56 Design Patterns interview questions with answers

All Design Patterns interview questions

• What is the Single Responsibility Principle (SRP) and why does it matter?

• What is the Open/Closed Principle (OCP)? Give a real-world example.

• What is the Liskov Substitution Principle (LSP)? When is it violated?

• What is the Interface Segregation Principle (ISP)? Example in TypeScript?

• What is the Dependency Inversion Principle (DIP)? How does it relate to Dependency Injection?

• What are DRY, KISS, and YAGNI? When should — and shouldn't — you apply them?

• Why does SOLID matter in real-world software development?

• How do you apply SOLID principles in a real Next.js/React project?

• What is the Singleton pattern? What problems does it cause in modern environments?

• What is the Factory Method pattern? How does it differ from Simple Factory?

• What is the Abstract Factory pattern? When should you use it instead of Factory Method?

• What is the Builder pattern? What advantages does it have over constructors with many parameters?

• What is the Prototype pattern? When should you use it in JavaScript/TypeScript?

• What is the Object Pool pattern? Real-world use cases in Node.js and Go?

• What is Dependency Injection (DI)? What are the types of DI and what frameworks support it?

• When should you use Creational patterns? How do you choose the right one?

• What is the Adapter pattern? Real-world example in TypeScript?

• What is the Bridge pattern? How does it differ from Adapter?

• What is the Composite pattern? When should you use it?

• What is the Decorator pattern? How does it differ from Inheritance? TypeScript example?

• What is the Facade pattern? How is it applied in frontend development?

• What is the Proxy pattern? What are the common types of Proxy?

• What is the Flyweight pattern? When should you use it?

• When should you use Structural patterns? How do you choose the right one?

• What is the Observer pattern? How is it applied in JavaScript/React?

• What is the Strategy pattern? How does it compare to if/else and switch/case?

• What is the Command pattern? How is Undo/Redo implemented with it?

• What is the Template Method pattern? How does it differ from Strategy?

• What is Chain of Responsibility? How is it applied in Express.js middleware?

• What is the State pattern? How does it differ from a Finite State Machine (FSM)?

• What is the Mediator pattern? How does it differ from Observer, and why does it matter in microservices?

• What is the Iterator pattern? How does the JavaScript Iterator Protocol work?

• What is the Visitor pattern? When is it the right choice?

• What are MVC, MVP, and MVVM? Key differences and applications in modern frontend?

• What are the most important anti-patterns to avoid in software development? Real-world examples?

• What are the four pillars of object-oriented programming (OOP)?

• What is the Memento pattern? How does it support undo/redo?

• What is the Null Object pattern? How does it eliminate null checks?

• How do Association, Aggregation, and Composition differ in object relationships (UML)?

• What are cohesion and coupling? Why aim for high cohesion and low coupling?

• My shipping-fee function is a long if/else over carrier type, and every new carrier means editing it again. How do I refactor?

• My constructor takes 8 parameters, several optional, and call sites look like `null, null, true`. How should I fix it?

• We initialize different storage/payment implementations based on environment config. When is Factory Method the right tool, and when Abstract Factory?

• Why is Singleton often called an anti-pattern, especially in code that has unit tests?

• When an order becomes paid I must send mail, reserve stock, write a log, and push a notification. `markPaid` keeps growing. What do I do?

• I need to add caching, retry, and timing to a running service without editing its class. How?

• We call a payment gateway SDK from all over the codebase, so switching providers later means editing everywhere. How should we wrap it?

• My signup controller calls six services in sequence (create user, create profile, send mail, write audit...). Should I group them, and with what?

• What problem does the Repository pattern solve, and when is it a useless extra layer?

• How does Dependency Injection differ from Service Locator, and why is Service Locator usually rated lower?

• Three import jobs (CSV, Excel, XML) share one processing skeleton but differ in parsing. Template Method or Strategy?

• An editor feature needs undo/redo, and some operations must run in the background through a queue. How does Command help, and what should I watch out for?

• Our order lifecycle (pending → paid → shipped → completed / cancelled / refunded) is enforced by scattered ifs. How do I model it as a state machine?

• What does applying Dependency Inversion look like in a real project? Is extracting an interface enough?

• How do you apply the Open/Closed Principle in practice without falling into premature abstraction?

• How do you spot a codebase that over-uses design patterns, and how do you improve it incrementally instead of rewriting?