Updating to next 8 break Date mocks in tests

Bug report

Describe the bug

Upgrading nextJS from 7.0.2 to the latest version of next 8.0.4 break tests that rely on mocking Date.now(). These tests are written in files that do not import nextjs

To Reproduce

Clone sample repo: https://github.com/ssylvia/next-test-date-mock

NextJS Update Breaks Date Mocks

Steps to reproduce:

1. yarn install
2. Run yarn test --no-cache.
3. Notice that all test pass
4. Upgrade to nextjs 8
5. Run yarn test --no-cache.
6. See failing tests because Date mock no longer works

Test that fails

Source method (src/utils/index.ts)

export const getCurrentTime = () => {
  const time = Date.now();
  return time;
};

Test (src/utils/index.ts)

test("getCurrentTime", () => {
  jest.spyOn(Date, "now").mockImplementation(() => 0);
  expect(getCurrentTime()).toBe(0);
});

Expected behavior

Date.now() should continue to be mocked

Screenshots

If applicable, add screenshots to help explain your problem.

System information

• OS: MacOS
• Version of Next.js: 8.0.4
• Node: v10.13.0

[lfades]

Removing corejs does fix the issue, I think it shouldn’t introduce other issues, at least not in test mode:

{
  "env": {
    "development": {
      "presets": ["next/babel"]
    },
    "production": {
      "presets": ["next/babel"]
    },
    "test": {
      "presets": [
        [
          "next/babel",
          {
            "preset-env": {
              "modules": "commonjs"
            },
            "transform-runtime": {
              "corejs": false
            }
          }
        ]
      ]
    }
  }
}

[SebastienGllmt]

I ran into this problem also today with latest versions of all related libs.

The bug seems be that Date and global.Date get treated differently.

Date.now = jest.spyOn(global.Date, 'now').mockImplementation(() => 5);
console.log(global.Date.now()); // returns 5 as  expected
console.log(Date.now()); // returns 1570401251960 (mock had no affect)

I changed my babel.config.js to

module.exports = function (api) {
  return {
      // other config omitted
      presets: [
        '@babel/plugin-transform-runtime',
        {
            // CoreJS breaks Jest mocks for some reason
            corejs: api.env('jest') ? false : 2,
            helpers: true,
            regenerator: true
        }
      ]
  }
}

and this seems to fixed it.