Jest: expect(jest.fn().mock.calls)toStrictEqual([]) fails

[lpinca]

🐛 Bug Report

The following expectation

expect(jest.fn().mock.calls).toStrictEqual([])

fails

To Reproduce

$ cat jest.config.js 
'use strict';

module.exports = {
  testEnvironment: 'node'
};
luigi@imac:mock$ cat test.js 
'use strict';

const fn = jest.fn();

test('`jest.fn().mock.calls` is an `Array`', function() {
  expect(Array.isArray(fn.mock.calls)).toBe(true);
  expect([]).toStrictEqual([]);
  expect(fn.mock.calls).toStrictEqual([]);
});
luigi@imac:mock$ npx jest
 FAIL  ./test.js
  ✕ `jest.fn().mock.calls` is an `Array` (5ms)

  ● `jest.fn().mock.calls` is an `Array`

    expect(received).toStrictEqual(expected)

    Difference:

    Compared values have no visual difference.

       6 |   expect(Array.isArray(fn.mock.calls)).toBe(true);
       7 |   expect([]).toStrictEqual([]);
    >  8 |   expect(fn.mock.calls).toStrictEqual([]);
         |                         ^
       9 | });
      10 | 

      at Object.toStrictEqual (test.js:8:25)

Test Suites: 1 failed, 1 total
Tests:       1 failed, 1 total
Snapshots:   0 total
Time:        0.374s, estimated 1s
Ran all test suites.

Expected behavior

Test passes.

Link to repl or repo (highly encouraged)

Please provide either a repl.it demo or a minimal repository on GitHub.

Issues without a reproduction link are likely to stall.

Run npx envinfo --preset jest

Paste the results here:

npx: installed 1 in 1.068s

  System:
    OS: macOS 10.14.3
    CPU: (16) x64 Intel(R) Xeon(R) W-2140B CPU @ 3.20GHz
  Binaries:
    Node: 11.10.0 - /usr/local/bin/node
    npm: 6.7.0 - /usr/local/bin/npm

[jeysal]

Just checked, indeed [].__proto__ !== jest.fn().mock.calls.__proto__, so I would guess this is probably another symptom of https://github.com/facebook/jest/issues/2549

[lpinca]

Forgot to mention that this was working as intended in jest@<24.

[SimenB]

I agree it's a bug, but it's recommended to not rely on the internal structure of the mocks. If you want to verify the mock hasn't been called, you can do expect(jest.fn()).not.toHaveBeenCalled().

Just checked, indeed [].__proto__ !== jest.fn().mock.calls.__proto__, so I would guess this is probably another symptom of #2549

Equality checks shouldn't care about the prototype... So I'd say this is an issue with toStrictEqual, and not jest.fn().mock

[jeysal]

Equality checks shouldn't care about the prototype

Sorry, my statement was inaccurate - toStrictEqual does not care about the .__proto__, but the .constructor.
And [].constructor !== jest.fn().mock.calls.constructor, even though it seems like jest-mock initializes calls with just [], so it might be #2549 after all?

[lpinca]

@SimenB I'm actually using it in place of

expect(fn).toHaveBeenCalledTimes(number);
expect(fn).toHaveBeenNthCalledWith(1, arg1, agr2);
expect(fn).toHaveBeenNthCalledWith(2, arg1, agr2);

so I can either use that or create a new array from mock.calls.

expect(Array.from(fn.mock.calls)).toStrictEqual([
  [arg1, arg2],
  [arg1, arg2]
]);

Both are ok but I think that the documentation should mention that toStrictEqual() fails when used against mockFn.mock.calls.

[SimenB]

It's definitely a bug that should be fixed 🙂