implementation-design-patterns

TypeScript Design Patterns Best Practices (Refactoring Guru)

Implementation reference for the 22 Gang of Four design patterns in TypeScript, distilled from refactoring.guru. Each of the 22 pattern files across 3 categories captures intent, problem, solution, applicability, a runnable TypeScript example, implementation steps, pros/cons, and relations to sibling patterns.

The patterns are a vocabulary for structural decisions, not a prescription. Reach for a pattern only when its applicability criteria match the problem at hand — every pattern entry includes a When NOT to Use section to guard against over-engineering.

When to Apply

• Refactoring a class that has grown unwieldy via inheritance — combinatorial subclasses, conditional branching on type, or a "god class" with many responsibilities
• Designing a new module whose collaborators are not yet fixed — you want to keep the interface stable while implementations vary
• Integrating an incompatible third-party API, library, or legacy class into existing code
• Modeling a tree-shaped domain (file systems, organization charts, expression ASTs, UI component trees) where leaves and branches must be treated uniformly
• Adding cross-cutting behavior at runtime — logging, caching, access control, decoration — without subclassing
• Selecting an algorithm or behavior variant at runtime based on configuration, user input, or environmental conditions
• Implementing undo/redo, history snapshots, transactional rollback, or scheduling/queueing of operations
• Coordinating many objects whose direct mutual references have become tangled — a hub that brokers communication
• Notifying many subscribers when something changes — event systems, reactive data flows
• Reviewing code that smells like a pattern is implicit (large switch on kind, parallel class hierarchies, identical algorithm skeletons across siblings) — make it explicit

Rule Categories

#	Category	Impact	Patterns	When to reach for this group
1	Creational	HIGH	5	Object construction is non-trivial, varies by configuration, or risks tight coupling to concrete classes
2	Structural	HIGH	7	Composing classes/objects into larger structures while keeping parts substitutable
3	Behavioral	HIGH	10	Distributing responsibility and defining how objects collaborate at runtime

How to Use

1. Recognize the shape. Read the Quick Reference below and identify which pattern's intent matches your problem. Most pattern-shaped problems sound like one of the listed phrases.
2. Read the pattern reference. Open references/{category}-{pattern}.md. Confirm intent, then read Applicability and When NOT to Use before adopting.
3. Adapt the example. The TypeScript example uses pedagogical names (ConcreteStrategyA, Receiver). Rename to domain terms before merging.
4. Check the relations. Each entry ends with Related Patterns — siblings worth considering for the same problem.

Quick Reference

1. Creational Patterns (object instantiation)

• creational-factory-method — Subclasses decide which concrete product to create. "I need to add new product types without touching the creator code." — HIGH
• creational-abstract-factory — Produce families of related objects together. "My code must work with multiple matching variants (chair+sofa+table) and shouldn't mix families." — MEDIUM-HIGH
• creational-builder — Construct complex objects step by step. "My constructor has 10+ parameters or I have a telescoping-constructor smell." — HIGH
• creational-prototype — Clone objects through their own clone() method. "I need to copy objects without depending on their concrete class." — MEDIUM
• creational-singleton — Guarantee a single shared instance with a global access point. "I need exactly one instance of this class — config, registry, pool." — MEDIUM

2. Structural Patterns (composition)

• structural-adapter — Translate one interface to another. "I need to use a library whose API doesn't match what my code expects." — HIGH
• structural-bridge — Split abstraction from implementation so they can vary independently. "I have two orthogonal dimensions and the subclass count is exploding." — MEDIUM
• structural-composite — Treat individual objects and compositions uniformly. "I have a tree (folders/files, groups/items, components/children) and want one interface for leaves and branches." — HIGH
• structural-decorator — Wrap an object to add behavior without subclassing. "I want to layer behaviors (logging + caching + auth) on the same interface at runtime." — HIGH
• structural-facade — Expose a simple interface over a complex subsystem. "My client code is tangled in initialization and orchestration of a third-party library." — HIGH
• structural-flyweight — Share common state across many objects to save memory. "I'm spawning millions of similar objects and running out of RAM." — LOW-MEDIUM
• structural-proxy — Substitute for another object to control access. "I need lazy loading, access control, caching, or logging without touching the real subject." — MEDIUM-HIGH

3. Behavioral Patterns (collaboration)

• behavioral-chain-of-responsibility — Pass a request along a chain of handlers. "I have a pipeline of validation / auth / parsing checks and want to add or reorder them dynamically." — MEDIUM-HIGH
• behavioral-command — Turn a request into a stand-alone object. "I need undo/redo, queueing, scheduling, macro recording, or to decouple invoker from receiver." — HIGH
• behavioral-iterator — Traverse a collection without exposing its representation. "I want clients to walk a structure without knowing if it's a list, tree, or graph." — HIGH
• behavioral-mediator — Centralize communication among components in a single hub. "My form fields all reference each other directly and the coupling is unmanageable." — MEDIUM
• behavioral-memento — Capture and restore an object's state without breaking encapsulation. "I need snapshots for undo/redo or transactional rollback." — LOW-MEDIUM
• behavioral-observer — Notify dependent objects when state changes. "Many objects need to react when one object changes — events, reactive UI, pub/sub." — CRITICAL
• behavioral-state — Alter behavior when internal state changes. "My class is a state machine with massive conditionals branching on a status field." — MEDIUM-HIGH
• behavioral-strategy — Make algorithms interchangeable at runtime. "I have multiple algorithms (sort, route, pay, compress) and want to pick one without conditionals." — HIGH
• behavioral-template-method — Fix an algorithm's skeleton in a base class; subclasses override steps. "Several classes share the same algorithm structure with minor step differences." — MEDIUM
• behavioral-visitor — Add operations to an object structure without modifying the classes. "I'd need to add 5 unrelated operations across an AST but I can't touch the node classes." — LOW-MEDIUM

How to Choose Between Similar Patterns

Several patterns share a structural shape but solve different problems. Read each pattern's Related Patterns section, then apply these distinctions:

• Adapter vs. Facade vs. Proxy vs. Decorator — all four wrap a target. Adapter changes the interface. Facade simplifies a subsystem. Proxy keeps the interface and controls access/lifecycle. Decorator keeps the interface and adds behavior recursively.
• Strategy vs. State — both swap a delegated object. Strategy objects are independent; the client picks one. State objects know each other and trigger transitions on the context.
• Strategy vs. Template Method — both vary parts of an algorithm. Strategy uses composition (swap at runtime). Template Method uses inheritance (fixed at compile time).
• Factory Method vs. Abstract Factory vs. Builder — Factory Method returns one product through a single method. Abstract Factory returns a family of related products through several methods. Builder assembles one complex product step by step.
• Composite vs. Decorator — both wrap children recursively. Composite sums or aggregates child results. Decorator adds responsibilities and passes through.
• Chain of Responsibility vs. Command vs. Mediator vs. Observer — all connect senders and receivers. CoR passes a request along a chain. Command makes the request a first-class object. Mediator centralizes mutual communication. Observer establishes one-publisher-to-many-subscribers notification.

Related Skills

• implementation-functional-patterns — TypeScript's functional answer (HOFs, lambdas, pipelines, streams, composition) for problems where this catalog reaches for a class. Most Strategy / Iterator / Command / Chain-of-Responsibility / Decorator / Template-Method shapes have a lighter functional form in idiomatic TS; consult it before introducing a new class hierarchy.

References

1. Refactoring Guru — Design Patterns Catalog
2. Refactoring Guru — TypeScript Examples
3. Refactoring Guru — Creational Patterns
4. Refactoring Guru — Structural Patterns
5. Refactoring Guru — Behavioral Patterns