nix.txt

Travis-Whitton's-MacBook-Pro:nix traviswhitton$ cat node.nix
# This imports the nix package collection,
# so we can access the `pkgs` and `stdenv` variables
with import <nixpkgs> {};

# Make a new "derivation" that represents our shell
stdenv.mkDerivation {
  name = "my-environment";

  # The packages in the `buildInputs` list will be added to the PATH in our shell
  buildInputs = [
    # see https://nixos.org/nixos/packages.html to search for more
    pkgs.yarn
    pkgs.nodejs-10_x
  ];

  shellHook = ''
    export DERP="foobingo"
  '';
}
Travis-Whitton's-MacBook-Pro:nix traviswhitton$ nix-shell node.nix

[nix-shell:~/Projects/nix]$ node --version
v10.12.0

[nix-shell:~/Projects/nix]$ yarn --version
1.12.3

[nix-shell:~/Projects/nix]$ exit
Travis-Whitton's-MacBook-Pro:nix traviswhitton$ yarn --version
-bash: yarn: command not found