Port 0 lets the kernel pick a free port

Today I learned that I can pass 0 as the port when starting a server, and the kernel will pick an available port for me.

I came across this while working on a local proxy in Node.js. Here’s a small demo—save it as port-zero.mjs:

import http from "node:http";

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

server.listen(0, "127.0.0.1", () => {
  console.log(`Listening at http://127.0.0.1:${server.address().port}`);
});

Run it with node port-zero.mjs. For example, it might print:

Listening at http://127.0.0.1:53147

Opening that URL returns Hello!. The port can change each time I restart the script. Press Ctrl+C to stop it.

I usually hardcode something like 3000 and change it if it’s already taken. Passing 0 asks the OS to choose and bind a free port in one step. Once the server is listening, server.address().port gives me the assigned number.

What I’d casually call a random port is really an OS-assigned unused port. Handy for temporary local proxies and tests running in parallel, where I don’t need a fixed port.

Node.js docs: server.listen()