💬 EN

Real-World Use Cases for This Testing Pattern

Why is this pattern useful beyond “hello world”? Here are practical scenarios where separating unit and integration tests, and using mocks as shown, pays off for real developers:

• Faster Feedback Loops By mocking dependencies (like getGreeting()), you can run tests in milliseconds instead of waiting for slow network calls or complex logic. This keeps your CI/CD pipeline fast and developers productive¹²³.
• Pinpointing Bugs If a test fails, you know exactly whether the problem is in your data-fetching logic (getGreeting) or your UI rendering (the component). This isolation makes debugging much easier¹².
• Safe Refactoring When you change how data is fetched (e.g., switch from a REST API to GraphQL), your component tests won’t break as long as the mocked interface remains the same. You can refactor internals with confidence¹².
• Testing Edge Cases You can easily simulate rare or hard-to-reproduce scenarios (like API errors, empty responses, or strange time values) by mocking return values, ensuring robust handling in your UI².
• Enforcing Business Rules Unit tests on helpers like getGreeting() ensure business logic (e.g., greeting by time of day) is correct, while component tests ensure that logic is displayed properly. This separation helps catch subtle errors (like the “Good buggy evening!” bug in your example)¹².
• Demonstrating Best Practices This pattern teaches junior devs the difference between unit and integration tests, and why mocking matters-skills that scale to large, real-world codebases¹²³.

Summary Table: When to Use This Pattern

Scenario	Why This Pattern Helps
Large, complex apps	Keeps tests fast and maintainable
APIs or asynchronous data	Avoids slow, flaky tests
Business logic changes	Prevents UI tests from breaking unnecessarily
Onboarding new team members	Makes code intent and test coverage clear
CI/CD pipelines	Reduces build times and false negatives

This approach is valuable for any team that wants reliable, maintainable, and fast tests-not just for toy demos, but for real-world Angular projects¹²³.

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But, like, can you give me some fun examples besides getGreeting and “wrapping greeting in <h1>”?

Absolutely! Here are some fun, concrete, real-world examples where this pattern (separating unit/integration tests and using mocks) is not just useful, but makes testing delightful and practical:

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1. Weather Widget

• Helper function: getWeatherEmoji(temp: number, condition: string): string
  • Returns ☀️ for sunny, ☔ for rain, ❄️ for snow, etc.
• Component: <app-weather [city]="city"></app-weather>
  • Fetches weather for a city and displays: It's 72°F in Austin! ☀️
• Unit test:
  • Mock the API response to test how the component renders ALL possible emojis, even if the real weather API never returns snow for Miami!
• Integration test:
  • Ensure the API fetch returns valid data for a real city.

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2. Shopping Cart Price Calculator

• Helper function: calculateTotal(items: CartItem[]): number
  • Applies discounts, taxes, etc.
• Component: <app-cart-summary [items]="cartItems"></app-cart-summary>
  • Shows: Subtotal: $100, Tax: $8, Total: $108
• Unit test:
  • Mock cartItems to simulate edge cases:
    • 0 items
    • 1000 items
    • Negative prices (bug!)
• Component test:
  • Mock calculateTotal to return a weird value, check that the UI displays "Total: $9999" correctly.

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3. User Avatar Uploader

• Helper function: isValidImage(file: File): boolean
  • Checks file type, size, dimensions.
• Component: <app-avatar-upload></app-avatar-upload>
  • Lets user pick an image, shows preview, disables submit if invalid.
• Unit test:
  • Mock File objects to test all validation paths (huge files, wrong format, etc).
• Component test:
  • Mock isValidImage to always return true or false, check UI reacts (shows error, disables button).

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4. Notification Bell

• Helper function: getUnreadCount(notifications: Notification[]): number
• Component: <app-notification-bell></app-notification-bell>
  • Shows 🔔 with badge: 🔔 3
• Unit test:
  • Test getUnreadCount with all-read, all-unread, and mixed arrays.
• Component test:
  • Mock getUnreadCount to return 99, check the badge shows 99+.

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5. Quiz App

• Helper function: scoreQuiz(answers: UserAnswer[], questions: Question[]): number
• Component: <app-quiz-result [score]="score"></app-quiz-result>
  • Shows: You scored 7 out of 10!
• Unit test:
  • Test scoreQuiz with perfect, zero, and partial scores.
• Component test:
  • Mock scoreQuiz to return a failing score, check the UI shows encouragement.

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Why This Is Fun and Useful

• Test anything, anytime: Want to see what happens if the user wins the lottery? Just mock it!
• Catch weird bugs: Like “negative prices” or “emoji for Martian weather.”
• Show off: Instantly demo how your UI looks for every possible scenario (even ones that are hard to trigger in real life).
• Speed: No waiting for real APIs or edge cases to happen.

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Bottom line: This pattern lets you test, demo, and bulletproof your app’s logic and UI for any situation you can imagine-not just the boring “hello world” stuff!

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Let’s do things that still require decoupling, but are a little more tightly scoped to within a pure SPA, no API backend.

Here are some tightly scoped, pure SPA (no backend) use cases where decoupling with mocks and separation of unit/integration tests are still valuable:

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1. Undo/Redo Functionality

• Helper/service: Manages the undo/redo stack.
• Component: Shows buttons and current state.
• Unit test: Mock the stack to simulate “no more undos” or “multiple redos,” ensuring the component disables/enables buttons correctly.
• Integration test: Use the real stack logic to ensure the UI updates as the user performs actions.

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2. Theme or Dark Mode Toggle

• Helper/service: Stores and retrieves the current theme from local storage.
• Component: Toggles theme and updates UI.
• Unit test: Mock the theme service to instantly switch themes, testing UI reaction without touching actual storage.
• Integration test: Use the real service to ensure the toggle persists and the correct classes are applied.

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3. In-App Notification System

• Helper/service: Queues and manages notifications.
• Component: Renders notification popups.
• Unit test: Mock the notification service to simulate bursts of notifications, ensuring the component displays and removes them as expected.
• Integration test: Use the real service to test notification flow as a user interacts with the app.

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4. Drag-and-Drop List Reordering

• Helper: Calculates new item order after a drag event.
• Component: Renders the list and handles drag events.
• Unit test: Mock drag events and test the helper’s logic for edge cases (e.g., dragging to the start/end).
• Integration test: Use real drag events to ensure the UI updates and the list order is correct.

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5. Form Validation and Error Display

• Helper: Validates fields and returns error messages.
• Component: Shows errors inline as the user types.
• Unit test: Mock validation results to check that the component displays the right errors for every possible case.
• Integration test: Use the real validation logic to ensure errors appear/disappear as the user interacts.

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These patterns keep your SPA codebase maintainable, let you test edge cases easily, and ensure UI and logic stay decoupled-even when everything runs in the browser⁴⁵⁶.