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Error: listen EADDRINUSE: address already in use

Exact error

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

What it means

Another process is already listening on the port your server is trying to use.

Likely causes

• A previous run of the server is still running.
• Two servers (e.g. API and frontend) configured for the same port.

How to fix it

1. Stop the previous process (in Replit, stop and re-run; locally find it):

   lsof -i :3000
   kill <PID>

2. Read the port from the environment with a fallback, and bind to 0.0.0.0 so hosted environments can reach it:

   const port = process.env.PORT || 3000;
   app.listen(port, '0.0.0.0', () => console.log('listening on', port));

Prevent this next time

• Commit package-lock.json or requirements.txt so every environment installs the same dependencies.
• Read the port from `process.env.PORT` and bind to 0.0.0.0 in hosted environments.

Guide: catch outages before your users do.

Related errors

• Error: Cannot find module 'express'