Testing with Jest

• Selective Auto-mocking
• Mock Class Methods

Selective Auto-mocking

Let's say we have a utilModule that looks like the following.

• The utilFunction is core to the functionality of the app and we want to maintain its behavior throughout our tests.
• callThirdPartyApi involves a http call to a third-party service. This is a method we want to mock to ease our automated tests.

export const utilModule = {
  utilFunction() {
    /**
     * a util function that is necessary for the app to work
     */
  }

  callThirdPartyApi() {
    /**
     * a http call to a third party
     */
  }
}

+++

We could auto-mock with the following:

jest.mock('aModule');

This will mock the entire module and replace all method implementations with jest.fn(). This isn't what we want, cause the original behavior of utilFunciton is lost.

+++

To solve this, we can set up jest auto-mock to retain selected functionalities:

jest.mock('aModule', () => {
  const originalModule = jest.requireActual('aModule');
  return {
    ...originalModule,
    callThirdPartyApi: jest.fn(),
  };
});

Mock Class Methods

In this example, we want to mock methodToMock but retain the behavior of utilFunction. The instance method methodToMock is called within the app and we want jest to mock its implementation to return a specific result.

class SomeClass {
  methodToMock() {}
}

export const moduleA = {
  SomeClass,
  utilFunction() {},
};

Using selective auto-mocking, we can preserve utilFunction. What we can do to mock instance class methods:

import { SomeClass } from 'aModule';
import { mocked } from 'ts-jest/utils';

jest.mock('aModule', () => {
  const originalModule = jest.requireActual('aModule');
  return {
    ...originalModule,
    SomeClass: jest.fn(),
  };
});

const mockedSomeClass = mocked(SomeClass, true);

// mock return value of class instance method
mockedSomeClass.prototype.methodToMock = jest.fn().mockReturnValue('return me');