The simple answer is web servers should never be run as root for well known security reasons, so this goes for npm commands as well.

To start fresh, remove prior Node.js and npm installs as well as these files/directories:

mv ~/.npmrc ~/.npmrc~prior
mv ~/.npm ~/.npm~prior
mv ~/tmp ~/tmp.~prior
mv ~/.npm-init.js ~/.npm-init.js~prior

Solution: Install Node.js (which comes with npm) as NON root (no sudo)

Download Source Code directly from https://nodejs.org/en/download/

Execute the below as yourself (Linux/OS X)

cd node-v8.1.2 # into expanded source dir

export NODE_PARENT=${HOME}/node-v8.1.2 # put this into your ~/.bashrc

Feel free to change above export to whatever location is appropriate

./configure --prefix=${NODE_PARENT}
make -j4 # for dual core ... use -j8 for quad core CPU
make install

which puts the binaries for Node.js and npm as well as its modules repository into $NODE_PARENT, a $USER owned dir which then allows you to issue subsequent npm install xxx commands as yourself.

To reach the binaries for node and npm alter your PATH environment variables in your ~/.bashrc:

export PATH=${NODE_PARENT}/bin:${PATH}
export NODE_PATH=${NODE_PARENT}/lib/node_modules

Then to install packages into that directory (global), as opposed to the current directory (local) always pass in the -g flag (global):

npm install -g someModule

NOTE - at no time are you executing anything npm or node related as root / sudo.