Node.js is a JavaScript runtime you use to develop modern Office Add-ins.
Install Node.js by downloading the latest recommended version from their website. Follow the installation instructions for your operating system.
npm is an open source software registry from which to download the packages used in developing Office Add-ins. It's usually installed automatically when you install Node.js. To check if you already have npm installed and see the installed version, run the following in the command line.
npm -v
If, for any reason, you want to install it manually, run the following in the command line.
npm install npm -g
Tip
You may wish to use a Node version manager to allow you to switch between multiple versions of Node.js and npm, but this isn't strictly necessary. For details on how to do this, see npm's instructions.