NPM Package Location

Global libraries

You can run npm list -g to see where global libraries are installed. On Unix systems they are normally placed in /usr/local/lib/node or /usr/local/lib/node_moduleswhen installed globally. If you set the NODE_PATH environment variable to this path, the modules can be found by node.

Windows XP:

%USERPROFILE%\Application Data\npm\node_modules

Windows 7

%AppData%\npm\node_modules

Non-global libraries

Non-global libraries are installed the node_modules sub folder in the folder you are currently in. You can run npm list to see the installed non-global libraries for your current location.

Reference

Answer copied from this Stackoverflow answer