Regarding NPM, you can do user-level "global" installs on NixOS, see toraritte's answer in the same Stackexchange question. NPM should work fine within a project, but if you need specific dynamic libraries that a Node package implicitly depends on, you can specify them using Nix, and this means you automate the process of installing system dependencies for anyone else who touches the project too.

While you could use nix-shell, it is IMO a dated and very manual way, preferably you'd use direnv or lorri to automatically load a default.nix or shell.nix from your project root directory where libraries have been specified in `buildInputs`.