I have installed nodejs using:
apt-get install nodejs
Then i have installed npm using:
apt-get install npm
<
While the accepted answer fixes the problem, the correct way to do that, at least with Debian Jessie and forward and Ubuntu 14.4 and forward1 is to install nodejs-legacy:
apt-get install nodejs-legacy
The reason is that Debian already had a package (node) providing /usr/bin/node, and the nodejs node binary had to be installed into /usr/bin/nodejs.
The nodejs-legacy package provides a symbolic link from /usr/bin/nodejs to /usr/bin/node (and conflicts with the node package).
Source: [CTTE #614907] Resolution of node/nodejs conflict and Debian bug #614907: node: name conflicts with node.js interpreter