Overview

This PR turns AxiosError into an instance of Error that returns true when checked with Node's utils.types.isNativeError.

Related Issue: #5394

Why this PR?

This PR is useful if axios is used in Node.js and the user wants to check if an instance of AxiosError, that moved across realms, is an Error. In practice this might happen if Node.js worker threads or vms are used.

A call to X instanceof Error compares X's prototype to the Error constructor in the current realm and thus returns false if the object was created in another realm. Node's isNativeError can be used to address this:

const assert = require('assert');
const vm = require('vm');
const {isNativeError} = require('util/types');

const context = vm.createContext({});
const differentRealmErr = vm.runInContext('new Error()', context);
assert(!(differentRealmErr instanceof Error)); // instanceof returns false
assert(isNativeError(differentRealmErr)); // isNativeError returns true

Node uses isNativeError(e) || e instanceof Error internally to check for errors.

What does the PR actually do?

Currently, the AxiosError constructor calls Error.call(this) but it does not return its result. With the change the result is returned. Here is a short illustration of the difference:

const assert = require('assert');
const {isNativeError} = require('util/types');

function MyErr() {
    return Error.call(this);
}
assert(isNativeError(new MyErr())); // instances of `MyErr` are native errors
assert(new MyErr() instanceof Error); // they are also instance of `Error`

function MyBrokenErr() {
    Error.call(this);
    this.__proto__ = Error.prototype;
}
assert(!isNativeError(new MyBrokenErr())); // instances of `MyBrokenErr` are _not_ native errors
assert(new MyBrokenErr() instanceof Error); // they are instance of `Error` however

Testing

I wrote a test which should only run in NodeJS environments. Sadly, I could not get the tests under test/specs running. The contributing guidelines mention grunt but I did not get it working. (Is this still up to date?) When running npm run test on a freshly checked-out repo I had several failures.