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List network interfaces

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Deno.networkInterfaces lists every network address of the machine. The most common use is finding the LAN address of the computer, for example to print a URL that teammates on the same network can open. The call needs the system permission, granted with -S (short for --allow-sys).

Each entry describes one address bound to one interface. An interface like en0 typically appears several times, once per IPv4 and IPv6 address.
const interfaces = Deno.networkInterfaces();
console.log(interfaces.length); // e.g. 15
An entry contains the interface name, address family, the address itself, plus the netmask, CIDR notation and MAC address.
const first = interfaces[0];
console.log(first.name, first.family, first.address); // e.g. lo0 IPv4 127.0.0.1
A small helper to find the LAN address: the first IPv4 address that is not on the loopback interface. Addresses starting with 127. only reach the machine itself, so they are skipped.
function lanAddress(): string | undefined {
  return Deno.networkInterfaces().find((ni) =>
    ni.family === "IPv4" && !ni.address.startsWith("127.")
  )?.address;
}

console.log(lanAddress()); // e.g. 192.168.1.239
With that address a local development server can announce a URL that other devices on the same network can open.
console.log(`http://${lanAddress()}:8000/`); // e.g. http://192.168.1.239:8000/
Code written for Node.js gets the same data from the os module, shaped as a record keyed by interface name instead of a flat list.
import os from "node:os";
const nodeInterfaces = os.networkInterfaces();
console.log(Object.keys(nodeInterfaces).length); // e.g. 12
console.log(nodeInterfaces["lo0"]?.[0]?.address); // e.g. 127.0.0.1

Run this example locally using the Deno CLI:

deno run -S https://docs.deno.com/examples/scripts/network_interfaces.ts

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