Updating npm is deleting npm? - node.js

Question

Running npm update -g npm is deleting npm from usr/local/bin.
It gives this error and when I try to run it again using sudo it is gone.
as-MacBook-Air:~ a$ npm update -g npm
npm ERR! Darwin 14.5.0
npm ERR! argv "/usr/local/bin/node" "/usr/local/bin/npm" "update" "-g" "npm"
npm ERR! node v4.2.4
npm ERR! npm v2.14.12
npm ERR! path /usr/local/lib/node_modules/npm
npm ERR! code EACCES
npm ERR! errno -13
npm ERR! syscall rmdir
npm ERR! Error: EACCES: permission denied, rmdir '/usr/local/lib/node_modules/npm'
npm ERR! at Error (native)
npm ERR! { [Error: EACCES: permission denied, rmdir '/usr/local/lib/node_modules/npm']
npm ERR! errno: -13,
npm ERR! code: 'EACCES',
npm ERR! syscall: 'rmdir',
npm ERR! path: '/usr/local/lib/node_modules/npm' }
npm ERR!
npm ERR! Please try running this command again as root/Administrator.
npm ERR! error rolling back Error: EACCES: permission denied, rmdir '/usr/local/lib/node_modules/npm'
npm ERR! error rolling back at Error (native)
npm ERR! error rolling back { [Error: EACCES: permission denied, rmdir '/usr/local/lib/node_modules/npm']
npm ERR! error rolling back errno: -13,
npm ERR! error rolling back code: 'EACCES',
npm ERR! error rolling back syscall: 'rmdir',
npm ERR! error rolling back path: '/usr/local/lib/node_modules/npm' }
npm ERR! Please include the following file with any support request:
npm ERR! /Users/a/npm-debug.log
as-MacBook-Air:~ a$

Answer 1

permission denied, rmdir '/usr/local/lib/node_modules/npm'
try the command again with sudo:
sudo npm update -g npm
reinstall npm :
First uninstall node and remove node_modules. then install it again by homebrew
rm -rf /usr/local/lib/node_modules
brew uninstall node
brew install node --without-npm
echo prefix=~/.node >> ~/.npmrc
curl -L https://www.npmjs.com/install.sh | sh
export PATH="$HOME/.node/bin:$PATH"
npm install -g http-server node-inspector forever nodemon

Answer 2

Try it with Sudo permissions or better try remove it and install a updated one.