cannot install npm i -g webpack webpack-dev-server - node.js

Question

npm ERR! Darwin 15.6.0
npm ERR! argv "/usr/local/bin/node" "/usr/local/bin/npm" "i" "-g" "webpack" "webpack-dev-server"
npm ERR! node v6.11.0
npm ERR! npm v3.10.10
npm ERR! path /usr/local/lib/node_modules
npm ERR! code EACCES
npm ERR! errno -13
npm ERR! syscall access
npm ERR! Error: EACCES: permission denied, access '/usr/local/lib/node_modules'
npm ERR! at Error (native)
npm ERR! { Error: EACCES: permission denied, access '/usr/local/lib/node_modules'
npm ERR! at Error (native)
npm ERR! errno: -13,
npm ERR! code: 'EACCES',
npm ERR! syscall: 'access',
npm ERR! path: '/usr/local/lib/node_modules' }
npm ERR!
npm ERR! Please try running this command again as root/Administrator.
npm ERR! Darwin 15.6.0
npm ERR! argv "/usr/local/bin/node" "/usr/local/bin/npm" "i" "-g" "webpack" "webpack-dev-server"
npm ERR! node v6.11.0
npm ERR! npm v3.10.10
npm ERR! path npm-debug.log.1837435588
npm ERR! code EACCES
npm ERR! errno -13
npm ERR! syscall open
npm ERR! Error: EACCES: permission denied, open 'npm-debug.log.1837435588'
npm ERR! at Error (native)
npm ERR! { Error: EACCES: permission denied, open 'npm-debug.log.1837435588'
npm ERR! at Error (native)
npm ERR! errno: -13,
npm ERR! code: 'EACCES',
npm ERR! syscall: 'open',
npm ERR! path: 'npm-debug.log.1837435588' }
npm ERR!
npm ERR! Please try running this command again as root/Administrator.
npm ERR! Please include the following file with any support request:
npm ERR! /Users/npm-debug.log
lnkimac:Users fernando$

Answer 1

When you install something gloabally (-g), you have to launch npm with administrator rights (in most cases).
You can do this by starting a Commandline with administrator rights (Search CMD in Windows, right click it -> start as administrator).
There you can execute the command again. This should work.