Mastering Jest with Page Object Model for Enhanced UI Testing

In modern web development, robust and maintainable end-to-end (E2E) tests are crucial for ensuring application quality. The Page Object Model (POM) is a powerful design pattern that significantly improves the readability, reusability, and maintainability of UI test automation code. This challenge will guide you through implementing POM principles within a Jest testing environment using TypeScript.

Problem Description

Your task is to implement a set of Jest tests for a simplified, hypothetical web application. You will leverage the Page Object Model to encapsulate the UI elements and user interactions for different pages of this application. This will allow your tests to be more declarative and less coupled to the specific implementation details of the UI.

Key Requirements:

1. Define Page Objects: Create TypeScript classes for each distinct page or significant component of the hypothetical application (e.g., LoginPage, HomePage, ProductDetailPage).
2. Encapsulate Locators and Actions: Within each page object class, define properties or methods for accessing UI elements (using selectors) and methods for performing common actions on those elements (e.g., enterUsername, clickLoginButton, getProductTitle).
3. Implement Jest Tests: Write Jest tests that utilize these page objects to simulate user flows and assert expected outcomes.
4. Use a Mocking Strategy: Since we won't be running a real browser, you'll need a strategy to mock the DOM and element interactions. For this challenge, we'll assume you're using a library like @testing-library/dom or a similar approach that allows you to select elements and simulate events within a simulated DOM environment.
5. Maintain Test Readability and Reusability: The goal is to have tests that are easy to understand and that the page objects can be reused across multiple test suites.

Expected Behavior:

Your tests should successfully simulate user interactions with the hypothetical application and verify that the application behaves as expected. For example, a login test should instantiate the LoginPage object, interact with its methods to enter credentials and click submit, and then assert that the user is redirected to the HomePage or sees a success message.

Edge Cases to Consider:

• Handling of missing elements (though for this mock-based challenge, this might be less critical if your mocking strategy is robust).
• Ensuring actions are performed in the correct order.

Examples

Let's consider a simplified hypothetical application with a login page and a home page.

Hypothetical Application Structure:

• Login Page:

  • Input field for username (#username)
  • Input field for password (#password)
  • Login button (#login-button)
  • Error message element (#error-message)

• Home Page:

  • Welcome message element (#welcome-message)
  • A list of products (represented by elements with class .product-item)

Example 1: Successful Login

Input (Test Scenario): A user attempts to log in with valid credentials.

Page Objects (Conceptual):

// LoginPage.ts
class LoginPage {
  // Mocking methods to find elements
  getUsernameInput(): HTMLInputElement | null { /* ... */ }
  getPasswordInput(): HTMLInputElement | null { /* ... */ }
  getLoginButton(): HTMLButtonElement | null { /* ... */ }
  getErrorMessage(): HTMLDivElement | null { /* ... */ }

  // Mocking methods to interact with elements
  enterUsername(username: string): void { /* ... */ }
  enterPassword(password: string): void { /* ... */ }
  clickLoginButton(): void { /* ... */ }
  getErrorMessageText(): string | null { /* ... */ }
}

// HomePage.ts
class HomePage {
  getWelcomeMessage(): HTMLHeadingElement | null { /* ... */ }
  getProductItems(): NodeListOf<Element> { /* ... */ }
}

Jest Test (Conceptual):

// login.test.ts
import { LoginPage } from './LoginPage';
import { HomePage } from './HomePage';

describe('Login Functionality', () => {
  it('should allow a user to log in with valid credentials', () => {
    // Setup mock DOM and initial state

    const loginPage = new LoginPage();
    loginPage.enterUsername('testuser');
    loginPage.enterPassword('password123');
    loginPage.clickLoginButton();

    // In a real scenario, this would involve checking for navigation or a state change
    // For this mock, we'll assume a successful login renders the home page elements
    const homePage = new HomePage();
    expect(homePage.getWelcomeMessage()?.textContent).toBe('Welcome, testuser!');
    expect(homePage.getProductItems().length).toBeGreaterThan(0); // Assume products are displayed
  });
});

Explanation: The test instantiates LoginPage, uses its methods to fill in the username and password fields, and clicks the login button. It then asserts that elements on the HomePage are visible and contain expected content, simulating a successful redirection or state change.

Example 2: Failed Login

Input (Test Scenario): A user attempts to log in with invalid credentials.

Jest Test (Conceptual):

// login.test.ts (continued)
describe('Login Functionality', () => {
  // ... (previous test)

  it('should display an error message for invalid credentials', () => {
    // Setup mock DOM and initial state

    const loginPage = new LoginPage();
    loginPage.enterUsername('wronguser');
    loginPage.enterPassword('wrongpassword');
    loginPage.clickLoginButton();

    expect(loginPage.getErrorMessage()?.textContent).toBe('Invalid username or password.');
    // Ensure the home page elements are NOT visible
    const homePage = new HomePage(); // Still need to be able to check for its absence/state
    expect(homePage.getWelcomeMessage()).toBeNull();
  });
});

Explanation: This test also uses LoginPage methods. After attempting login with incorrect credentials, it asserts that the error message element on the LoginPage is visible and displays the correct error text. It also verifies that the elements expected on the HomePage are not present, indicating a failed login.

Constraints

• You must use TypeScript.
• Your solution should be structured into at least two page object classes and one Jest test file.
• Your tests should be designed to be readable and understandable without deep knowledge of the underlying DOM manipulation.
• You are free to choose your preferred method for mocking the DOM and element interactions (e.g., jest-dom, @testing-library/dom, manual DOM manipulation with Jest's JSDOM environment).
• Focus on the structure and implementation of the Page Object Model, not on the intricacies of a specific DOM mocking library.

Notes

• DOM Mocking: For this challenge, imagine you have a function renderDOM(htmlString: string) that sets up your JSDOM environment with the provided HTML. You would then use standard DOM APIs (document.querySelector, element.click(), input.value = ..., etc.) or a library like @testing-library/dom to interact with these elements.
• Selectors: For simplicity, use CSS selectors (e.g., #id, .class).
• Actions: Think about common user actions: typing into an input, clicking a button, selecting from a dropdown, etc.
• Assertions: Use Jest's built-in matchers (.toBe, .toEqual, .toHaveTextContent, .toBeNull, .toBeTruthy, etc.) to verify outcomes.
• Structure: Organize your page objects and tests into logical files.