Close nodejs app running in another shell.
Restart the terminal and run the program again.
Another server might be also using the same port that you have used for nodejs. Kill the process that is using nodejs port and run the app.
To find the PID of the application that is using port:8000
$ fuser 8000/tcp
8000/tcp: 16708
Here PID is 16708 Now kill the process using the kill [PID] command
$ kill 16708