Up to date
Published
3 min read

Trevor I. Lasn

Staff Software Engineer, Engineering Manager

Error.isError(): A Better Way to Check Error Types in JavaScript

Why the new Error.isError() method solves important cross-realm issues and provides more reliable error identification than instanceof

JavaScript’s error handling system has long had a blind spot when dealing with errors across different execution contexts. The new Error.isError() method addresses this limitation, providing developers with a more reliable way to identify error objects.

The traditional approach to checking if a value is an Error has been using the instanceof operator.

JavaScript
try {
// Code that might throw
} catch (e) {
if (e instanceof Error) {
// Handle error
}
}

This approach has two significant limitations:

  1. Cross-realm errors aren’t correctly identified. When an error originates from another realm (like an iframe or VM module), instanceof Error returns false because each realm has its own Error constructor.

This can lead to situations where errors are not handled correctly, as the instanceof check fails.

  1. Fake errors can pass the test. Objects with Error.prototype in their prototype chain but lacking error characteristics will be incorrectly identified as errors.

These issues can lead to inconsistent error handling and difficult-to-diagnose bugs. The new Error.isError() method provides a solution:

Instead of checking the prototype chain, Error.isError() uses a simpler and more reliable approach. It looks for a special internal marker (like a hidden ID tag) that gets added to every genuine Error object when it’s created.

This method works better than instanceof for two reasons:

  1. It correctly identifies errors even when they come from different contexts (like iframes or modules)
  2. It rejects fake objects that try to pretend they’re errors by manipulating the prototype

Think of it like checking for a manufacturer’s watermark instead of just looking at the label - it’s much harder to fake.

Typing Error.isError()

Here’s one way you could type the Error.isError() method in TypeScript:


Found this article helpful? You might enjoy my free newsletter. I share dev tips and insights to help you grow your coding skills and advance your tech career.

Interested in supporting this blog in exchange for a shoutout? Get in touch.


Liked this post?

Check out these related articles that might be useful for you. They cover similar topics and provide additional insights.

Javascript
6 min read

AggregateError in JavaScript

Handle multiple errors at once

Sep 2, 2024
Read article
Javascript
4 min read

What is the JavaScript Pipeline Operator |>

A deep dive into how pipeline operators can make your code more readable and maintainable

Oct 29, 2024
Read article
Javascript
5 min read

Precise Decimal Math in JavaScript with Fraction.js

How to handle exact decimal calculations in JavaScript when floating-point precision isn't good enough

Nov 16, 2024
Read article
Javascript
4 min read

The Only Widely Recognized JavaScript Feature Ever Deprecated

The 'with' statement is the only feature ever deprecated in JavaScript

Aug 22, 2024
Read article
Javascript
3 min read

Float16Array in JavaScript

Understanding the new 16-bit floating point array in JavaScript

Apr 14, 2025
Read article
Javascript
4 min read

Understanding Bitwise Shifts in JavaScript: << and >>

A practical guide to left and right shift operators in JavaScript

Nov 12, 2024
Read article
Javascript
4 min read

Embrace Intermediate Variables and Early Returns in JavaScript

Early returns and intermediate variables make your code easier to reason about

Aug 30, 2024
Read article
Javascript
4 min read

JavaScript compile hints: what they are and when to use them

V8's compile hints let you control which JavaScript gets compiled immediately during page load

May 12, 2025
Read article
Javascript
9 min read

Exploring JavaScript Symbols

Deep dive into JavaScript Symbols - what they are, why they matter, and how to use them effectively

Nov 15, 2024
Read article

This article was originally published on https://www.trevorlasn.com/blog/error-iserror-javascript. It was written by a human and polished using grammar tools for clarity.