How to install npm packages in NixOS?

In NixOS, I installed the package yarn as usual by running $ nix-env -i yarn. Now I am attempting to run yarn via $ yarn start. But this leads me to the following error. $ yarn start yarn start v0.20.3 $ webpack-dev-server --env dev sh: webpack-dev-server: command not found error Command failed with exit code 127. When I try to install webpack-dev-server in my usual NixOS way I get a 'matches no derivations' error. $ nix-env -i webpack-dev-server error: selector ‘webpack-dev-server’ matches no derivations I read that webpack-dev-server is an npm package, and am unsure of a couple questions regarding the relevance of that in this case. 1. Does it make sense to use npm, a different package manager than nix, under Nix? 2. If answer to (1) is yes, then how to install npm on NixOS? I do not see npm available when searching via nix-env, as $ nix-env -qa npm also matches no derivations. **What is the correct way to install webpack-dev-server on NixOS?** ------------------------------------ EDIT I attempted to install webpack-dev-server following the commented link and was able to install node2nix, but am not able to follow through on step 2 listed in the readme there. I located the file referenced in step 2 in /nix/store at /nix/store/sgk7sxgqxrv2axkxjwc3y15apcqbrv1z-nixos-17.03.1482.1b57bf274a/nixos/pkgs/development/node-packages/node-packages.json I can open that file to view the npm packages listed, but the permissions are read-only, even running with sudo -- so I would need to edit it's permissions in order to alter it. It seems that I should not be editing this /nix/store file directly and should instead be manipulating it indirectly via nix. **Am I correct that I should not be editing this file directly?** If so, **how else can I complete step 2 by using nix or something to add webpack-dev-server to it?**