FAQ

Unhandled rejection Error: EISDIR: illegal operation on a directory

sudo on Mac:

sudo chown -R $(whoami) ~/.npm
sudo chown -R $(whoami) /usr/local/lib
sudo chown -R $(whoami) /usr/local/bin

Then try your command again without sudo. e.g. (react-devtools in your case)

npm install -g npm@latest

This solved the provblem for me after upgrading to npm v6.9.0