Errors

JavaScript · Reference cheat sheet

📋 Overview

Errors interrupt normal control flow. Use try / catch / finally, typed error classes, and rethrow carefully. Prefer throwing Error (or subclasses) over strings so stacks and cause work.

🔧 Core concepts

Built-ins: Error, TypeError, RangeError, SyntaxError, URIError, AggregateError.
Properties: name, message, stack, cause (ES2022).
Throw: throw new Error("msg", \{ cause \}).
Catch: bind the error; optional binding omission in some engines for unused catches.
finally: always runs (cleanup), even on return.
Async: rejections surface at await or .catch.

try {
  JSON.parse("{");
} catch (err) {
  console.error(err.name, err.message);
} finally {
  console.log("done");
}

💡 Examples

class HttpError extends Error {
  constructor(status, message, options) {
    super(message, options);
    this.name = "HttpError";
    this.status = status;
  }
}

async function getJson(url) {
  try {
    const res = await fetch(url);
    if (!res.ok) {
      throw new HttpError(res.status, res.statusText);
    }
    return await res.json();
  } catch (err) {
    throw new Error(`getJson failed: ${url}`, { cause: err });
  }
}

// Narrowing
function isHttpError(e) {
  return e instanceof HttpError;
}

try {
  await getJson("/api");
} catch (e) {
  if (isHttpError(e) && e.status === 404) {
    console.warn("missing");
  } else {
    throw e; // rethrow unknown
  }
}

// AggregateError (Promise.any)
try {
  await Promise.any([Promise.reject("a"), Promise.reject("b")]);
} catch (e) {
  if (e instanceof AggregateError) {
    console.log(e.errors);
  }
}

// Optional catch binding (when unused)
try {
  risky();
} catch {
  /* ignore */
}

// AbortError from fetch
const ac = new AbortController();
fetch(url, { signal: ac.signal }).catch((err) => {
  if (err.name === "AbortError") return;
  console.error(err);
});
ac.abort();

⚠️ Pitfalls

Catching everything and swallowing errors hides bugs — rethrow unknowns.
throw "string" loses stack traces — throw Error instances.
finally can override a returned/thrown value if it returns or throws itself.
Async errors outside try around await become unhandled rejections.
instanceof fails across realms/iframes — check name as fallback.

promise.md — rejection handling
async.md — await + try/catch
fetch.md — HTTP failures
console.md — logging errors
api.md — AbortController

Errors

Errors

📋 Overview

🔧 Core concepts

💡 Examples

⚠️ Pitfalls

🔗 Related

On this page