Find (and kill) process locking port 3000 on Mac

1. You can try netstat

   netstat -vanp tcp | grep 3000
   

2. For macOS El Capitan and newer (or if your netstat doesn't support -p), use lsof

   lsof -i tcp:3000
   

Find:

sudo lsof -i :3000

Kill:

kill -9 <PID>

PLEASE NOTE: -9 kills the process immediately, and gives it no chance of cleaning up after itself. This may cause problems. Consider using -15 (TERM) or -3 (QUIT) for a softer termination which allows the process to clean up after itself.