Обработка ошибок: try/catch, async-ошибки и понятные сообщения

export function safeParseJson(text) {
  try {
    return { ok: true, value: JSON.parse(text) };
  } catch (error) {
    return { ok: false, error };
  }
}

const results = ['{"qty": 2}', "{qty: 2}"].map(safeParseJson);
for (const result of results) {
  if (result.ok) console.log("value:", result.value);
  else console.log("invalid JSON:", result.error.message);
}

$ node safe-parse.mjs
value: { qty: 2 }
invalid JSON: Expected property name or '}' in JSON at position 1 (line 1 column 2)

function parseQty(input) {
  const qty = Number(input);
  if (!Number.isInteger(qty) || qty < 1) {
    throw new RangeError(`qty must be a positive integer, got "${input}"`);
  }
  return qty;
}

for (const input of ["2", "0", "abc"]) {
  try {
    console.log("try:", parseQty(input));
  } catch (error) {
    if (!(error instanceof RangeError)) throw error;
    console.log("catch:", error.name, "-", error.message);
  } finally {
    console.log("finally: input", JSON.stringify(input), "checked");
  }
}

$ node basics.mjs
try: 2
finally: input "2" checked
catch: RangeError - qty must be a positive integer, got "0"
finally: input "0" checked
catch: RangeError - qty must be a positive integer, got "abc"
finally: input "abc" checked

export class AppError extends Error {
  constructor(message, options) {
    super(message, options);
    this.name = new.target.name;
  }
}

export class NetworkError extends AppError {}

export class HttpError extends AppError {
  constructor(status, url, options) {
    super(`HTTP ${status} for ${url}`, options);
    this.status = status;
  }
}

export class DataError extends AppError {}

import { DataError, HttpError, NetworkError } from "./errors.mjs";

export async function loadMenu(url) {
  let res;
  try {
    res = await fetch(url, { signal: AbortSignal.timeout(3000) });
  } catch (error) {
    throw new NetworkError(`Request to ${url} failed`, { cause: error });
  }

  if (!res.ok) throw new HttpError(res.status, url);

  let data;
  try {
    data = await res.json();
  } catch (error) {
    throw new DataError(`Response from ${url} is not JSON`, { cause: error });
  }

  if (!Array.isArray(data.items)) {
    throw new DataError(`Response from ${url} has no "items" array`);
  }
  return data.items;
}

import { loadMenu } from "./load-menu.mjs";

try {
  await loadMenu("http://127.0.0.1:8082/menu");
} catch (error) {
  let current = error;
  while (current) {
    console.log(`${current.name}: ${current.message}${current.code ? ` (${current.code})` : ""}`);
    current = current.cause;
  }
}

$ node cause-chain.mjs
NetworkError: Request to http://127.0.0.1:8082/menu failed
TypeError: fetch failed
Error: connect ECONNREFUSED 127.0.0.1:8082 (ECONNREFUSED)

import { loadMenu } from "./load-menu.mjs";

function refreshMenu() {
  loadMenu("http://127.0.0.1:8082/menu");
  console.log("refresh started");
}

refreshMenu();
setTimeout(() => console.log("next refresh in 1 s"), 1000);

$ node floating.mjs
refresh started
file:///…/load-menu.mjs:8
    throw new NetworkError(`Request to ${url} failed`, { cause: error });
          ^

NetworkError: Request to http://127.0.0.1:8082/menu failed
    at loadMenu (file:///…/load-menu.mjs:8:11)
    at process.processTicksAndRejections (node:internal/process/task_queues:104:5) {
  [cause]: [TypeError: fetch failed] {
    [cause]: Error: connect ECONNREFUSED 127.0.0.1:8082
        at TCPConnectWrap.afterConnect [as oncomplete] (node:net:1706:16) {
      errno: -4078,
      code: 'ECONNREFUSED',
      syscall: 'connect',
      address: '127.0.0.1',
      port: 8082
    }
  }
}

Node.js v24.18.0

import { loadMenu } from "./load-menu.mjs";
import { toUserMessage } from "./user-message.mjs";

async function refreshMenu() {
  try {
    const items = await loadMenu("http://127.0.0.1:8082/menu");
    console.log("menu:", items.join(", "));
  } catch (error) {
    console.error("[log]", error.name, error.message);
    console.log("User sees:", toUserMessage(error));
  }
}

await refreshMenu();
setTimeout(() => console.log("next refresh in 1 s"), 1000);

$ node fixed-refresh.mjs
[log] NetworkError Request to http://127.0.0.1:8082/menu failed
User sees: No connection to the server. Check your internet and try again.
next refresh in 1 s

process.on("unhandledRejection", (reason) => {
  console.error("[fatal] unhandled rejection:", reason?.name, reason?.message);
  console.error("[fatal] cause:", reason?.cause?.message ?? "-");
  process.exitCode = 1;
});

await import("./floating.mjs");

$ node safety-net.mjs; echo "exit=$?"
refresh started
[fatal] unhandled rejection: NetworkError Request to http://127.0.0.1:8082/menu failed
[fatal] cause: fetch failed
next refresh in 1 s
exit=1

import { createServer } from "node:http";

export function startTestServer(port) {
  const server = createServer((req, res) => {
    if (req.url === "/menu") {
      res.writeHead(200, { "Content-Type": "application/json" });
      return res.end(JSON.stringify({ items: ["latte", "flat white"] }));
    }
    if (req.url === "/broken") {
      res.writeHead(500, { "Content-Type": "text/plain" });
      return res.end("Internal Server Error");
    }
    if (req.url === "/html") {
      res.writeHead(200, { "Content-Type": "text/html" });
      return res.end("<!doctype html><h1>Maintenance</h1>");
    }
    res.writeHead(404).end();
  });
  return new Promise((resolve) => server.listen(port, "127.0.0.1", () => resolve(server)));
}

import { loadMenu } from "./load-menu.mjs";
import { toUserMessage } from "./user-message.mjs";
import { startTestServer } from "./test-server.mjs";

const server = await startTestServer(8081);
const urls = [
  "http://127.0.0.1:8081/menu",
  "http://127.0.0.1:8081/broken",
  "http://127.0.0.1:8081/html",
  "http://127.0.0.1:8082/menu",
];

for (const url of urls) {
  console.log("→", url, "(spinner on)");
  try {
    const items = await loadMenu(url);
    console.log("  OK:", items.join(", "));
  } catch (error) {
    console.error("  [log]", error.name, error.message, "| cause:", error.cause?.message ?? "-");
    console.log("  User sees:", toUserMessage(error));
  } finally {
    console.log("  spinner off");
  }
}

server.close();

$ node main.mjs
→ http://127.0.0.1:8081/menu (spinner on)
  OK: latte, flat white
  spinner off
→ http://127.0.0.1:8081/broken (spinner on)
  [log] HttpError HTTP 500 for http://127.0.0.1:8081/broken | cause: -
  User sees: The menu is temporarily unavailable. Please try again in a minute.
  spinner off
→ http://127.0.0.1:8081/html (spinner on)
  [log] DataError Response from http://127.0.0.1:8081/html is not JSON | cause: Unexpected token '<', "<!doctype "... is not valid JSON
  User sees: The menu could not be displayed. We are already looking into it.
  spinner off
→ http://127.0.0.1:8082/menu (spinner on)
  [log] NetworkError Request to http://127.0.0.1:8082/menu failed | cause: fetch failed
  User sees: No connection to the server. Check your internet and try again.
  spinner off

import { DataError, HttpError, NetworkError } from "./errors.mjs";

export function toUserMessage(error) {
  if (error instanceof NetworkError) {
    return "No connection to the server. Check your internet and try again.";
  }
  if (error instanceof HttpError && error.status >= 500) {
    return "The menu is temporarily unavailable. Please try again in a minute.";
  }
  if (error instanceof HttpError && error.status === 404) {
    return "Menu not found. Please refresh the page.";
  }
  if (error instanceof DataError) {
    return "The menu could not be displayed. We are already looking into it.";
  }
  return "Something went wrong. Please try again later.";
}

async function saveOrder(order) {
  throw new Error(`DB is read-only, order ${order.id} not saved`);
}

const order = { id: 42 };
try {
  await saveOrder(order);
} catch {}
console.log("Order accepted!");

$ node antipattern.mjs
Order accepted!