Javascript Design Patterns Interview Questions

The Singleton pattern ensures a class has only one instance and provides global access to it. In modern JavaScript, it's implemented using a combination of class definition and module system. The implementation should include private instance storage, lazy initialization, and proper export mechanics. Consider thread safety in concurrent environments, implement proper instance checking, and ensure proper cleanup in testing environments. Handle proper initialization timing and instance access synchronization.

The Observer pattern enables objects to notify multiple observers about changes in their state. Implementation involves maintaining a list of observers, methods for adding/removing observers, and notification mechanics. This pattern is fundamental for event-driven programming and reactive systems. Consider memory management for observer references, implement proper error handling in notification chains, and ensure proper cleanup of observer relationships.

The Factory pattern provides an interface for creating objects without specifying their exact classes. It's particularly useful when object creation involves complex logic or configuration. The pattern encapsulates instantiation logic, enables runtime object creation decisions, and promotes loose coupling. Consider inheritance hierarchies, implement proper error handling for invalid configurations, and document factory interface requirements.

The Module pattern uses closures and immediately invoked function expressions (IIFE) to create private scope and expose public API. Modern JavaScript implements this through ES modules with import/export statements. The pattern enables information hiding, state encapsulation, and dependency management. Consider proper exposure of public interface, implement proper initialization, and handle cleanup requirements.

The Strategy pattern enables selecting algorithms at runtime by defining a family of interchangeable algorithms. Implementation involves creating a strategy interface and concrete strategy implementations. This pattern promotes code reuse and maintainability by isolating algorithm implementations. Consider proper strategy selection mechanism, implement error handling for invalid strategies, and document strategy requirements.

The Decorator pattern allows adding new behaviors to objects dynamically by placing them inside wrapper objects. Implementation involves creating a common interface and series of decorator classes. This pattern provides flexible alternative to subclassing for extending functionality. Consider proper composition of decorators, implement proper method delegation, and maintain proper object state.

The Command pattern encapsulates method invocation by treating it as an object. Implementation involves creating command objects with execute methods and optional undo functionality. This pattern enables operation queuing, logging, and undo mechanisms. Consider proper command state management, implement error handling, and handle command lifecycle properly.

The Proxy pattern provides a surrogate object that controls access to another object. Implementation in JavaScript often uses the Proxy object for intercepting operations. This pattern enables access control, lazy initialization, and logging. Consider performance implications, implement proper validation, and handle method delegation appropriately.

The Chain of Responsibility pattern passes requests along a chain of handlers until one processes the request. Implementation involves creating handler classes with next references. This pattern decouples senders and receivers and enables dynamic handler chains. Consider proper chain construction, implement error handling, and handle chain modification properly.

The Mediator pattern provides a central point for object communication, reducing direct connections between objects. Implementation involves creating a mediator object that coordinates interactions. This pattern reduces coupling and simplifies object relationships. Consider proper event handling, implement error management, and maintain proper object references.

The Composite pattern treats individual objects and compositions uniformly through a common interface. Implementation involves creating component interface and composite/leaf classes. This pattern simplifies client code when working with tree structures. Consider proper child management, implement traversal methods, and handle proper cleanup.

The State pattern allows objects to alter behavior when internal state changes by encapsulating state-specific behavior in separate classes. Implementation involves creating state classes and context class. This pattern localizes state-specific behavior and makes state transitions explicit. Consider proper state transition management, implement proper cleanup, and handle invalid states.

The Template Method pattern defines algorithm skeleton in base class while letting subclasses override specific steps. Implementation involves creating abstract base class with template method. This pattern enables code reuse and ensures consistent algorithm structure. Consider proper abstraction level, implement hooks for customization, and document override requirements.

The Adapter pattern converts interface of one class into interface expected by clients. Implementation involves creating adapter class that implements target interface. This pattern enables integration of incompatible components. Consider proper interface mapping, implement error handling, and maintain proper object lifecycle.

The Iterator pattern provides way to access elements of collection sequentially without exposing underlying representation. Implementation involves creating iterator interface with next/hasNext methods. This pattern simplifies collection traversal and enables different iteration strategies. Consider proper iteration state management, implement proper cleanup, and handle concurrent modification.

The Builder pattern constructs complex objects step by step through fluent interface. Implementation involves creating builder class with configuration methods. This pattern separates construction from representation and enables fine-grained object creation control. Consider proper validation, implement error handling, and maintain proper object state.

The Facade pattern provides unified interface to complex subsystem. Implementation involves creating facade class that wraps subsystem functionality. This pattern reduces coupling between clients and subsystem components. Consider proper abstraction level, implement error handling, and maintain proper documentation.

The Flyweight pattern shares common state between multiple objects to reduce memory usage. Implementation involves separating intrinsic and extrinsic state. This pattern is useful when dealing with large numbers of similar objects. Consider proper state separation, implement proper caching, and handle object creation efficiently.

The Bridge pattern separates abstraction from implementation allowing them to vary independently. Implementation involves creating abstraction and implementor hierarchies. This pattern is useful when dealing with platform-specific implementations. Consider proper interface design, implement proper delegation, and maintain proper documentation.

The Visitor pattern lets you add operations to objects without modifying them by placing operation logic in separate visitor objects. Implementation involves creating visitor interface and concrete visitors. This pattern enables separation of concerns and open-closed principle. Consider proper double dispatch, implement error handling, and maintain proper documentation.

The Memento pattern captures and externalizes object's internal state for later restoration. Implementation involves creating memento class and caretaker for managing mementos. This pattern enables undo/redo functionality and state preservation. Consider proper state encapsulation, implement proper cleanup, and handle memory management.

The Abstract Factory pattern provides interface for creating families of related objects without specifying concrete classes. Implementation involves creating abstract factory interface and concrete factories. This pattern ensures object compatibility within product families. Consider proper factory selection, implement error handling, and maintain proper documentation.

The Prototype pattern creates new objects by cloning existing ones rather than through instantiation. Implementation involves defining clone method in prototype objects. This pattern enables flexible object creation without coupling to specific classes. Consider deep vs shallow cloning, implement proper state copying, and handle circular references.

The Object Pool pattern manages set of reusable objects rather than creating and destroying them. Implementation involves creating pool manager for object lifecycle. This pattern reduces overhead of frequent object creation. Consider pool size management, implement proper cleanup, and handle concurrent access.

The Interpreter pattern defines grammar representation and interpreter for language. Implementation involves creating abstract syntax tree and interpreter methods. This pattern is useful for parsing and evaluating expressions. Consider proper grammar definition, implement error handling, and maintain proper documentation.

The Null Object pattern provides default object behavior instead of null references. Implementation involves creating null object class implementing same interface. This pattern simplifies client code by eliminating null checks. Consider proper interface compliance, implement meaningful default behavior, and document null object usage.

The Lazy Loading pattern defers initialization of object until first use. Implementation involves creating proxy or wrapper for delayed initialization. This pattern improves application startup time and resource usage. Consider proper initialization timing, implement error handling, and handle cleanup properly.

The Dependency Injection pattern provides dependencies to objects instead of having them create or find dependencies. Implementation involves creating injector/container for managing dependencies. This pattern improves modularity and testability. Consider proper dependency resolution, implement lifecycle management, and handle circular dependencies.

The Repository pattern mediates between domain and data mapping layers using collection-like interface. Implementation involves creating repository interface and concrete implementations. This pattern centralizes data access logic and enables substitution of data sources. Consider proper abstraction level, implement error handling, and maintain proper documentation.

The Unit of Work pattern maintains list of business objects affected by transaction and coordinates changes and resolution of problems. Implementation involves creating unit of work class managing object states. This pattern ensures data consistency and transaction management. Consider proper state tracking, implement rollback mechanism, and handle concurrent modifications.