Fix "Port Already in Use" in Node.js

The Error Message

When you start a Node.js server and see this error, it means another process is already listening on that port:

Error: listen EADDRINUSE: address already in use :::3000

Or in Express:

Error: listen EADDRINUSE: address already in use 0.0.0.0:3000

Or in Next.js:

Port 3000 is already in use.
Use `next dev --port 3001` to use a different port.

The most common reason: a previous instance of your server is still running in the background, either in another terminal tab or because it crashed without releasing the port.

One-Command Fix with portrm

The fastest way to fix this in any Node.js project:

$ ptrm fix 3000 -y

  Port 3000 in use
  Node.js (PID 42891) - Express server
  SAFE - dev server
  Killed PID 42891
  Port 3000 is now free

Or fix and restart in one step:

$ ptrm fix 3000 --run "npm run dev"

Install: npm install -g portrm or try instantly with npx portrm fix 3000

Fix for Express

Express apps typically listen on port 3000. When the port is taken:

$ ptrm fix 3000 --run "node app.js"

Or manually:

$ lsof -ti :3000 | xargs kill -9
$ node app.js

To use a different port in Express:

const PORT = process.env.PORT || 3000;
app.listen(PORT, () => console.log(`Server on port ${PORT}`));

Then: PORT=3001 node app.js

Fix for Next.js

Next.js defaults to port 3000 and shows a helpful message when it is taken:

$ ptrm fix 3000 --run "npx next dev"

Or change the port:

$ npx next dev --port 3001

Fix for Vite / React

Vite defaults to port 5173. If that is taken:

$ ptrm fix 5173 --run "npm run dev"

Vite usually auto-increments to the next available port, but if you need a specific port, add to vite.config.js:

export default defineConfig({
  server: { port: 3000, strictPort: true }
})

Change the Default Port

Prevent the Error Programmatically

Add graceful shutdown to your Node.js application so it releases the port when it exits:

const server = app.listen(3000);

process.on('SIGTERM', () => {
  server.close(() => process.exit(0));
});

process.on('SIGINT', () => {
  server.close(() => process.exit(0));
});

Or use portrm to manage your dev stack and handle this automatically:

$ ptrm init                     # create .ptrm.toml
$ ptrm up                       # start all services with graceful management
$ ptrm watch 3000               # auto-restart on crash