Fix "Something is Already Running on Port 3000"

The error

When you run npm start in a Create React App project, you see:

? Something is already running on port 3000.
Would you like to run the app on another port instead? (Y/n)

If you press Y, React starts on port 3001, 3002, etc. But you probably want port 3000 back.

Fix 1: portrm (instant)

$ ptrm fix 3000
  Port 3000 in use
  node (PID 91234) - react-scripts start
  Killed PID 91234. Port 3000 is free.

$ npm start  # starts on port 3000

Fix 2: Kill manually

# macOS
$ lsof -ti :3000 | xargs kill -9

# Linux
$ fuser -k 3000/tcp

# Windows
> netstat -ano | findstr :3000
  TCP  0.0.0.0:3000  0.0.0.0:0  LISTENING  91234
> taskkill /PID 91234 /F

Fix 3: Use a different port

Set the PORT environment variable to use a different port:

# macOS / Linux
$ PORT=3001 npm start

# Windows (PowerShell)
> $env:PORT=3001; npm start

# Or in .env file
PORT=3001

Fix 4: Disable the prompt

If you want React to always use the next available port without asking:

# .env
BROWSER=none
PORT=3000

Or set CI=true to skip the interactive prompt (useful in scripts).

Why does this keep happening?

CRA proxy conflict

If you use proxy in package.json to forward API calls, make sure the proxy target is not also on port 3000:

// package.json
{
  "proxy": "http://localhost:4000"  // API on 4000, React on 3000
}

Auto-clear port in npm scripts

Add a prestart script so the port is always freed before React starts:

// package.json
{
  "scripts": {
    "prestart": "ptrm fix 3000",
    "start": "react-scripts start"
  }
}