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NPM self-reporting error, how to solve it
There is an open issue with installing global packages using sudo leaving behind files with the wrong ownership.
The short version is run this to fix the ownership, then try your install again:
sudo chown -R $(whoami) ~/.npm
The long version and canonical bug is: Global installs (sudo npm i -g) fail on Mac after 6.5 upgrade. Works fine after 6.4.1 downgrade.