EADDRINUSE: Port Already in Use Error

Identify the process ID using the port and terminate it.

# Windows
netstat -ano | findstr :3000
taskkill /PID <PID> /F

# Linux/macOS
lsof -i :3000
kill -9 <PID>
# or
fuser -k 3000/tcp

Change the port your application listens on.

# Node.js: use environment variable
const PORT = process.env.PORT || 3000
app.listen(PORT)

# Next.js
npx next dev -p 3001

# Docker: map different host port
docker run -p 8080:3000 myapp

Allow immediate port reuse after process termination.

// Node.js: enable SO_REUSEADDR
const server = app.listen(3000)
server.on("listening", () => {
  server.address()
})
// By default, Node.js enables SO_REUSEADDR on macOS and Linux

Automatically try the next available port if the default is taken.

function startServer(port, maxAttempts = 10) {
  const tryPort = (p) => {
    app.listen(p).on("error", (err) => {
      if (err.code === "EADDRINUSE" && p < port + maxAttempts) {
        console.log(`Port ${p} in use, trying ${p + 1}`)
        tryPort(p + 1)
      } else {
        throw err
      }
    })
  }
  tryPort(port)
}