How to use jest to mock out a private variable

Question

I am trying to write a unit test for a function like this:

export class newClass {
    private service: ServiceToMock;

    constructor () {
    this.service = new ServiceToMock()
    }

    callToTest () {
        this.service.externalCall().then(()=> {
        //Code to test
        })
    }
}

In order to test this piece of code I need to mock out service because it calls a function outside of the class but the problem is it's private.

How exactly do I mock out a private variable with jest? The class creates its own instance of it so is it even possible to mock out?

Answer 1

In your implementation you either import the service

implementation.js

import ServiceToMock from './serviceToMock.js';

implementation.spec.js

// import the already mocked service
import ServiceToMock from './serviceToMock.js';
import newClass from './implementation';

// auto-mock the service
jest.mock('./serviceToMock.js');

describe('newClass', () => {
  describe('somMethod', () => {
    beforeAll(() => {
      // it is recommended to make sure
      // the previous calls are cleared
      // before writing assertions
      ServiceToMock.prototype.externalCall.mockClear()

      // jest's auto-mocking will create jest.fn()
      // for each of the service's methods
      // and you will be able to use methods like .mockResolvedValue
      // to modify the mock behavior
      ServiceToMock.prototype.externalCall.mockResolvedValue(data);

      // call your method
      (new newClass).someMethod();
    });

    it('should call ServiceToMock.externalCall', () => {
      // and you can write assertions for the mocked methods
      expect(ServiceToMock.prototype.externalCall).toHaveBeenCalledWith();
    });
  });
});

working example without Types

Or have the implementation within the file

In that case you'll have to test the both classes as that is your Unit