Global libraries

You can run npm list -g to see which global libraries are installed and where they're located. Use npm list -g | head -1 for truncated output showing just the path. If you want to display only main packages not its sub-packages which installs along with it - you can use - npm list --depth=0 which will show all packages and for getting only globally installed packages, just add -g i.e. npm list -g --depth=0.

On Unix systems they are normally placed in /usr/local/lib/node or /usr/local/lib/node_modules when installed globally. If you set the NODE_PATH environment variable to this path, the modules can be found by node.

Windows XP - %USERPROFILE%\AppData\npm\node_modules
Windows 7, 8 and 10 - %USERPROFILE%\AppData\Roaming\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.

When installing use -g option to install globally

npm install -g pm2 - pm2 will be installed globally. It will then typically be found in /usr/local/lib/node_modules (Use npm root -g to check where.)

npm install pm2 - pm2 will be installed locally. It will then typically be found in the local directory in /node_modules

The command npm root will tell you the effective installation directory of your npm packages.

If your current working directory is a node package or a sub-directory of a node package, npm root will tell you the local installation directory. npm root -g will show the global installation root regardless of current working directory.

Example:

$ npm root -g/usr/local/lib/node_modules

See the documentation.

For globally-installed modules:

The other answers give you platform-specific responses, but a generic one is this:

When you install global module with npm install -g something, npm looks up a config variable prefix to know where to install the module.

You can get that value by running npm config get prefix

To display all the global modules available in that folder use npm ls -g --depth 0 (depth 0 to not display their dependencies).

If you want to change the global modules path, use npm config edit and put prefix = /my/npm/global/modules/prefix in the file or use npm config set prefix /my/npm/global/modules/prefix.

When you use some tools like nodist, they change the platform-default installation path of global npm modules.