use it like $(nix-build npm2nix.nix)/bin/npm2nix -i package.json -d -5 -c npm.nix

npm2nix.nix

with (import <nixpkgs> {});
let
  npm2nixRepo = fetchFromGitHub {
    owner = "svanderburg";
    repo = "npm2nix";
    rev = "758b286acfb077051a17aa9579adc558deeebbd4";
    sha256 = "0gmvpxvp75fqw7w7x0c1ywc1zghd1hl6iimbv4xnjwlxgh704yg6";
  };
  npm2nixOut = (import "${npm2nixRepo}/release.nix" {}).package."${builtins.currentSystem}".outPath;
in stdenv.mkDerivation {
  name = "npm2nix";
  buildInputs = [];
  src = npm2nixRepo;
  phases = "buildPhase";
  buildPhase = ''
    mkdir -p $out/bin
    ln -s ${npm2nixOut}/bin/npm2nix $out/bin/npm2nix
  '';
}

Is this expression used to install npm2nix?

What you can also do for convenience is creating a git clone:

git clone https://github.com/svanderburg/npm2nix.git -b reengineering2

Opening the git folder:

cd npm2nix

And running the following command-line instruction to install it:

nix-env -f default.nix -iA package