All the dependencies/modules for a node project are in the ./node_modules directory. If you copy this directory to a different project all the dependencies will stay in tact. However, be advised that the origin environment and destination environment must match quite closely. Changes to the OS, architecture, node version, etc. can break many commonly used modules. NPM will take care of this for you so it is recommended.
If you are building a library and trying to share it outside of NPM, you might consider
npm-pack.
HTH,
Mikkel