What causes EADDRINUSE: address already in use?

A previous instance of the same server process did not exit cleanly and is still holding the port. A different application (a database, another dev server, a proxy) is already listening on that port. Hot-reload or watch mode restarted the process before the old TCP socket was fully released. Running multiple test suites or server instances in parallel without assigning different ports. The same conflict shows up outside Node.js too — a Python server, a Windows service, or anything else trying to bind a port someone else already has.

← Errors

JavaScript

EADDRINUSE: address already in use: What It Means and How to Fix It

Error reference

EADDRINUSE: address already in use

Language

JavaScript

Severity

High

When it happens

EADDRINUSE happens when a process tries to bind to a port that is already occupied by another process.

Potential fixes

Find and kill the process using the port: lsof -i :<port> on macOS/Linux, or netstat -ano | findstr :<port> on Windows. Use an environment variable for the port number so different environments can use different ports. Add an error handler on the server object to fail with a clear message instead of an unhandled crash.

Related errors

ENOENT: no such file or directory

Deep dive

EADDRINUSE happens when a process tries to bind to a port that is already occupied by another process. It is not specific to Node.js — the operating system only allows one process to listen on a given port at a time, whether that process is a Node server, a Python script, or any other program, on Windows, macOS, or Linux.

Why it happens

•A previous instance of the same server process did not exit cleanly and is still holding the port.
•A different application (a database, another dev server, a proxy) is already listening on that port.
•Hot-reload or watch mode restarted the process before the old TCP socket was fully released.
•Running multiple test suites or server instances in parallel without assigning different ports.
•The same conflict shows up outside Node.js too — a Python server, a Windows service, or anything else trying to bind a port someone else already has.

Potential fixes

Find and kill the process using the port: lsof -i :<port> on macOS/Linux, or netstat -ano | findstr :<port> on Windows. Use an environment variable for the port number so different environments can use different ports. Add an error handler on the server object to fail with a clear message instead of an unhandled crash.

Examples

JavaScript

Code that triggers the error

const http = require("http");

const server = http.createServer((req, res) => {
  res.end("Hello");
});

server.listen(3000, () => console.log("Listening on port 3000"));

Error output

Error: listen EADDRINUSE: address already in use :::3000
    at Server.setupListenHandle [as _listen2] (node:net:1739:16)

Fixed code

const http = require("http");

const PORT = process.env.PORT || 3000;

const server = http.createServer((req, res) => {
  res.end("Hello");
});

server.on("error", (err) => {
  if (err.code === "EADDRINUSE") {
    console.error(`Port ${PORT} is in use. Kill the process or use a different port.`);
    process.exit(1);
  }
});

server.listen(PORT, () => console.log(`Listening on port ${PORT}`));

Practice in English

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"I hit an error...

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