Mocks broken after updating to Jest 26

Question

I just updated react-scripts to 4.0 which includes Jest@26 and some accompanying test packages here's the package.json diff:

After upgrading, some Jest mocks have begun to fail. It seems like the mocked return value is just undefined? Am I missing something? Here's one of the failing mocks

import useFetch from "use-http";

jest.mock("use-http", () => ({
    __esModule: true,
    default: jest.fn()
}));

describe("the user context", () => {
    beforeAll(() => {
        useFetch.mockReturnValue({
            get: async () => Promise.resolve({ foo: 666 }),
            response: { ok: true }
        });
    });

Tests that try to utilize the 'get' method fail with:

TypeError: Cannot destructure property 'get' of '(0 , _useHttp.default)(...)' as it is undefined.

And another that isn't default, doesn't import the package for one-time mocks:

jest.mock("_hooks", () => ({
    useBaseUrls: jest.fn().mockReturnValue({
        app: "bar"
    })
}));

Tests that access the 'app' property throw TypeError: Cannot read property 'app' of undefined

Answer 1

Jest 26 changed the default behavior of resetMocks to true, which resets mock state before each test.

You can revert to the prior behavior by disabling resetMocks in package.json

  "jest": {
    "resetMocks": false
  }

A discussion to change the default back again is currently an open issue on their Github: https://github.com/facebook/create-react-app/issues/9935