toMatchObject not working to assert on the message of an error

[alem0lars]

What is the current behavior?

While discussing on issue #3601, we found that the method toMatchObject isn't working to assert on the message of a Error.

In fact, the following expectation doesn't work:

it("foo", () => {
  expect(new Error("octopus")).toMatchObject({
    message: expect.stringMatching(/oct/)
  });
  return expect(Promise.reject(new Error("octopus"))).rejects.toMatchObject({
    message: expect.stringMatching(/oct/)
  });
});

What is the expected behavior?

The expectation described above should pass.

[SimenB]

If using toMatchObject({message: 'octopus'}), it fails with a different error:

    Expected value to match object:
      {"message": "octopus"}
    Received:
      [Error: octopus]
    Difference:
    Compared values have no visual difference.

So I think this is a matter of toMatchObject to accepting Error as Object.

(we also should fix our message, as the diff is misleading with the current expect.stringMatching(/oct/) - it matches)

[SimenB]

Ironically, git bisect blames #4339.

[kpsroka]

Hi,
I got to the bottom of the issue: the comparison of Error objects and regular objects doesn't work because of a check in subsetEquality, which extended to return undefined for Errors in #4339. When comparing Errors with ordinary Object, the subsetEquality returns undefined and the flow continues to check on Object.prototype.toString equality, from where it returns false.

What I suggest, and will send a PR soon to include this, is to remove the !isObjectWithKeys(object) part of the check in subsetEquality. This will essentially change the behavior of this method to return true to match not only own properties of the passed object, but also of its prototype's properties. Since toMatchObject's docs don't specify whether comparison depends only on an object's own properties.

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