Only treat native errors as errors

* Remove is-error dependency
* Document edge case where `error instanceof Error` can be true, yet AVA does not recognize `error` as an error

See also #2911 for an earlier attempt.

docs/08-common-pitfalls.md

@@ -4,6 +4,18 @@ Translations: [Français](https://github.com/avajs/ava-docs/blob/main/fr_FR/docs
 
 If you use [ESLint](https://eslint.org), you can install [eslint-plugin-ava](https://github.com/avajs/eslint-plugin-ava). It will help you use AVA correctly and avoid some common pitfalls.
 
+## Error edge cases
+
+The `throws()` and `throwsAsync()` assertions use the Node.js built-in [`isNativeError()`](https://nodejs.org/api/util.html#utiltypesisnativeerrorvalue) to determine whether something is an error. This only recognizes actual instances of `Error` (and subclasses).
+
+Note that the following is not a native error:
+
+```js
+const error = Object.create(Error.prototype);
+```
+
+This can be surprising, since `error instanceof Error` returns `true`.
+
 ## AVA in Docker
 
 If you run AVA in Docker as part of your CI, you need to fix the appropriate environment variables. Specifically, adding `-e CI=true` in the `docker exec` command. See [#751](https://github.com/avajs/ava/issues/751).

lib/assert.js

@@ -1,5 +1,6 @@
+import {isNativeError} from 'node:util/types';
+
 import concordance from 'concordance';
-import isError from 'is-error';
 import isPromise from 'is-promise';
 
 import concordanceOptions from './concordance-options.js';
@@ -163,7 +164,7 @@ function validateExpectations(assertion, expectations, numberArgs) { // eslint-d
 // Note: this function *must* throw exceptions, since it can be used
 // as part of a pending assertion for promises.
 function assertExpectations({assertion, actual, expectations, message, prefix, savedError}) {
-	if (!isError(actual)) {
+	if (!isNativeError(actual)) {
 		throw new AssertionError({
 			assertion,
 			message,

lib/serialize-error.js

@@ -5,7 +5,6 @@ import {isNativeError} from 'node:util/types';
 
 import cleanYamlObject from 'clean-yaml-object';
 import concordance from 'concordance';
-import isError from 'is-error';
 import StackUtils from 'stack-utils';
 
 import {AssertionError} from './assert.js';
@@ -160,7 +159,7 @@ export function tagWorkerError(error) {
 const isWorkerError = error => workerErrors.has(error);
 
 export default function serializeError(origin, shouldBeautifyStack, error, testFile) {
-	if (!isError(error) && !isWorkerError(error)) {
+	if (!isNativeError(error) && !isWorkerError(error)) {
 		return {
 			avaAssertionError: false,
 			nonErrorObject: true,

package.json

@@ -104,7 +104,6 @@
 		"globby": "^13.2.1",
 		"ignore-by-default": "^2.1.0",
 		"indent-string": "^5.0.0",
-		"is-error": "^2.2.2",
 		"is-plain-object": "^5.0.0",
 		"is-promise": "^4.0.0",
 		"matcher": "^5.0.0",

types/assertions.d.cts

@@ -103,12 +103,12 @@ export type Assertions = {
 	snapshot: SnapshotAssertion;
 
 	/**
-	 * Assert that the function throws [an error](https://www.npmjs.com/package/is-error). If so, returns the error value.
+	 * Assert that the function throws a native error. If so, returns the error value.
 	 */
 	throws: ThrowsAssertion;
 
 	/**
-	 * Assert that the async function throws [an error](https://www.npmjs.com/package/is-error), or the promise rejects
+	 * Assert that the async function throws a native error, or the promise rejects
 	 * with one. If so, returns a promise for the error value, which must be awaited.
 	 */
 	throwsAsync: ThrowsAsyncAssertion;
@@ -295,7 +295,7 @@ export type SnapshotAssertion = {
 
 export type ThrowsAssertion = {
 	/**
-	 * Assert that the function throws [an error](https://www.npmjs.com/package/is-error). If so, returns the error value.
+	 * Assert that the function throws a native error. If so, returns the error value.
 	 * The error must satisfy all expectations. Returns undefined when the assertion fails.
 	 */
 	<ErrorType extends ErrorConstructor | Error>(fn: () => any, expectations?: ThrowsExpectation<ErrorType>, message?: string): ThrownError<ErrorType> | undefined;
@@ -306,13 +306,13 @@ export type ThrowsAssertion = {
 
 export type ThrowsAsyncAssertion = {
 	/**
-	 * Assert that the async function throws [an error](https://www.npmjs.com/package/is-error). If so, returns the error
+	 * Assert that the async function throws a native error. If so, returns the error
 	 * value. Returns undefined when the assertion fails. You must await the result. The error must satisfy all expectations.
 	 */
 	<ErrorType extends ErrorConstructor | Error>(fn: () => PromiseLike<any>, expectations?: ThrowsExpectation<ErrorType>, message?: string): Promise<ThrownError<ErrorType> | undefined>;
 
 	/**
-	 * Assert that the promise rejects with [an error](https://www.npmjs.com/package/is-error). If so, returns the
+	 * Assert that the promise rejects with a native error. If so, returns the
 	 * rejection reason. Returns undefined when the assertion fails. You must await the result. The error must satisfy all
 	 * expectations.
 	 */