Sorry if this question has already been asked ( I could not find however).
I installed npm dependencies by running on moodle dir:
npm install
I also installed grunt cli as global and npx as well. Commands are working. Everything is fine except when I run 'npx grunt' adm/src/my.js files are being transpiled in same (src) directory. I think it is supposed to that grunt should output .js files inside 'build' directory.