To install NodeJs and NPM we gonna use Homebrew. If you already haven't installed Homebrew please check this post
What is NPM?
Going straight to the point
npm is a command-line tool that interacts with a repository, that repository that enables the installation of packages. So, a package manager :D
To install is easy as opening the terminal and run the follow command:
brew install node
To verify if
npm is already installed: