Design Patterns quiz — 37 questions with answers and explanations

Answer: B

When many parties need to react to a change but the emitter doesn't need to know who they are: an event source, and subscribers that listen. Adding a new listener doesn't require changing the source.

Answer: C

When the concrete class is chosen at runtime based on data: createPaymentGateway(method) returns the right gateway. The caller only knows the interface, and adding a new payment method doesn't require changing the caller.

Answer: C

Provides a single simple interface over a multi-step system: checkout(cart) internally calls inventory check, fee calculation, order creation, email sending. Callers don't need to know the order or the details of each step.

Answer: B

One side may get undefined because the other module hasn't finished loading when it reads. Failures depend on load order, so they show up inconsistently and often surface somewhere unrelated to the cause.

Answer: A

No class should be forced to depend on methods it does not use. Large interfaces should be split into smaller role-based interfaces so each consumer only sees the part relevant to it.

Answer: A

A unit of code should be open for extension but closed for modification: add new behavior by writing new code rather than changing existing code. This is usually achieved through interfaces and polymorphism, not necessarily inheritance.

Answer: C

A class receives its dependencies from outside — usually via the constructor — instead of instantiating them internally. As a result it does not depend on a concrete implementation, can be swapped at the composition point, and can be tested with a fake.

Answer: C

An entity has its own identity and keeps that identity even as its fields change. A value object has no identity, two objects with the same value are considered equal, and it is usually designed to be immutable.

Answer: A

It separates traversal from storage structure: the caller uses a common interface to walk through elements without knowing whether the internals are an array, tree, or linked list. Changing the internal structure does not affect consumers.

Answer: C

Once created, an object does not change value, so sharing it across places and threads is safe without synchronization. To get a different value, create a new instance. The trade-off is higher memory cost under continuous updates.

Answer: C

Strategy: define a ShippingCalculator interface, one implementing class per carrier, then pick the class based on input data. Adding a new carrier is adding one file, not touching a function that already works.

Answer: B

It's hidden global state: a function using a singleton doesn't declare that dependency in its parameters, so reading the function signature doesn't tell you what it depends on. Tests are also hard because state can leak from a previous test.

Answer: B

Adapter (a.k.a. port/adapter): declare an interface shaped to your needs (sendSms(phone, message)), then make each provider a class implementing it. Switching providers is just injecting a different class.

Answer: C

The business layer doesn't depend on infrastructure details; both depend on an abstraction. For example, OrderService depends on the OrderRepository interface, while the Postgres class implements that interface.

Answer: C

Decorator: withLogging(withTiming(handler)) — each wrapper adds one concern that the original function knows nothing about. In JS this is a higher-order function, and Express or Next middleware is exactly this pattern.

Answer: C

Two practical benefits: queries are centralized in one place (when the schema changes you know where to fix it, and query names express business meaning), and the service is testable with a fake repository without a real DB.

Answer: C

A module should have one reason to change — that is, serve one group of users or one concern. Reading it as "does only one thing" leads to fragmenting into hundreds of files that each say nothing.

Answer: D

Take an object with named fields (createServer({ port, host, timeout })) — the caller reads and understands, doesn't worry about order, and adding a new option doesn't break the signature. A builder fits when construction has many steps or needs incremental validation.

Answer: B

State machine: declare a table of states and allowed transitions (pending → paid → shipped), and every change must go through a function that checks that table. Invalid transitions are blocked in a single place.

Answer: C

When the problem the pattern solves doesn't exist yet. An interface with exactly one implementation, a factory that creates exactly one type, an event bus for two modules — all are abstractions paid for in advance for a need that may never come.

Answer: B

Prefer composing small components with clear responsibilities (an object holding its collaborators) over multi-level inheritance. Inheritance forces you to take on all of the parent's behavior, including the parts you don't need.

Answer: B

God object: one class carrying every responsibility on a topic. Concrete consequences: every change touches it (constant merge conflicts), and it depends on so many things that any test has to set up the whole world.

Answer: D

It shapes data crossing a boundary (API responses, incoming payloads), separate from internal entities. That way adding a DB column doesn't automatically leak into the API, and changing the API doesn't force changing the model.

Answer: A

When substituting the parent class with a subclass breaks working code: the subclass narrows the accepted conditions, throws where the parent doesn't, or returns something outside the contract. Classic example: Square extends Rectangle and then setting width also changes height.

Answer: A

Proxy: an object implementing the same interface, handling the side concerns (cache, permission checks, timing, lazy loading) before forwarding the call to the real object. Callers don't know a proxy sits in between.

Answer: A

This is an anemic domain model: domain classes hold only data, while business rules live in services. The same rule gets duplicated across services and drifts apart, because there is no single place that owns it.

Answer: D

Mediator: components don't call each other directly but notify a central object, which coordinates them. The number of links drops from quadratic to linear, and each component can be tested independently.

Answer: B

Repository handles access to each type of object; Unit of Work tracks all changes in a business operation and persists them in one transaction. This lets many repositories participate in one operation while preserving integrity.

Answer: C

Adapter converts the interface of an existing class to match what the caller expects — the problem is a mismatch. Facade provides a simple interface in front of a complex subsystem — the problem is that it is too cumbersome to use directly.

Answer: D

When creation cost far exceeds usage cost — database connections, threads, network connections. For lightweight objects, pooling and reuse is usually slower and more complex than letting the GC do its job.

Answer: D

It inverts what is easy to extend: adding a new operation only requires a new visitor, but adding a new element type forces changes to every existing visitor. It fits when the set of element types is stable while the number of operations grows.

Answer: D

Observer calls directly within the same process and is usually synchronous: if a receiver fails, the publisher is affected, and there is no delay. Pub/sub goes through a broker, so the two sides are fully decoupled, but it adds latency and possible duplication.

Answer: C

It violates the Law of Demeter: an object should only talk to its immediate neighbors, not reach through multiple layers. This call chain depends on the internal structure of four classes, so any of them changing its structure breaks it.

Answer: B

Composite: individual elements and groups of elements implement the same interface, and a group can contain other groups. The caller treats both the same way, so recursive code doesn't need to distinguish cases.

Answer: C

When the two pieces are coincidentally similar rather than the same business rule. Merging them creates a dependency between two unrelated modules, and whenever one side changes, the shared function needs another conditional parameter, ending up harder to understand than two separate copies.

Answer: A

When the error rate crosses a threshold, it stops calling and fails immediately for a period of time. That way the caller doesn't pile up threads waiting to time out, and the failing side gets no extra load so it has a chance to recover.

Answer: A

Dependencies show up in the constructor signature, so you can't create an object missing what it needs, and the object is always valid right after creation. Tests can instantiate it directly, with no framework or reflection to set fields.