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# EADDRINUSE error ("port already in use"): how to fix

## What the EADDRINUSE error means

The `EADDRINUSE` error (from *Error: Address already in use*) happens when your application tries to "listen" on a port that is **already being used by another process**. In Node.js, the message usually looks like `Error: listen EADDRINUSE: address already in use :::3000`.

In short: port 3000 (or any other) is already taken, and two processes can't use the same port at the same time.

## Why it happens

* You started the application twice without stopping the first one.
* A previous process crashed and got "stuck" holding the port.
* Another application on the same server already uses that port.
* You hardcoded a port (e.g., `app.listen(3000)`) instead of using the one provided by the environment.

## How to fix it on your computer (local)

**1. Identify the process using the port**

On Linux or macOS:
`lsof -i :3000`

On Windows:
`netstat -ano | findstr :3000`

**2. Kill the process** (replace PID with the number that showed up)

On Linux or macOS: `kill -9 PID`

On Windows: `taskkill /PID PID /F`

## The definitive fix: use the environment's port

Hardcoding the port is the most common cause of this problem in production. Instead of `app.listen(3000)`, use the `PORT` environment variable:

```
const port = process.env.PORT || 3000;
app.listen(port);
```

That way the hosting environment sets the right port automatically and the conflict disappears.

## How Square Cloud avoids this problem

On Square Cloud, each application runs in an isolated, dedicated environment — no other process "steals" your port. Just have your application respect the `PORT` variable and set the port in the `squarecloud.app` file; the deploy takes care of the rest.

## Still getting the error?

If the application still won't start, review the start command in [My application won't start: Verifying the start script](https://help.squarecloud.app/en-us/article/my-application-wont-start-verifying-the-start-script-147pgxu/).