Hmm interesting it doesn't trigger a rebuild of all nodePackages. Thanks for your approval @marsam.

I have declared nodePackages = pkgs.nodePackages_13_x in my overlays.
There is no npm in nodePackages_13_x.
This is inconvenient, especially if i don't want to have npm in my nodePackages at all.

tutnix ~ 
% nixos-rebuild test
building Nix...
building the system configuration...
error: attribute 'npm' missing, at /home/alab/ws/nixpkgs/nixos/modules/programs/npm.nix:19:19
(use '--show-trace' to show detailed location information)
tutnix ~ 
% nix-build \<nixpkgs\> --no-out-link --no-build-output -A nodePackages.npm
error: attribute 'npm' in selection path 'nodePackages.npm' not found
tutnix ~ 
% nix-build \<nixpkgs\> --no-out-link --no-build-output -A nodePackages_10_x.npm
/nix/store/6mkiyvgphqahlzvj6w4f95qpal5jp1gd-node_npm-6.14.4
tutnix ~ 
% nix-build \<nixpkgs\> --no-out-link --no-build-output -A nodePackages_12_x.npm
error: attribute 'npm' in selection path 'nodePackages_12_x.npm' not found
tutnix ~ 
% nix-build \<nixpkgs\> --no-out-link --no-build-output -A nodePackages_13_x.npm
error: attribute 'npm' in selection path 'nodePackages_13_x.npm' not found

@doronbehar maybe you could change this to

diff --git a/nixos/modules/programs/npm.nix b/nixos/modules/programs/npm.nix
index f101a44587a..52810f526cd 100644
--- a/nixos/modules/programs/npm.nix
+++ b/nixos/modules/programs/npm.nix
@@ -14,9 +14,9 @@ in
       enable = mkEnableOption "<command>npm</command> global config";
 
       package = mkOption {
-        type = types.path;
+        type = types.path or null;
         description = "The npm package version / flavor to use";
-        default = pkgs.nodePackages.npm;
+        default = if hasAttr "npm" pkgs.nodePackages then pkgs.nodePackages.npm else null;
         example = literalExample "pkgs.nodePackages_13_x.npm";
       };

or add npm to both

pkgs/development/node-packages/node-packages-v12.json
pkgs/development/node-packages/node-packages-v13.json

and update with pkgs/development/node-packages/generate.sh
And while you are at it you could also add the missing standard gyp tools

  "node-gyp",
  "node-gyp-build",
  "node-pre-gyp",

Can't you use:

    programs.npm.package = pkgs.nodejs_latest;

?

This is inconvenient, especially if i don't want to have npm in my nodePackages at all.

I'm not sure I understand what kind of ability does the current module is missing. If you feel there are missing attributes to some of the nodePackages* attribute sets, then please fix it in a different PR. This one is merged & done. I'm afraid I don't have the necessary time to do apply your suggestions my self or review them once made. Then please don't try to request my review 🙏 . Thanks.

I could :) The point is i do not use programs.npm so why should i need to set this?