Testing with Jest and Typescript using Mocks

Question

I am working with Typescript and Jest to try to test some components for my Angular and Ionic application, but the issue is not limited to Angular or Ionic. As such, I am trying to get the mock functionality of Jest to work.

I am simply creating a dummy class that I want to try to mock the responses of functions to see if I can override the behaviour.

jest-mock.ts

export class AClass {
    constructor() { }

    GetOne():any {
        return  1;
    }

    GetTwo():any {
        return 2;
    }
}

jest-mock.spec.ts

import { AClass } from './jest-mock';

// const mockGet = jest.fn( () => { return 3; } );  // Tried this to return 3?
const mockGet = jest.fn();
jest.mock('./jest-mock', () => {
    return jest.fn().mockImplementation( () => {
        return { GetOne: mockGet };
    });
});

describe('Testing Jest Mock is working', () => {
    it('should support mocking out the component', () => {
        expect(mockGet).toBeTruthy();
        expect(mockGet).toBe(3);                // Mocked Value
    });
});

I am simply trying to create a test that can change the result of the function, so that my mock will be used by other real test code to provide results for testing.

When I try to create a class from the mock TestObject = new AClass();

TypeError: _jestMock.AClass is not a constructor

With the test defined above, I get the following error:

expect(received).toBe(expected)
    Expected value to be (using Object.is):
      3
    Received: 
      [Function mockConstructor]
    Difference:
       Comparing two different types of values. Expected number but received function.

Answer 1

While checking other references, I did manage to get the mock test working. I changed the jest-mocks.spec.ts to be:

jest.mock('./jest-mock', () => {
    return {                          // Define Function Mock Return Values
        GetOne: jest.fn( () => 3 )
    }
});
const MockObject = require('./jest-mock');

describe('mock function', () => {
    it('should create mock', () => {
        expect(jest.isMockFunction(MockObject.GetOne)).toBeTruthy();
    });

    it('should return mock values', () => {
        expect(MockObject.GetOne()).toBe(3);
        expect(MockObject.GetOne).toHaveBeenCalled();
        expect(MockObject.GetTwo).toBeUndefined();
    });
});