For those using Windows, a possible solution is to utilize the following command in your console:

npx kill-port 3000

I faced a similar issue and this method proved effective for me. As for MAC users, the compatibility is uncertain.

To begin, launch the command prompt by opening cmd and entering the following:

netstat -a -o -n

This will provide you with a list of active connections. Next, locate the number 3000 by using the shortcut

Cntrl + f

https://i.sstatic.net/aL1NZ.jpg

Identify the PID of that particular port and execute the following command:

taskkill /F /PID PID_of_port

https://i.sstatic.net/oK5ui.png

Edit Please note that this guide is specifically for Windows users.

Following @khurram khan's advice, ending the process could be the optimal solution for your situation. I personally found success with this method on my Linux system:

$ lsof -i tcp:3000
$ kill -9 PID

The initial command will provide you with the PID number that you will need to input into the second command as "PID."

Terminate the Node.Js process through Task Manager.

Step 1: Access Task Manager by pressing ctrl+shift+delete

Step 2: Navigate to the Processes tab

Step 3: Locate the Node.JS process, right click on it, and select End Task

Step 4: You can now restart as needed.

For Windows users experiencing issues with the port not showing up when running the command netstat -a -o -n on the command prompt, follow these steps:

Open your command prompt in administrator mode and enter the following command:

net stop winnat

You should see this response:

The Windows NAT Driver service was stopped successfully.

Next, run the command:

net start winnat

Upon successful completion, you will see:

The Windows NAT Driver service was started successfully.

After completing these steps, start the React server and it should work properly. This also applies if your backend server is not running on port 3000.

Facing an issue on my Mac, I was able to resolve it by executing the following command:

npx kill-port 3000

After grappling with this issue for several hours, I finally managed to uncover the solution. It turns out there was a faulty mapping to the local host in the hosts file that had nothing to do with the assigned port. The incorrect entry looked something like this:

10.2.224.130    localhost 

All it took was changing it back to the correct entry:

127.0.0.1        localhost 

For those facing similar troubles, the hosts file can usually be found at:

This method consistently solves the issue for me on macOS:

sudo kill -9 $(sudo lsof -t -i:3000)

If you happen to stumble upon this and the solutions above didn't work, double-check that your /etc/hosts file includes:

127.0.0.1 localhost

It seems that this line was removed from my hosts file, causing issues with CRA's port detection tool (detect-port-alt). This tool checks localhost:[PORT] to determine availability. If it encounters an error, you will consistently receive the "Something is already running..." message when checking a specific port. However, this error does not occur when using a random port since it is selected by the dependency.

To resolve the issue, terminate the node.js process running in the background. If that doesn't work, consider rebooting your machine as an alternative solution.

Solving this issue is quite straightforward. Just follow these 2 simple steps.

1. Firstly, make sure to inspect your environment variables for any key/entry labeled as "PORT".
2. If you locate it, either remove the entry entirely or change its name to something different.

It appears that another application is utilizing this particular variable. Typically, when launching react-scripts, it will search for an environment variable named PORT.

If you are unable to see anything on localhost:3000, it could be because there is no webapp or server running on that port at the moment. The port may be occupied by a process unrelated to a website or application.

To check if there is a process running on port 3000, you can easily do so depending on your operating system. For MacOS, you can open a terminal and use the command `sudo lsof -i ':3000'` to view the current processes on that port. Remember to run this command with sudo privileges.

Once you identify the process, take note of the PID and then use the command `kill -9 {PID}` to terminate that specific process using port 3000. After doing this, running `npm start` should build and serve your app on localhost:3000 as expected.

If you had previously started a `npm start` process which was either interrupted or closed in the foreground, you can bring it back to the foreground by using the `fg` command in your terminal. If this opens a `npm start` instance, make sure to properly end the session to ensure all processes on port 3000 are terminated.

Happy coding!

If you happen to be using a Linux operating system, one possible solution is to use the command below:

pkill -f node

This will help terminate the current process.

When a task is left running on a port without being properly terminated, an error can occur. This issue may arise on both your React app and your Node server.

To resolve this issue, you can execute the following commands.

netstat -anp tcp | grep 3000

Running this command in the terminal will display the list of activities on port 3000.

npx kill-port 3000

This command will terminate all running servers on port 3000.

If you encounter the same problem with your Node server, simply follow the same steps.

netstat -anp tcp | grep 5000

Then

npx kill-port 5000

The first command is not necessary, as it just lists the running activities on the port.

You can change the port number to address any other port issues.

After executing the commands, this is the output I received using Git Bash on Windows.

Date: Sat 07, October 2020
Windows: Microsoft Windows 10 Pro Build 19041
Node: 12.16.1
NPM: 6.14.8

I encountered a similar issue on my Windows system and tried various solutions recommended on StackOverflow and elsewhere.

In my situation, I was following instructions from a tutorial, which advised adding --script-version 1.1.5 (as shown below) while creating a new React application.

create-react-app app-name --scripts-version 1.1.5

Below are the script versions found in the package.json file:
React: 17.0.1
React DOM: 17.0.1
React Scripts: 1.1.5

When I executed the command npm start and entered y to switch ports, the terminal would hang indefinitely until I terminated it manually.

https://i.sstatic.net/K7wjg.png

Subsequently, I created another React application without specifying

--scripts-version 1.1.5</code. Even though I encountered the error <code>Something is already running on port 3000

, after typing y, the application ran smoothly on a different port.

https://i.sstatic.net/ehTRt.png

Here are the updated script versions in the package.json file (after successful execution):
React: 17.0.1
React-Dom: 17.0.1
React-Scripts: 4.0.0

Here's how you can set the port for Windows in your package.json file: "start": "set PORT=3006 && react-scripts start"

If you're using Linux or MacOS, use this configuration instead: "start": "PORT=3006 react-scripts start"

For MAC users:

To check for the specific port, use command lsof -i :3000 https://i.sstatic.net/3uxp9.png

To kill the process occupying the port, use command kill -9 7233

After killing the process, the port should be available again. Verify using the command lsof -i :3000.