Comparing errors through deepEqual

[rpepato]

I'm implementing tests for my js project and I need to compare an error returned from a rejected promise with an expected one.

It turns out that, no mater what is the content of the error object, assert.deepEqual always returns true as long as the two instances being compared are Errors.

The same behaviour does not occur when comparing strings, for example (node version 4.1.1):

node --version
v4.1.1

> let assert = require("assert");
undefined

> assert.deepEqual(new Error("a"), new Error("b"));
undefined

> assert.deepEqual(new String("a"), new String("b"));
AssertionError: [String: 'a'] deepEqual [String: 'b']
    at repl:1:8
    at REPLServer.defaultEval (repl.js:164:27)
    at bound (domain.js:250:14)
    at REPLServer.runBound [as eval] (domain.js:263:12)
    at REPLServer.<anonymous> (repl.js:393:12)
    at emitOne (events.js:82:20)
    at REPLServer.emit (events.js:169:7)
    at REPLServer.Interface._onLine (readline.js:210:10)
    at REPLServer.Interface._line (readline.js:549:8)
    at REPLServer.Interface._ttyWrite (readline.js:826:14)

Reading the documentation for the deepEqual assertion, I didn't found any mentions regarding something special when comparing Errors.

Is this behaviour intended by design? Am I missing something important or should a consider another strategy to deeply compare two Error objects (instances + messages)?

[Trott]

This looks like it's caused by code in lib/assert.js that uses Object.keys() to get all the keys in the Error objects and then compares them. The Error objects have non-enumerable keys so Object.keys(new Error('a')); returns an empty array. So the errors are determined to be equal.

[Trott]

Possible fix: #3124

[Trott]

I'm afraid a fix for this is unlikely to land in Node.js. The API is now locked and users are encouraged to employ userland assertion libraries instead.

If you don't want to use a userland assertion library for whatever reason, a workaround for your case might look like this:

assert.deepEqual(error1.message, error2.message);
assert.deepEqual(error1.name, error2.name);

Or, more thoroughly (like, if you want the stack traces to match exactly, which you typically probably don't):

var ka = Object.getOwnPropertyNames(error1).concat(Object.getOwnPropertySymbols(error1));
var kb = Object.getOwnPropertyNames(error1).concat(Object.getOwnPropertySymbols(error2));

ka.sort();
kb.sort();

assert.deepStrictEqual(ka, kb);

for (var i = ka.length - 1; i >= 0; i--) {
  console.log(ka[i]);
  assert.deepEqual(error1[ka[i]], error2[ka[i]]); // use assert.deepStrictEqual() if that's what you want
}