test.nix

# nix-build test.nix --arg config '{ my.option = "foobar!"; }' && cat result
{ pkgs ? import <nixpkgs> {}
, config ? {}
}:

let
  inherit (pkgs) lib;
  eval = config: lib.evalModules {
    modules = [
      {
        options = with lib; {
          my = {
            option = mkOption {
              type = types.str;
              default = "hi";
            };
          };
        };
      }
      config
    ];
  };
in
  pkgs.writeText "config.json" (builtins.toJSON (eval config).config)