Node.js fetch with a proxy: undici, environment variables and the errors

// npm i undici: use undici's own fetch with its ProxyAgent, so the versions match.
import { fetch, ProxyAgent } from "undici";

const USER = process.env.PROXY_USER;
const PASS = process.env.PROXY_PASS;

// One ProxyAgent keeps its connection, so it keeps one IP. A new agent, a new IP.
function agent(targeting = "") {
  const login = `${encodeURIComponent(USER + targeting)}:${encodeURIComponent(PASS)}`;
  return new ProxyAgent(`http://${login}@via.proxies.vision:9000`);
}

// The proxy's status is two causes deep: "Proxy response (407) !== 200 when HTTP Tunneling".
function proxyStatus(err) {
  const match = /Proxy response \((\d+)\)/.exec(err?.cause?.cause?.message ?? "");
  return match ? Number(match[1]) : null;
}

async function get(url, targeting = "", attempts = 3) {
  for (let i = 1; ; i++) {
    try {
      return await fetch(url, { dispatcher: agent(targeting), signal: AbortSignal.timeout(30000) });
    } catch (err) {
      const status = proxyStatus(err);
      // 407 and 403 won't change on a retry; dropped exits and 465 might.
      if (status === 407 || status === 403 || i === attempts) {
        throw new Error(status ? `proxy refused with ${status}` : "request failed", { cause: err });
      }
    }
  }
}

for (let i = 0; i < 3; i++) {
  const data = await (await get("https://ipinfo.io/json", "-country-de")).json();
  console.log(data.ip, data.country, data.org);
}

try {
  await get("https://ipinfo.io/json", "-country-usa");
} catch (err) {
  console.log(err.message);
}