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implementation-design-patterns

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Implementation guide for the 22 Gang of Four design patterns in TypeScript, distilled from refactoring.guru. Use this skill when writing, refactoring, or reviewing TypeScript that exhibits a pattern-shaped problem — class-explosion from inheritance, conditionals switching on type, tight coupling to concrete classes, tree-shaped models, runtime algorithm selection, undo/redo, snapshot-and-restore, state-dependent behavior, subscriber notification, or hiding subsystem complexity. Each pattern entry includes intent, problem, solution, applicability (when to use AND when NOT to use), a runnable TypeScript example, implementation steps, pros/cons, and relations to sibling patterns. Trigger even when no pattern is named — cues like "class getting unwieldy," "giant switch," "swap implementations at runtime," "combinatorial subclasses," "need undo," or "traverse a tree" are pattern-shaped. Covers all 5 Creational, 7 Structural, and 10 Behavioral GoF patterns.

skills/.curated/implementation-design-patterns/SKILL.md

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# 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
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