Comment by ndriscoll

Why not? Make a bash alias for `npm` that runs it with `bwrap` to isolate it to the current directory, and you don't have to think about it again. Distributions could have a package that does this by default. With nix, you don't even need npm in your default profile, and can create a sandboxed nix-shell on the fly so that's the only way for the command to even be available.

Most of your programs are trusted, don't need isolation by default, and are more useful when they have access to your home data. npm is different. It doesn't need your documents, and it runs untrusted code. So add the 1 line you need to your profile to sandbox it.