Node / Express: EADDRINUSE, Address already in use – how can I stop the process using the port?

First, you would want to know which process is using port 3000

sudo lsof -i :3000

this will list all PID listening on this port, once you have the PID you can terminate it with the following:

kill -9 <PID>

where you replace <PID> by the process ID, or the list of process IDs, the previous command output.