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You should not be using su with npm install or bower install.

However, we understand when you see errors such as npm ERR! Error: EACCES, mkdir .., you will relent to using your superadmin power..

In bower case, if you do, you will see warning message:

Since bower is a user command, there is no need to execute it with superuser permissions.
If you’re having permission errors when using bower without sudo, please spend a few minutes learning more about how your system should work and make any necessary repairs.

The fix is simple:

sudo chown -R $USER:$GROUP ~/.npm
sudo chown -R $USER:$GROUP ~/.config

This makes the directory where npm and bower is using when installing packages to be given the correct permissions.