I was not able to install npm packages globally in mac os(using npm install -g), but able to install locally in the project. Installing locally in every project will increase the total node_modules folder size and for every project we need to install the same packages that will be tedious and unnecessary.

Here are the steps I performed.

I was using Homebrew to install node and npm. So,

1. Used command npm config get prefix - check where the npm actually points to. If this points to /opt/homebrew

2. Check if node_modules are installed in cd /usr/local/lib

3. If (2) is true, brew remove npm and then brew install npm

4. Now see if npm install -g express works

ref: https://stackoverflow.com/questions/22562041/global-installation-with-npm-doesnt-work-after-mac-os-x-mavericks-update