Create a flake compiling and outputing node package with Serokell/nix-npm-buildpackage

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Hi,
I’m trying to create a flake that output a node package binary, i’m a little lost with nix-npm-buildpackage as overlay.

The node file to git clone (with recursion because there is module) and package is here : GitHub - graphlab-fr/cosma-cli: Cosma is a document graph visualization tool. It modelizes interlinked Markdown files and renders them as an interactive network in a web interface. This is the command-line version of Cosma.

I use nix-npm-buildpackage to try to do this job : GitHub - serokell/nix-npm-buildpackage: Build nix packages that use npm/yarn

I will use the output of this binary flakes as input for another flakes.

My actual try is bottom, i don’t understand how i could pass the result of cosmabinary variable to defaultPackage

{
  description = "packaging cosma cli";

  inputs = {
    flake-utils.url = "github:numtide/flake-utils";
    nixpkgs.url = "github:nixos/nixpkgs";
    nix-npm-buildpackage.url = "github:serokell/nix-npm-buildpackage";
    };

  outputs = { self, nixpkgs, flake-utils, nix-npm-buildpackage }:
    flake-utils.lib.eachsystem [ "x86_64-linux" ] (system:
      let
        overlays = [nix-npm-buildpackage.overlay
                    (final: prev: {
                      cosmabinary = prev.buildnpmpackage {
                        src = final.fetchfromgithub
                        {
                          name = "cosma";
                          owner = "cosmateam";
                          repo = "graphlab-fr/cosma-cli";
                          rev = "5fdf7f99fec50218070a2f871eb9da9160ef0f2c";
                          sha256 = "";
                        };

                       postinstall = ''
                       npm pack
                       cp graphlab-fr-cosma-1.1.0.tgz $out
                       '';
                      };
                    })
                   ];
    pkgsfor = import nixpkgs { inherit system overlays; };
    flake = pkgsfor.cosmabinary;
    in flake // {
   inherit overlays;
   defaultpackage = flake.packages;
    });

}