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

If you have to install an older version of a package, just specify it

npm install <package>@<version>

For example: npm install express@3.0.0

You can also add the --save flag to that command to add it to your package.json dependencies, or --save --save-exact flags if you want that exact version specified in your package.json dependencies.

The install command is documented here: https://docs.npmjs.com/cli/install

If you're not sure what versions of a package are available, you can use:

npm view <package> versions

And npm view can be used for viewing other things about a package too. https://docs.npmjs.com/cli/view