Mocking and testing a class (static, instance and constructor methods) in Jest (Typescript)

app.test.ts

import { handler } from '../src/app'; // Actual part that will be tested
import { Model } from '../src/module'; // A module that will actually be imported as a mock object

jest.mock('../src/module'); // Jest will automatically hoist jest.mock calls to the top of the module (before any imports)

afterEach(() => { // Resetting the state after each test
  jest.resetAllMocks();
  jest.resetModules();
});

const input = {
  id: 'test',
  name: 'Mike',
};

describe('Mocking a class usage', () => {
  it('Should test a static method call', async () => {
    // Mocking the return of the call
    (Model.findOne as jest.Mock).mockResolvedValue({});

    await handler(input); // Calling the testable part

    // Asserting a static method call
    expect(Model.findOne).toHaveBeenCalledWith(expect.objectContaining({ id: input.id }));
  });

  it('Should test an instance method call', async () => {
    await handler(input); // Calling the testable part

    // Asserting an instance method call by accessing the prototype
    expect(Model.prototype.save).toHaveBeenCalled();
  });

  it('Should test a constructor', async () => {
    await handler(input); // Calling the testable part

    // Asserting the class constructor
    expect(Model).toHaveBeenCalledWith(expect.objectContaining({ name: input.name }));
  });

  it('Should test failure of instance method', async () => {
    // Mocking an exception being thrown from an instance method
    (Model.prototype.save as jest.Mock).mockRejectedValue(new Error('Something went wrong'));

    // Asserting that the rejection happened
    await expect(handler(input)).rejects.toThrow(Error);

    // Asserting an instance method call by accessing the prototype
    expect(Model.prototype.save).toHaveBeenCalled();
  });
});

app.ts

import { Model } from '../src/module'; // Importing a 3rd party module that needs to be mocked out

export async function handler(input: any): Promise<void> {
  const item = Model.findOne({ id: input.id }); // Calling a static method
  const instance = new Model({ name: input.name }); // Creating a class instance
  instance.save(); // Calling an instance method
}

module.ts

export class Model {
  constructor(values: any) {
    console.log('Creating an instance', values);
  }

  static findOne(filter: any): any {
    console.log('Finding an item', filter);
    return {
      id: 'example',
    };
  }

  save(): boolean {
    return true;
  }
}