Cannot access before initialization js ошибка

None of the answers above solved my problem, so here’s my solution:

var mockMyMethod: jest.Mock;

jest.mock('some-package', () => ({
  myMethod: mockMyMethod
}));

Something about using const before the imports feels weird to me. The thing is: jest.mock is hoisted. To be able to use a variable before it you need to use var, because it is hoisted as well. It doesn’t work with let and const because they aren’t.

answered Jun 21, 2021 at 19:31

The accepted answer does not handle when you need to spy on the const declaration, as it is defined inside the module factory scope.

For me, the module factory needs to be above any import statement that eventually imports the thing you want to mock.
Here is a code snippet using a nestjs with prisma library.

// app.e2e.spec.ts
import { Test, TestingModule } from '@nestjs/testing';
import { INestApplication } from '@nestjs/common';
import * as request from 'supertest';
import mockPrismaClient from './utils/mockPrismaClient'; // you can assert, spy, etc. on this object in your test suites.

// must define this above the `AppModule` import, otherwise the ReferenceError is raised.
jest.mock('@prisma/client', () => {
  return {
    PrismaClient: jest.fn().mockImplementation(() => mockPrismaClient),
  };
});

import { AppModule } from './../src/app.module'; // somwhere here, the prisma is imported

describe('AppController (e2e)', () => {
  let app: INestApplication;

  beforeEach(async () => {
    const moduleFixture: TestingModule = await Test.createTestingModule({
      imports: [AppModule],
    }).compile();

    app = moduleFixture.createNestApplication();
    await app.init();
  });
)};

answered Apr 15, 2021 at 19:07

MattMatt

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The problem that the documentation addresses is that jest.mock is hoisted but const declaration is not. This results in factory function being evaluated at the time when mocked module is imported and a variable being in temporal dead zone.

If it’s necessary to access nested mocked functions, they need to be exposed as a part of export object:

jest.mock('./my_class', () => {
  const mockMethod1 = jest.fn();
  const mockMethod2 = jest.fn();
  return {
    __esModule: true,
    mockMethod1,
    mockMethod2,
    default: {
      ...

This also applies to manual mocks in __mocks__ where variables are accessible inside a mock only.

answered Jan 4, 2021 at 7:43

Ricardo NoldeRicardo Nolde

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Example of using TypeScript with Jest and mockDebug.js module

jest.mock('debug', () => {
  global.mockDebug = jest.fn();
  return () => global.mockDebug;
});

// usage
describe('xxx', () => {
    test('xxx', () => {
        expect(global.mockDebug.mock.calls.toString()).toContain('ccc');
    })
});