Find a free port
Test servers and helper processes should not fight over hardcoded ports. The getAvailablePort function from @std/net asks the operating system for a free port, and Deno.serve with port 0 does the same thing in one step.
import { getAvailablePort } from "jsr:@std/net";getAvailablePort binds port 0, reads the port the OS picked, and closes the probe listener. Optionally pass { preferredPort: 8000 } to try a fixed port first and fall back to a random one.
const port = getAvailablePort();
console.log(port); // e.g. 61678
const server = Deno.serve(
{ port, onListen: ({ port }) => console.log(`listening on ${port}`) },
() => new Response("Hello from a random port\n"),
);Note there is a small race: between getAvailablePort returning and the server starting, another process could grab the port. Asking Deno.serve itself for port 0 has no such window, the listener that gets the random port is the one you keep. Read the assigned port from server.addr. In Node the same trick is server.listen(0) plus server.address().port.
const server2 = Deno.serve(
{ port: 0, onListen: () => {} },
() => new Response("Hello from port 0\n"),
);
console.log(server2.addr.port); // e.g. 61679Both servers answer normally on their assigned ports.
const res = await fetch(`http://localhost:${server2.addr.port}/`);
console.log(await res.text()); // Hello from port 0
await server.shutdown();
await server2.shutdown();Run this example locally using the Deno CLI:
deno run -N https://docs.deno.com/examples/scripts/find_free_port.ts