[Bug]: variable beginning with 'mock' ist NOT hoisted.

[ikilinc]

Version

27.1.3

Steps to reproduce

const mock = jest.fn();
jest.mock('@sailplane/logger', () => {
  const mockLogger = jest.fn().mockImplementation(() => {
    return {
      debugObject: mock,
      debug: jest.fn(),
    };
  });

  return {
    Logger: mockLogger,
  };
});

Expected behavior

Test runs without error.

Actual behavior

throws error.

ReferenceError: Cannot access 'mock' before initialization

Debug log

● Test suite failed to run

ReferenceError: Cannot access 'mock' before initialization

  57 |   const mockLogger = jest.fn().mockImplementation(() => {
  58 |     return {
> 59 |       debugObject: mock,
     |                    ^
  60 |       debug: jest.fn(),
  61 |     };
  62 |   });

Additional context

No response

Environment

System:
    OS: Linux 5.10 Ubuntu 20.04.3 LTS (Focal Fossa)
    CPU: (8) x64 Intel(R) Xeon(R) CPU E3-1505M v5 @ 2.80GHz
  Binaries:
    Node: 16.14.0 - ~/.nvm/versions/node/v16.14.0/bin/node
    npm: 8.3.1 - ~/.nvm/versions/node/v16.14.0/bin/npm

[ahnpnl]

By default, jest.mock always hoists. This is not applied to variables starting with mock. Do you have any references saying that variable starting with mock should be the same?

EDITED: reference link jestjs/jest#8148 (comment)

this is a bug indeed

[smbkr]

This was discussed previously in 2020: #1088

Essentially, jest doesn't hoist variables starting with mock, but when hoisting mocked functions during initialisation, does not check if variables starting with mock are accessible when setting up the mocks.

@ahnpnl in the linked issue, you indicated that you were not interested in supporting this in ts-jest. Is this no longer the case? If so, where does this fit on your roadmap, and is any help required with this issue?

It's currently preventing me from migrating my projects from jest+babel to ts-jest. I'm creating mocks in ways like the following:

const mockSSM = {
    getParameter: jest.fn().mockReturnThis(),
    putParameter: jest.fn().mockReturnThis(),
    promise: jest.fn(),
}

jest.mock('aws-sdk', () => ({
    SSM: jest.fn().mockImplementation(() => mockSSM),
}))

[ahnpnl]

It's not about not interested in supporting this in ts-jest:) This can be achieved via modifying our AST transformer https://github.com/kulshekhar/ts-jest/blob/main/src/transformers/hoist-jest.ts to have this behavior. Any helps are welcome.

[smbkr]

Thanks, I've opened a PR #3400 with a failing e2e test, however I'm not entirely sure where to go from here - the tests for the AST transformer seem to already check for this behaviour.

[janeklb]

The following is a decent workaround until ts-jest supports this directly (if ever -- im still not sure if it's possible given how this behaviour is achieved in vanilla jest)

The key is to declare the variable using var (not let), and then initialise the value inside the mock factory

// eslint-disable-next-line no-var
var mockThing: jest.Mock;

jest.mock('./path-to/module', () => {
  mockThing = jest.fn();
  return {
    thing: mockThing,
  };
});

// OR more succinctly

jest.mock('./path-to/module', () => ({
  thing: (mockThing = jest.fn()),
}));

then you can do whatever you want in your test / beforeEach functions etc.

beforeEach(() => {
  mockThing.mockRejectedValue(new Error('bang!'));
});