Testing Strategies And Best Practices

Thank you for providing detailed information about your software testing needs. Given your scenario, here is a recommended testing strategy for your JavaScript web application built with React:
Unit Testing: Since your team is already capable of performing unit tests, continue to do so, but aim to increase your test coverage. Focus particularly on the User Authentication Component. Unit tests should cover various scenarios such as successful login, incorrect password, invalid username, and other potential edge cases.
Jest, the testing tool you're using, works well with React and should serve your needs. Consider using the react-testing-library to write more maintainable tests by testing components "as a user would".
import { render, fireEvent } from '@testing-library/react';
import Login from './Login';
test('allows the user to login successfully', async () => {
  const fakeUserResponse = { token: 'fakeusertoken' };
  jest.spyOn(window, 'fetch').mockImplementationOnce(() => {
    return Promise.resolve({
      json: () => Promise.resolve(fakeUserResponse),
    });
  });
const { getByLabelText, getByText } = render();
fireEvent.change(getByLabelText(/username/i), {
    target: { value: 'chuck' },
  });
  fireEvent.change(getByLabelText(/password/i), {
    target: { value: 'norris' },
  });
fireEvent.click(getByText(/submit/i));
// Let's assert that the fetch function got called correctly
  expect(window.fetch).toHaveBeenCalledWith('/api/login', {
    body: JSON.stringify({
      username: 'chuck',
      password: 'norris',
    }),
    headers: {
      'Content-Type': 'application/json',
    },
    method: 'POST',
  });
// … other assertions
});
This is a simple example demonstrating how you can mock API responses using Jest and simulate user events with react-testing-library.
Performance Testing: Since you've expressed concerns about slow responses on the login page, implementing some form of performance testing could be beneficial.
Tools like Google Lighthouse or WebPageTest can be used for performance testing of your web application. These tools can provide insights into load times, time to interactive, and other key performance indicators that can help you pinpoint where you need to optimize your code.
Integration Testing: Although your team is currently focused on unit testing, you might consider exploring integration testing as well. These tests would help you understand how different components of your application work together, and they could provide valuable insight for a complex feature like User Authentication.
With React, you can use the same react-testing-library to simulate user workflows spanning multiple components. Jest, the testing tool you're already using, is a good tool for running these tests.
Automated Regression Testing: To ensure that new code changes do not break existing functionalities, establish a process for automated regression testing. Jest can also be used for this purpose. You can set it up to automatically run your test suite before you merge new changes into your main codebase.
Remember, the goal of testing isn't to eliminate every single bug—that's often not feasible in complex applications. Instead, aim to mitigate risk and catch the most important issues that could affect your users. Keep refining your testing strategy as your application grows and evolves.
You mentioned that you're under time constraints and have intermediate expertise. Given these factors, start with the recommendations that you feel most comfortable with, and incrementally adopt others as your team gains more confidence and skill in testing.
Happy testing!