denoland-deno/cli/tests/echo_server.ts

const { args, listen, copy } = Deno;
const addr = args[1] || "0.0.0.0:4544";
const [hostname, port] = addr.split(":");
const listener = listen({ hostname, port: Number(port) });
console.log("listening on", addr);
listener.accept().then(
  async (conn): Promise<void> => {
    await copy(conn, conn);
    conn.close();
    listener.close();
  }
);