NodeJS 0.12 on Ubuntu 15.04

First make sure that you have this repo list: edit (create) /etc/apt/sources.list/nodesource.list:

deb https://deb.nodesource.com/node_0.12 utopic main
deb-src https://deb.nodesource.com/node_0.12 utopic main

then run

apt-get install nodejs

if your npm version is <2.7.6, run

npm install npm@2.7.6 -g

This must be done since earlier versions had issues with private repos.