Normal installation would be sudo apt install nodejs to install Node.js and then sudo apt install npm to install Node Package Manager. However, upon doing so, npm -v says 3.5.2. To upgrade normally, I would do sudo npm install -g npm, which updates to the latest version (which, at the time of writing this article, is 6.0.1).
When I do a which npm, I get /usr/local/bin/npm, however apt installs a symlink at /usr/bin/npm. If I sudo apt purge npm to remove npm, it still leaves the npm version of npm at /usr/local/bin/npm, however npm -v says -bash: /usr/bin/npm: No such file or directory.
Many articles say to use a PPA to install nodejs, but I think there should be a native way to do this through apt.
DigitalOcean instructions on installation normally and through PPA: https://www.digitalocean.com/community/tutorials/how-to-install-node-js-on-ubuntu-18-04
TecAdmin instructions on installation through PPA: https://tecadmin.net/install-latest-nodejs-npm-on-ubuntu/