React native ERROR Packager can't listen on port 8081

On a mac, run the following command to find id of the process which is using port 8081
sudo lsof -i :8081
Then run the following to terminate process:
kill -9 23583

Here is how it will look like

You can run the packager on another port.

$ react-native start --port=8088

Alternatively, find out what is using which ports on Windows with netstat.

$ netstat -a -b -o

Netstat gives you a PID, which you can use to kill the process.

$ taskkill /pid 1234

This error is coming because some process is already running on 8081 port. Stop that process and then run your command, it will run your code. For this first list all the process which are using this port by typing

lsof -i :8081  

This command will list the process id (PID) of the process and then kill the node process by using

kill -9 <PID>  

Here PID is the process id of the node process.