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How to handle EADDRINUSE errors

One of the most common errors raised when listening is EADDRINUSE. This happens when another server is already listening on the requested port/path/handle:

node:events:489
      throw er; // Unhandled 'error' event
      ^

Error: listen EADDRINUSE: address already in use :::8080
    at Server.setupListenHandle [as _listen2] (node:net:1829:16)
    at listenInCluster (node:net:1877:12)
    at Server.listen (node:net:1965:7)

One way to handle this would be to catch the error and retry after a certain amount of time:

const { createServer } = require("node:http");
const { Server } = require("socket.io");

const httpServer = createServer();
const io = new Server(httpServer);

const PORT = process.env.PORT || 8080;

io.on("connection", (socket) => {
  // ...
});

httpServer.on("error", (e) => {
  if (e.code === "EADDRINUSE") {
    console.error("Address already in use, retrying in a few seconds...");
    setTimeout(() => {
      httpServer.listen(PORT);
    }, 1000);
  }
});

httpServer.listen(PORT);

Reference: https://nodejs.org/api/net.html#serverlisten

tip

When testing, you might not need to use a specific port. You can simply omit the port and the operating system will automatically pick an arbitrary unused port for you:

const { createServer } = require("node:http");
const { Server } = require("socket.io");

const httpServer = createServer();
const io = new Server(httpServer);

httpServer.listen(() => {
  const port = httpServer.address().port;
  // ...
});

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