When you run npm install -g somepackage, you may get an EACCES error asking you to run the command again as root/Administrator. It’s a permissions issue.
It’s easy to fix, open your terminal (Applications > Utilities > Terminal)
sudo chown -R $USER /usr/local
** I strongly recommend you to not use the package management with sudo (sudo npm -g install something), because you can get some issues later **
Reference: http://foohack.com/2010/08/intro-to-npm/
** Update **
The reason is because this can cause permissions problems with lots of other apps, so I’d suggest not doing this.
A better solution when you are installing globally:
sudo chown -R `whoami` ~/.npm