how to deal with jest mock function of a module and typescript type

Question

I use ts-jest and jest to write my testing files with typescript.

I am confused how to typing the mock function of a module.

Here is my code:

./module.ts:

import {IObj} from '../interfaces';

const obj: IObj = {
  getMessage() {
    return `Her name is ${this.genName()}, age is ${this.getAge()}`;
  },

  genName() {
    return 'novaline';
  },

  getAge() {
    return 26;
  }
};

export default obj;

./module.test.ts:

import * as m from './module';

describe('mock function test suites', () => {

  it('t-1', () => {
    // I think the jest.Mock<string> type here is not correct.
    m.genName: jest.Mock<string> = jest.fn(() => 'emilie'); 
    expect(jest.isMockFunction(m.genName)).toBeTruthy();
    expect(m.genName()).toBe('emilie');
    expect(m.getMessage()).toEqual('Her name is emilie, age is 26');
    expect(m.genName).toHaveBeenCalled(); 

  });

});

how to type the mock function genName of module m?

typescript give me an error here:

Error:(8, 7) TS2540:Cannot assign to 'genName' because it is a constant or a read-only property.

Answer 1

This is how I have solved the same problem and how I do all of my mocking and spying now.

import * as m from './module';

describe('your test', () => {
  let mockGenName;

  beforeEach(() => {
    mockGenName = jest.spyOn(m, 
      'genName').mockImplemetation(() => 'franc');
  })

  afterEach(() => {
    mockGenName.mockRestore();
  })


  test('your test description', () => {
    // do something that calls the genName function
    expect(mockGenName).toHaveBeenCalledTimes(1);
  })

})

With this setup, you can programmatically change the implementation of the mock for different tests, and then assert that the function was called and what it was called with, all while clearing your mock in between tests and after all tests.