expect.objectContaining(...) mutates properties of the argument

[floyd-may]

🐛 Bug Report

When upgrading from react-scripts version 3.4.3 to version 4.0.0, jest was also upgraded. Tests that were behaving reasonably began failing. Upon closer inspection, I discovered that expect.objectContaining(...) was mutating properties of the argument! For example, this code would fail in the indicated location:

test('adds 1 + 2 to equal 3', () => {
  const thing = { foo: [{ val: 1}], bar: 2 };
  const originalThing = { ...thing };

  const json = JSON.stringify(originalThing);
  expect(originalThing).toEqual(expect.objectContaining(thing));

  expect(json).toEqual(JSON.stringify(thing)); // kaboom!
});

Expected behavior

The argument to objectContaining(...) is not mutated.

Link to repl or repo (highly encouraged)

Note: this link contains the very code described above 😄

https://repl.it/repls/TomatoVengefulAngles#example.test.js

[dylang]

This is the PR that seems to have broken it.
#10508

Looks like we're mutating the object?

this.sample[property] = objectContaining(
  this.sample[property] as Record<string, unknown>,
);

[danfriesen383]

Mikeenns74@gmail.com

…

On Fri, Oct 23, 2020, 5:24 PM Floyd May, ***@***.***> wrote: 🐛 Bug Report When upgrading from react-scripts version 3.4.3 to version 4.0.0, jest was also upgraded. Tests that were behaving reasonably began failing. Upon closer inspection, I discovered that expect.objectContaining(...) was mutating properties of the argument! For example, this code would fail in the indicated location: test('adds 1 + 2 to equal 3', () => { const thing = { foo: [{ val: 1}], bar: 2 }; const originalThing = { ...thing }; const json = JSON.stringify(originalThing); expect(originalThing).toEqual(expect.objectContaining(thing)); expect(json).toEqual(JSON.stringify(thing)); // kaboom! }); Expected behavior The argument to objectContaining(...) is not mutated. Link to repl or repo (highly encouraged) Note: this link contains the very code described above 😄 https://repl.it/repls/TomatoVengefulAngles#example.test.js — You are receiving this because you are subscribed to this thread. Reply to this email directly, view it on GitHub <#10689>, or unsubscribe <https://github.com/notifications/unsubscribe-auth/AOH4EJHBJZUZ3MHDMHATHGLSMHYCVANCNFSM4S5BVV5Q> .

[floyd-may]

Thank you all for addressing this so quickly! Do you know when I can expect a release that contains this fix?

[SimenB]

today or tomorrow, need to fix #10690 first

[peter-mouland]

fyi: for those waiting for a fix, for now, you can pin the expect package in the package.json :

  "resolutions": {
    "expect": "26.6.0"
  }

[SimenB]

Hoping to have a fix for #10690 this weekend, but if not I'll cut a release without it