This comprehensive guideline assists in decision-making regarding testing at different levels - from Unit to End-to-end. It offers practical practices, rules of thumb, and heuristics to answer key questions, such as when to create a Unit Test versus an End-to-end test and the criteria for End-to-end tests. Focus on Unit Tests for Testing Individual Units or Modules Start by writing unit tests that accompany the code, testing at the functional unit level. Unit tests, which mock or fake external services, are faster to execute, offering quick feedback for easier issue debugging. Example: test('calculates total correctly', () => {
  const result = calculateTotal(10, 20);
  expect(result).toBe(30);
}); Use End-to-End UI Tests for Testing the Entire Application Flow End-to-end tests mimic end-user actions, testing the application holistically, including the user interface. While slower, these tests help identify issues with the application flow. Example: test('user can successfully complete checkout process', () => {
  navigateToCheckout();
  fillOutShippingDetails();
  proceedToPayment();
  completePayment();
  expect(orderConfirmation()).toBeVisible();
}); Consider the Complexity of the Feature Being Tested The complexity of the feature determines whether to rely on unit tests or opt for end-to-end UI tests. Simple features may be adequately tested with unit tests, while complex features with multiple module interactions benefit from end-to-end testing. Balance the Time and Resources Available for Testing While end-to-end UI tests offer comprehensive coverage, they can be time-consuming. Developers and testers should weigh the time investment against the additional testing coverage benefits. When to Use Unit Tests Testing individual units or modules of the application Faster execution and quick feedback Suitable for testing simple features When to Use End-to-End UI Tests Testing the entire application flow Mimicking end-user actions Slower execution but provides comprehensive testing Suitable for testing complex features and critical functionalities Prioritize based on risk and critical paths of the application Suitable for covering error-prone areas not apparent through other testing forms Steps to Define E2E Test Scenarios Based on User Story Identify critical user stories to pinpoint essential features for end-to-end testing. Start with user stories: Identify and test critical paths users take through the application for end-to-end scenarios. Consider critical paths: Prioritize testing based on risk, focusing on high-risk features or areas prone to errors. Evaluate risk (Leverage Risk-Based Testing): Concentrate end-to-end testing efforts on critical functionalities or high-risk features. Balance time and resources: Prioritize user goals, ensuring empathy in testing efforts to identify user-centric issues. Think like an end-user: Example: javascriptCopy code// E2E Test Scenario Example
test('user can easily navigate and complete purchase', async () => {
  await navigateToProductPage();
  await addToCart();
  await navigateToCheckout();
  await fillOutPaymentDetails();
  await completePurchase();
  expect(orderConfirmation()).toBeVisible();
}); By following these guidelines, developers and testers can make informed decisions about when and how to conduct unit and end-to-end testing, optimizing their testing efforts.

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