Demonstration of deep matching issues with errors

error.js

'use strict';

const chai = require('chai');
const expect = chai.expect;

const err1 = new Error('test error');
const err2 = new Error('test error');

it('should match exact errors', function() {
  expect(err1).to.equal(err1);
});

it('should NOT match exact different errors', function() {
  expect(err1).not.to.equal(err2);
});

it('should deep match different errors', function() {
  expect(err1).to.deep.equal(err2);
});

$ node --version
v6.11.2
$ npm list --depth=0
(redacted)
├── chai@4.1.1
├── mocha@3.5.0
├── mongoose@4.11.6
├── sinon@2.4.1
└── sinon-chai@2.12.0

$ mocha error.js 


  ✓ should match exact errors
  ✓ should NOT match exact different errors
  1) should deep match different errors

  2 passing (9ms)
  1 failing

  1)  should deep match different errors:

      AssertionError: expected [Error: test error] to deeply equal [Error: test error]
      + expected - actual

      
      at Proxy.assertEqual (node_modules/chai/lib/chai/core/assertions.js:1020:19)
      at Proxy.methodWrapper (node_modules/chai/lib/chai/utils/addMethod.js:57:25)
      at Context.<anonymous> (error.js:18:24)