Your translation is incomplete. You're ending with: > ({ pkgs }) => ({ deps = [ pkgs.cowsay ]; }) But the syntax for declaring objects in javascript differs

Your translation is incomplete. You're ending with: > ({ pkgs }) => ({ deps = [ pkgs.cowsay ]; }) But the syntax for declaring objects in javascript differs. It would be: ({ pkgs }) => ({ deps: pkgs.cowsay }) The fact that '{ x = 1; }' is declaring an object, identical to '{x: 1}' in javascript, is important. It's also worth pointing out that, while '{ pkgs }:' is idiomatic nix, idiomatically in javascript you'd probably just take multiple arguments (so '(pkgs) =>', not '({ pkgs }) =>' in js). The reason nix uses an object there is because a nix function only takes one argument, so there's a fairly common convention to make that argument an attr in order to simulate multiple arguments. That convention doesn't exist in quite the same way in js.