NPM is incorrect version on latest Ubuntu (18.04) installation

TLDR: This problem is caused by Bash caching the path of the npm command, and can be solved by hash -d npm. You don't even need to deal with apt purge unless you want to.

Explanation

Here were my steps for getting a new npm version on Ubuntu. First, do the installation as OP describes:

$ sudo apt-get install npm
(...apt installation of npm was successful...)
$ npm -v
3.5.2
$ command -v npm
/usr/bin/npm
$ sudo npm install -g npm
(...npm installation of npm was successful...so far, so good)

You can see that the new version is already working fine in /usr/local/bin/npm, but unfortunately the Bash cache still has /usr/bin/npm:

$ /usr/local/bin/npm -v
6.4.1
$ npm -v
3.5.2
$ command -v npm
/usr/bin/npm
$ type npm
npm is hashed (/usr/bin/npm)

To fix the problem, clear it from the Bash cache (do this in all open shells):

$ hash -d npm

Now the new version works as desired:

$ npm -v
6.4.1
$ command -v npm
/usr/local/bin/npm

The way I found is to purge npm through sudo apt purge npm, then simply recreate a symlink to the global installation via ln -s /usr/local/bin/npm /usr/bin/npm. After that fix, npm -v returns 6.0.1 as expected.

To have control on installed npm version, I always use nvm (node version control). You can install it through the instructions here: https://github.com/creationix/nvm Then by following command install the latest npm on your computer:

nvm install node