Short note with quick solution.
For some time my react app was constantly refusing to run on localhost:3000. For some time I thought that maybe there's some forgotten node process still running in the background.
After some investigation I found that something changed on MacOS High Sierra networking setup and localhost is often not resolved to 127.0.0.1.
Adding 127.0.0.0.1 localhost into /etc/hosts seems like a solution for this issue.