The Strategy pattern enables interchangeable algorithm implementations.

The Strategy Pattern is a behavioral design pattern that allows dynamic selection of algorithm implementations at runtime by encapsulating algorithms into separate strategy classes, making them interchangeable to enhance code flexibility and maintainability. The core idea is to encapsulate algorithms into strategy classes, where clients invoke strategies through a context without concerning themselves with specific implementations. E-commerce discount calculation is a classic application scenario: the context holds a reference to the strategy and provides a switching method. Compared to conditional statements, the Strategy Pattern is easier to extend, supports dynamic switching, and enables strategy composition. Form validation is another common use case, where performance optimization should be considered, and it can be combined with functional programming. While structurally similar to the State Pattern, their intents differ—the Strategy Pattern involves the client actively selecting algorithms.

State pattern (State) manages object behavior changes

The State pattern is a behavioral design pattern that allows an object to alter its behavior when its internal state changes. It encapsulates state into separate classes and delegates requests to the current state object, avoiding excessive conditional branches. In JavaScript, it is commonly used to manage complex state logic, such as UI interactions or game behaviors. The core idea is to delegate behavior to an object representing the current state. Key roles include the Context, State interface, and Concrete States. A basic implementation is demonstrated using a light switch example. In UI interactions, such as file upload components, it helps organize state-related logic clearly. It is closely related to the concept of finite state machines, as illustrated by a traffic light example. Advantages of the State pattern include adherence to the Single Responsibility Principle and elimination of conditional statements, while a potential drawback is over-engineering. The State pattern shares a similar structure with the Strategy pattern but serves a different purpose. In React, it can be used to manage complex state logic, such as form submission processes.

The difference between the Observer pattern and Publish/Subscribe.

The observer pattern and the publish-subscribe pattern are both design patterns that handle one-to-many dependencies, but they differ in implementation and coupling. In the observer pattern, the subject and observers interact directly, with the publisher being aware of the subscribers' existence. In contrast, the publish-subscribe pattern decouples the two through an event channel, where neither party knows about the other. The observer pattern is simple and straightforward, suitable for small systems or scenarios requiring immediate responses. The publish-subscribe pattern is more flexible, ideal for large systems or cross-component communication. Both patterns have their pros and cons in terms of memory usage, execution efficiency, and scalability. In practice, the choice between them depends on factors like system scale, component relationships, and message processing requirements. Sometimes, the two patterns are mixed to leverage their respective strengths.

The state recovery mechanism of the Memento pattern

The Memento pattern is a design pattern that captures and externalizes an object's internal state without violating encapsulation, suitable for scenarios requiring undo operations or history recording. It consists of three roles: Originator, Memento, and Caretaker. The Originator is responsible for creating and restoring state, the Memento stores the state, and the Caretaker saves the Memento. Practical applications include undo/redo functionality in rich text editors. Performance optimization can involve incremental saving or limiting the number of historical records. This pattern can be combined with the Command pattern. In browser environments, special handling is required for DOM state serialization strategies, which must account for complex objects. Time-travel debugging can be achieved by saving complete state history. Compared to the Prototype pattern, the Memento pattern focuses more on state history recording and restoration, while the Prototype pattern emphasizes creating new objects through cloning.

Decoupling component communication with the Mediator pattern

The mediator pattern is a behavioral design pattern that reduces direct dependencies between components by encapsulating their interaction logic, making it particularly suitable for complex component communication scenarios. In JavaScript, implementing the mediator pattern can elegantly handle many-to-many relationships among UI components, modules, or services. Its core idea is to define a mediator object that encapsulates interactions between groups of objects, transforming a web of dependencies into a star-shaped structure. The key advantage is reducing direct coupling between objects and centralizing control over interaction logic. JavaScript offers three mainstream implementation approaches: event-based, command-based, and hybrid. Practical applications include complex form validation and cross-component state synchronization. When using this pattern, it’s important to avoid excessive notifications, clean up subscriptions promptly, and prevent circular dependencies. Compared to the observer pattern, the mediator pattern emphasizes bidirectional communication and coordinating complex interactions. In React, state lifting is essentially a simplified version of the mediator pattern.

The relationship between the Iterator pattern and generator functions

The iterator pattern is an important behavioral design pattern in JavaScript, enabling sequential data access through iterable objects and iterators. Generator functions serve as syntactic sugar that simplifies iterator implementation. Both have distinct advantages in synchronous traversal and asynchronous programming. Generators support lazy evaluation via the `yield` keyword, offering higher memory efficiency and superior performance with large datasets. They can also be composed to build data processing pipelines. Their error handling mechanism is more intuitive than traditional iterators, and they integrate deeply with language features like `yield*` delegation for complex control flow while automatically maintaining execution state. Browser APIs also adopt similar patterns. In modern JavaScript development, combining both approaches efficiently solves various traversal challenges.

The language parsing implementation of the Interpreter pattern

The Interpreter pattern is a behavioral design pattern used to define language grammar and provide an interpreter to process this grammar, particularly suitable for domain-specific languages such as mathematical expressions, query languages, or markup languages. Its core idea involves representing grammar rules as classes, constructing a syntax tree through composition, and traversing it to perform operations. In JavaScript, dynamic language features can simplify implementation. The pattern consists of components like abstract expressions, terminal expressions, non-terminal expressions, context, and clients. It is commonly used for boolean expressions, mathematical expression parsing, and often combined with the Composite pattern, such as in HTML tag parsers, where a syntax tree is built to enable interpretation and execution. It is well-suited for relatively simple language processing, while more complex languages require integration with other techniques.

Encapsulation and undo operations in the Command pattern

The Command pattern is a behavioral design pattern that encapsulates requests as objects, decoupling the invoker from the receiver. By encapsulating operations, it supports parameterization and deferred execution. The core advantages of this pattern include request encapsulation, decoupling of invocation and execution, and support for composite commands, making it particularly suitable for implementing undo operations. By storing state information, operations can be rolled back. In practical applications, it maintains an undo stack to support multi-step undo and redo. In front-end development, it is commonly used to handle user interactions such as button clicks and menu selections. The Command pattern also supports command queues for deferred execution, making it suitable for animations and batch operations. Variants include simple commands, undoable commands, transactional commands, and macro commands. Typical application scenarios include GUI components, transaction processing, progress bar operations, multi-level undo, logging, and task scheduling systems.

The request processing flow of the Chain of Responsibility pattern.

The Chain of Responsibility pattern is a behavioral design pattern that allows a request to be passed along a chain of handlers, where each handler can either process the request or pass it to the next handler, decoupling the sender and receiver. In JavaScript, it is typically represented as objects containing references to other objects, forming a chain. The classic structure includes an abstract handler, concrete handlers, and a client. The abstract handler defines the interface, concrete handlers implement the interface, and the client creates the handler chain and submits requests. There are multiple implementation approaches, including the classic implementation, array-based implementation, and function-based implementation. The classic implementation forms a chain by setting successors, the array-based implementation stores handlers in an array and processes them in a loop, while the function-based implementation leverages JavaScript's functional features for a more concise approach. Application scenarios include event bubbling mechanisms, middleware mechanisms (such as Express or Koa middleware), and form validation. Event bubbling propagates from specific elements upward, middleware like Express or Koa handles requests in a pipeline, and form validation is suitable for complex validation logic.

Best practices for the Parasitic Combination inheritance pattern

The composite inheritance pattern combines the advantages of prototype chain inheritance and constructor inheritance while avoiding the drawbacks of using them separately. Traditional inheritance methods suffer from issues like shared reference-type properties among instances or the inability to inherit parent class prototype methods. Composite inheritance addresses these by borrowing constructors to inherit properties and using a mixin approach to inherit methods, ensuring instance property independence and method sharing. However, it introduces inefficiency by calling the parent class constructor twice. Parasitic composite inheritance optimizes this by using `Object.create` to call the parent constructor only once while maintaining prototype chain integrity. ES6 class inheritance is syntactic sugar for composite inheritance. Practical applications, such as UI component development, suit scenarios where methods are shared but properties remain independent. Performance optimizations include avoiding deep inheritance chains, placing method definitions on the prototype, and using `Object.freeze` to prevent modifications. It can also be combined with other patterns like the factory pattern or mixin pattern. Modern JavaScript simplifies inheritance implementation further with `Reflect` and `Proxy`.