If you have a shitty internet, it may take a little longer.
First, access Node.js website and click the big green "Download" button.
Download finished? Now run the installation. While the installation goes by, grab yourself a coffee or drink some water, then get back at your chair. By now you should have Node.js and NPM (Node's package manager) installed.
Now, lets install n
to manage (and update) Node versions in seconds on your machine. How to install it? Simply run: