Address already in use - bind(2) for 127.0.0.1:4000 (Errno::EADDRINUSE) code example

Example 1: Address already in use - bind(2) for "127.0.0.1" port 3000 (Errno::EADDRINUSE)

lsof -wni tcp:3000 
kill -9 PID_number

Example 2: listen EADDRINUSE: address already in use 127.0.0.1:8000

kill -9 {PID}