Install Nodejs and NPM Ubuntu 17

Step 1: Run the command to install LTS Release version.

 sudo apt-get install python-software-properties  

curl -sL https://deb.nodesource.com/setup_9.x | sudo -E bash -  

sudo apt-get install -y nodejs

Then verify your npm and nodejs installation.

 node -v   
 npm -v  

Useful link
  Nodejs