknown-ghcjs-cabal build all