It is common in Node.js and other languages to run a script on a certain port. Since ports can come in and out of use, it's also common to get the following error:

    Error: listen EADDRINUSE: address already in use :::3000
    at Server.setupListenHandle [as _listen2] (net.js:1318:16)
    at listenInCluster (net.js:1366:12)
    at Server.listen (net.js:1452:7)
    at Function.listen (/application.js:618:24)
    at file:///httpdocs/index.js:149:5
    Emitted 'error' event on Server instance at:
        at emitErrorNT (net.js:1345:8)
        at processTicksAndRejections (internal/process/task_queues.js:80:21) {
        code: 'EADDRINUSE',
        errno: -98,
        syscall: 'listen',
        address: '::',
        port: 3000
    }

This error says that there is currenlty something running on port 3000, so we can't use it. Fortunately this is quite easy to fix.

Linux and Mac

To solve this issue on linux or on a mac, you first want to find out the process ID or PID currently running on the port (in our case :3000). To do that, you can use lsof. If you want a way to remember lsof, it is that it means 'list of'.

    lsof -i :3000

This will return processes running on port :3000. The next step is to kill the processes on that port. Note the PID, and then run the following command, replacing [PID] with your PID:

    kill -15 [PID]

Why -15? -15 refers to the message your computer will send. You should try -15 the first time, as this will lead to an orderly shutdown of port 3000. If that doesn't work, then try:

    kill -9 [PID]

Now there will be nothing running on port :3000. This will also work for any other port you're having issues with, i.e. :8080, :1337, or any other number.

For Windows Users

For any windows users out there, it is slightly different, but I have put this here as it may also prove useful. First off, find what is running at the port you are looking for (i.e. 3000). The PID can be found at the end of the line when you run this process:

    netstat -ano | findstr :3000

Then, similar to linux and mac, kill what is on that port, replacing PID with the PID for that process from the statement above.

    taskkill /PID PID /F