As of npm version 3.8.6, you can use

npm install --prefix ./install/here <package>

to install in the specified directory. NPM automatically creates node_modules folder even when a node_modules directory already exists in the higher up hierarchy. You can also have a package.json in the current directory and then install it in the specified directory using --prefix option:

npm install --prefix ./install/here

As of npm 6.0.0, you can use

npm install --prefix ./install/here ./

to install the package.json in current directory to "./install/here" directory. There is one thing that I have noticed on Mac that it creates a symlink to parent folder inside the node_modules directory. But, it still works.

NOTE: NPM honours the path that you've specified through the --prefix option. It resolves as per npm documentation on folders, only when npm install is used without the --prefix option.

Generally speaking, loading arbitrary data from a URL into a root shell session is not a good idea and I wish people would stop peddling it as a solution for everything - "Please just run this script I'm sending you, and also while we're at it - I have a bridge you'd probably be interested in purchasing".

As an alternative, here's the "Ubuntu Way" of doing the same, where you can see how the system is being updated and know what repositories and what keys are added to your system configuration:

curl https://deb.nodesource.com/gpgkey/nodesource.gpg.key | sudo apt-key add -
sudo apt-add-repository "deb https://deb.nodesource.com/node_7.x $(lsb_release -sc) main"
sudo apt-get update
sudo apt-get install nodejs

And here's the "post deprecation of apt-key way" of doing the same thing:

curl https://deb.nodesource.com/gpgkey/nodesource.gpg.key | gpg --dearmor | sudo tee /etc/apt/keyrings/nodesource.gpg
echo "deb [signed-by=/etc/apt/keyrings/nodesource.gpg] https://deb.nodesource.com/node_7.x $(lsb_release -sc) main" | sudo tee /etc/apt/sources.list.d/nodejs.list
sudo apt update
sudo apt install nodejs

This is for the latest (at time of writing) Nodejs version 7. Other versions can also be gotten with a simple change to the repo URL - consult nodesource.com documentation for details.