$ sudo apt install nodejs
$ sudo apt install npm

{
  description = "Node.js + npm dev environment";

  inputs = {
    nixpkgs.url = "github:NixOS/nixpkgs/nixos-unstable";
  };

  outputs = { self, nixpkgs }:
    let
      system = "x86_64-linux"; # or aarch64-darwin, etc.
      pkgs = import nixpkgs { inherit system; };
    in {
      devShells.${system}.default = pkgs.mkShell {
        packages = [
          pkgs.nodejs_20  # includes npm
        ];
      };
    };
}

$ nix develop

# check it
node --version
npm --version