Error.isError

An ESnext spec-compliant Error.isError shim/polyfill/replacement that works as far down as ES3.

This package implements the es-shim API interface. It works in an ES3-supported environment and complies with the proposed spec.

Getting started

npm install --save error.iserror

Usage/Examples

const isError = require('error.iserror');
var assert = require('assert');

assert.equal(isError(undefined), false);
assert.equal(isError(null), false);
assert.equal(isError({}), false);
assert.equal(isError([]), false);
assert.equal(isError(Error), false);
assert.equal(isError({ __proto__: Error.prototype, constructor: Error }), false);

assert.equal(isError(new Error()), true);
assert.equal(isError(new EvalError()), true);
assert.equal(isError(new RangeError()), true);
assert.equal(isError(new ReferenceError()), true);
assert.equal(isError(new SyntaxError()), true);
assert.equal(isError(new TypeError()), true);
assert.equal(isError(new URIError()), true);

if (typeof AggregateError === 'function') {
    assert.equal(isError(new AggregateError([])), true);
}
if (typeof SuppressedError === 'function') {
    assert.equal(isError(new SuppressedError()), true);
}

// note: engines that have `Symbol.toStringTag`, and lack structuredClone, and lack other brand-checking
// mechanisms, are only capable of brand-checking Error objects when they lack a `Symbol.toStringTag` property in their
// prototype chain. An object with it will give the wrong answer.
// This affects node 6 - 9, Chrome 49 - 97, Safari 10 - 14, Edge 15 - 18

assert.equal(isError({ __proto__: Error.prototype, constructor: Error, [Symbol.toStringTag]: 'Error' }), false);

const err = Object.assign(new Error(), { [Symbol.toStringTag]: 'Non-Error' });
Object.setPrototypeOf(err, null);
assert.equal(isError(err), true);

var shimIsError = require('error.iserror/shim');
const getPolyfill = require('error.iserror/polyfill');
var assert = require('assert');
/* when Error.isError is not present */
delete Error.isError;
var shimmed = shimIsError();

assert.equal(shimmed, getPolyfill());
assert.deepEqual(Error.isError(new Error()), isError(new Error()));

var shimIsError = require('error.iserror/shim');
var assert = require('assert');
/* when Error.isError is present */
var shimmed = shimIsError();

assert.equal(shimmed, Error.isError);
assert.deepEqual(Error.isError(new Error()), isError(new Error()));

Tests

Simply clone the repo, npm install, and run npm test