Architecture & Design Patterns

Question 1

What is the MVC pattern?

Answer 1

MVC (Model-View-Controller) is a design pattern that separates an application into three components: Model (data), View (UI), and Controller (logic).

Question 2

What is a Service Layer?

Answer 2

A service layer encapsulates business logic, decoupling it from controllers and models for better modularity and testability.

Question 3

What is the Singleton pattern?

Answer 3

Singleton ensures a class has only one instance and provides a global point of access to it.

Question 4

What is the Factory pattern?

Answer 4

Factory is a creational pattern that provides an interface for creating objects without specifying their exact class.

Question 5

What is the Middleware pattern?

Answer 5

Middleware is a pattern used in web frameworks like Express where functions are chained to process requests/responses.

Question 6

What is Dependency Injection?

Answer 6

Dependency Injection is a technique where dependencies are passed into a component rather than being created inside it.

Question 7

What is Layered Architecture?

Answer 7

Layered Architecture organizes an application into layers like presentation, business, and data, each with specific responsibilities.

Question 8

What are common folder structures in Node.js?

Answer 8

Typical folder structures include folders for routes, controllers, services, models, middleware, and config.

Question 9

What is an advantage of MVC?

Answer 9

MVC separates concerns, making the application easier to manage, scale, and test.

Question 10

What is a disadvantage of MVC?

Answer 10

It can lead to bloated controllers and complex data flow if not implemented carefully.

Question 11

What is an advantage of the service layer?

Answer 11

Improves modularity by isolating business logic, making testing and reusability easier.

Question 12

What is a disadvantage of the Singleton pattern?

Answer 12

It introduces global state which can make testing and debugging harder.

Question 13

What is a best practice for using factories?

Answer 13

Use factories when object creation logic is complex or dependent on dynamic input.

Question 14

What is a use case for the Singleton pattern?

Answer 14

Database connections or logging instances where only one instance should exist.

Question 15

What is a use case for middleware?

Answer 15

Authentication, logging, and input validation in Express.js applications.

Question 16

What is the impact of Dependency Injection on system design?

Answer 16

It promotes loosely coupled and testable components, making systems more flexible and maintainable.

Question 17

Give an example of layered architecture.

Answer 17

An Express app with separate folders for API (routes/controllers), services (business logic), and models (database).

Question 18

What are architectural implications of using Singleton?

Answer 18

Can make system state shared globally, reducing flexibility and increasing risk of side effects.

Question 19

What is a performance benefit of the Singleton pattern?

Answer 19

Prevents repeated instantiation of heavy objects like DB clients, improving resource efficiency.

Question 20

What is a debugging challenge with the Singleton pattern?

Answer 20

Hidden global state can make it hard to track issues or reproduce bugs.

Question 21

What is a real-world tradeoff with MVC?

Answer 21

Easy to get started but can become rigid or confusing with complex apps if layering isn’t well-maintained.

Question 22

What is a potential gotcha with Dependency Injection?

Answer 22

Improper use can lead to overengineering or excessive indirection, making code harder to follow.

Question 23

What is a common interview question about design patterns?

Answer 23

Explain the difference between Singleton and Factory patterns and when you’d use them.

Question 24

What is a best practice for folder structures?

Answer 24

Organize by feature/module rather than type (e.g., users/ with its own routes, controllers, models).

Question 25

What is the role of controllers in MVC?

Answer 25

Controllers handle HTTP requests, interact with services/models, and return responses to the client.

Question 26

What is the role of services in a service layer?

Answer 26

Services contain business logic and can be reused across different controllers or apps.

Question 27

How does middleware improve fault tolerance?

Answer 27

Centralized middleware can catch and handle errors globally, improving app stability.

Question 28

What is a disadvantage of layered architecture?

Answer 28

May introduce complexity and performance overhead if overused for small apps.

Question 29

What is a use case for factory pattern in Node.js?

Answer 29

Creating database adapters or loggers based on environment or config at runtime.

Question 30

How does dependency injection improve testability?

Answer 30

Allows mocking of dependencies during tests, making unit testing easier.

Question 31

What is a folder structure gotcha in Node.js apps?

Answer 31

Over-nesting or inconsistent structure can confuse developers and hinder maintainability.

Question 32

What is a monitoring tip for layered architecture?

Answer 32

Log entry/exit points in each layer to trace request flow and pinpoint issues.

Question 33

What’s a common folder structure in Express apps?

Answer 33

/routes, /controllers, /services, /models, /middlewares, /utils, /config

Question 34

What is a common interview question on Dependency Injection?

Answer 34

How does dependency injection differ from service locator pattern?

Question 35

What is the impact of design patterns on scalability?

Answer 35

Well-applied patterns create modular, scalable architectures that are easier to maintain and extend.

Question 36

What’s a tradeoff when using the service layer?

Answer 36

Adds complexity for small apps but pays off in large, complex systems with reusable logic.

Question 37

What’s a benefit of centralized middleware?

Answer 37

Reduces duplication and keeps concerns like logging or validation separate from business logic.

Question 38

What’s a risk of improper Singleton usage?

Answer 38

Can accidentally introduce shared mutable state across requests, causing race conditions.

Question 39

How does MVC affect performance?

Answer 39

Adds abstraction layers, but proper separation improves long-term performance and maintainability.

Question 40

What is an example of dependency injection in JavaScript?

Answer 40

Passing a logger or DB client into a service class constructor instead of creating it inside.

Question 41

How does middleware chaining work in Express?

Answer 41

Each middleware calls `next()` to pass control to the next function in the stack.

Question 42

What’s a debugging tip for layered architecture?

Answer 42

Log each layer’s input and output, and use error-handling middleware for global issues.

Question 43

What’s the benefit of using factory pattern with configuration?

Answer 43

Allows easy switching between implementations (e.g., using Mongo or PostgreSQL) at runtime.

Question 44

What’s a disadvantage of deeply nested folder structures?

Answer 44

Can slow down onboarding and make file navigation cumbersome.

Question 45

What is an Express middleware function signature?

Answer 45

`(req, res, next)` – where `next` passes control to the next middleware.

Question 46

How does DI help in testing services?

Answer 46

You can inject mock dependencies to isolate and test service behavior without side effects.

Question 47

How does Factory pattern support the Open/Closed Principle?

Answer 47

By encapsulating object creation, you can add new types without modifying existing code.

Question 48

What’s the impact of folder structure on system design?

Answer 48

A well-thought structure improves clarity, maintainability, and team collaboration.