JEST: React Testing Library

SPY on a class constructor using Jest

Today, I would like to document and share with the community a few tips on testing.

As a developer working for a company with a large engineering team divided into sub-teams or squads where every member is working on the same project or code base but different features, having to test your work before merging is a high priority.

And so, I have been pretty involved in writing unit tests for my lines of code recently.

I had worked on a bug on the Login component and needed to mock CognitoUser for the unit test.

CognitoUser is a constructor class, and I need to spy on it.

firstly I import with an alias from amazon-cognito-identity-js

import * as CognitoClasses from 'amazon-cognito-identity-js';

then in my assertion, create a constcognitoUserSpy as in the code below, and notice how I had to mock return values for setAuthenticationFlowType and authenticateUser.

const cognitoUserSpy = jest
    .spyOn(CognitoClasses, 'CognitoUser')
    .mockReturnValue({
        setAuthenticationFlowType: () => {},
        authenticateUser: () => {}
    });

// write your expect here
// expect()

// restore/clear all mocks

Finally, don’t forget to restore the mock created with jest.spyOn , else it causes memory leak. You can do this inside the afterEach(() => {}) or after your assert or expect method.

cognitoUserSpy.mockRestore();

Thanks.