What does instanceof Error do?

JavaScriptJunior~1 min read

The instanceof Error operator in JavaScript is used to check whether an object is an instance of the Error class (or one of its subclasses - TypeError, RangeError, ReferenceError, etc.).

In short

obj instanceof Error returns true if the object obj is an error created via Error (or inheriting from it).

Syntax

javascript

object instanceof Error

Returns:

true - if the object is an error;
false - if it is not.

Example 1. Checking a plain error

javascript

const err = new Error("Something went wrong");

console.log(err instanceof Error); // true

The err object is created via new Error(), so the check returns true.

Example 2. Checking built-in error types

javascript

console.log(new TypeError() instanceof Error);       // true
console.log(new RangeError() instanceof Error);      // true
console.log(new ReferenceError() instanceof Error);  // true
console.log(new SyntaxError() instanceof Error);     // true

All of them inherit from Error, so instanceof Error returns true for every standard error type.

Example 3. Checking a non-error value

javascript

console.log("Error" instanceof Error); // false
console.log({ message: "Error" } instanceof Error); // false
console.log(404 instanceof Error); // false

These values are not created via Error, so they are not considered errors, even if they "look like" one.

Example 4. Checking custom (user-defined) errors

javascript

class ValidationError extends Error {}

const err = new ValidationError("Invalid email");

console.log(err instanceof ValidationError); // true
console.log(err instanceof Error);           // true

Any class that inherits from Error also returns true when checked with instanceof Error.

Example 5. Checking before logging

This is a common pattern in production code:

javascript

try {
  throw new Error("Request error");
} catch (e) {
  if (e instanceof Error) {
    console.error("Error:", e.message);
  } else {
    console.error("Unexpected exception:", e);
  }
}

This lets you safely handle only real errors, instead of accidentally thrown strings or numbers.

Example 6. Checking when working with promises

javascript

Promise.reject(new TypeError("Error")).catch(e => {
  if (e instanceof Error) {
    console.log("A real error:", e.name);
  } else {
    console.log("Something odd:", e);
  }
});

Why this matters

JavaScript allows you to throw any value:

javascript

throw "string";
throw 404;
throw { message: "error" };

But such values are not instances of Error, so instanceof Error lets you tell "real" errors apart from arbitrary data.

Summary

Value being checked	Result of `instanceof Error`	Reason
`new Error()`	true	an Error instance
`new TypeError()`	true	an Error subclass
`new ValidationError()`	true	a custom Error
`"Error"`	false	a plain string
`{ message: "Error" }`	false	just an object
`null` / `undefined`	false	not an object

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