Software Installation – How to Install Latest NodeJS and NPM Versions

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

Node.js package is available in two releases: LTS release and the current release. You can install any version you want. I am installing LTS version for this answer.

1. Add Node.js PPA

You will need curl to download the script, so first, install curl by typing sudo apt install curl in the terminal.

Once, curl is installed successfully, run the command to download and install the installer.

curl -sL https://deb.nodesource.com/setup_12.x | sudo -E bash -

2. Install nodejs

Now, run the below command install node on and Ubuntu using apt. This commmand will also install npm with nodejs. It will also install dependencies.

sudo apt install nodejs

Note: To check the version of node run the command

node -v 

To check the version of npm run the command

npm -v