Let's create an empty directory for our project named test, navigate to that directory, and install the underscore library there, using npm. Run the following commands:

mkdir test
cd test
npm install underscore

The last command will tell npm to run the install subcommand with the argument underscore, which in turn will install the package underscore locally. npm will output some progress information as it downloads and installs the package.

When installing a package locally, npm creates a subdirectory in the current directory named node_modules. Inside that directory, it creates another directory for the installed package. In this case, the underscore package will be placed inside the underscore directory.

Some npm packages are designed to be installed globally. Global packages add new functionality to the operating system. For example, the coffee-script package can be installed globally, which will cause the command coffee to become available on our system.

To install global packages we use the -g switch. Have a look at the following example:

npm install -g coffee-script

On some systems it's necessary to request the administrative privilege to run this program. You can do that by using the sudo command:

sudo npm install -g coffee-script

npm will download and install coffee-script along with all its dependencies. After the process is complete, we can start using the command coffee, which is now available on our system. We can now run coffee-script code. Lets say we want to run a simple hello-world script written in-line; we can use the -e switch for that. Have a look at the following example:

coffee -e "echo 'Hello world'"

To learn more in the global package about npm subcommands, we can use npm's help subcommand. For example, to learn more about the install subcommand, run the following command:

npm help install

More information about the latest version of npm can be found on the official npm documentation at https://npmjs.org/doc/.