I have a flake.nix with this derivation in the outputs:

packages.svg = pkgs.stdenv.mkDerivation rec {
  name = "generate-svg";
  src = self;
  buildInputs = [ pkgs.nodePackages.nodejs ];
  buildPhase = ''
    export NODE_PATH=${src}/node_modules:${src}
    node ${src}/generate.js
  '';

  installPhase = ''
    mkdir -p $out
    cp output.svg $out/
  '';
};

You can see the whole file (and others) here: https://github.com/MatrixManAtYrService/buildsvg

My goal is that the following command should run generate.js which will use d3.js to build a svg file defined by the contents of generate.js.

$ nix build 'github:MatrixManAtYrService/buildsvg#svg'
$ ls result
    output.svg

Instead, the nix build command fails with this error:

Error: Cannot find package '/nix/store/a76p6qgrxlx0safvxrqx5aknhszvg703-source/node_modules/opentype.js/package.json' imported from /nix/store/a76p6qgrxlx0safvxrqx5aknhszvg703-source/generate.js
    at legacyMainResolve (node:internal/modules/esm/resolve:215:26)
    at packageResolve (node:internal/modules/esm/resolve:841:14)
    at moduleResolve (node:internal/modules/esm/resolve:927:18)
    at defaultResolve (node:internal/modules/esm/resolve:1157:11)
    at ModuleLoader.defaultResolve (node:internal/modules/esm/loader:390:12)
    at ModuleLoader.resolve (node:internal/modules/esm/loader:359:25)
    at ModuleLoader.getModuleJob (node:internal/modules/esm/loader:234:38)
    at ModuleWrap.<anonymous> (node:internal/modules/esm/module_job:87:39)
    at link (node:internal/modules/esm/module_job:86:36) {
  code: 'ERR_MODULE_NOT_FOUND'
}
Node.js v20.12.2

The top of generate.js looks like this:

import fs from 'fs';
import * as d3 from 'd3';
import opentype from 'opentype.js';  // <--- failure is here
import D3Node from 'd3-node';

So I know that some of my modules are being imported correctly.
Also, If I use a nix devshell, it works fine:

devShells.default = pkgs.mkShell {
  buildInputs = [
    pkgs.nodePackages.nodejs
  ];
  shellHook = ''
    export NODE_PATH=$PWD/node_modules
  '';
};

$ nix develop
$ node generate.js
SVG generated successfully.

My package.json indicates these dependencies, and I’ve run npm install which caused them to be downloaded into node_modules (pardon if this is obvious, I’m a nodejs newbie).

I have sprinkled ls statements into the buildPhase and verified that /nix/store/a76p6qgrxlx0safvxrqx5aknhszvg703-source/node_modules/opentype.js/package.json does indeed exist, despite what the error says.

Why is my nix build failing, while the same build works in a devshell?