Fixing npm On Mac OS X for Homebrew Users
If you just want to fix the issue quickly, scroll down to the "solution" section below.
Explanation of the issue
If you're a Mac Homebrew user and you installed node via Homebrew, there is a major philosophical issue with the way Homebrew and NPM work together. If you install node with Homebrew and then try to do
npm update npm -g, you will see an error like this:
$ npm update npm -g