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implementation-functional-patterns

TypeScript's functional answers to the 22 Gang of Four classes — factory functions (Factory Method, Abstract Factory, Prototype, Memento), module-scope singletons, fluent immutable builders, wrapper functions (Adapter, Facade), native Proxy, WeakMap caches (Flyweight), discriminated unions with exhaustive match (State, Visitor, Composite), event emitters and signals (Mediator, Observer), pipelines and composition (CoR, Decorator), stream methods (Iterator), closures-as-commands, higher-order strategies, lambda placement. Use when reviewing TypeScript that has a class-shaped problem the GoF catalog solves with a hierarchy but where idiomatic TS reaches for a function, a tagged union, or a data structure. Each rule names the GoF pattern(s) it replaces and when the class form still wins. Trigger on "factory class", "singleton getInstance", "state machine class", "observer pattern", "AST visitor", "where do I put this lambda". Sibling to implementation-design-patterns.

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

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# TypeScript Functional Patterns

Implementation reference for the functional shapes that supersede the Gang of Four catalog in idiomatic TypeScript. Sibling to [`implementation-design-patterns`](../implementation-design-patterns/SKILL.md): read that one when the answer is a class, this one when the answer is a function, a tagged union, or a small data structure.

TypeScript has first-class functions, discriminated unions, structural typing, and zero ceremony around closures. That means the 22 GoF patterns — written in the catalog as class hierarchies because the source material targets Java/C# — collapse to far fewer functional shapes in real TS code. Several patterns share a functional answer: tagged unions cover State, Visitor, and Composite; factory functions cover Factory Method, Abstract Factory, Prototype, and Memento; event emitters cover Mediator and Observer; wrapper functions cover Adapter and Facade. This skill names those collapses, the placement rules they imply, and the performance trade-offs.

## When to Apply

- Refactoring a Factory class hierarchy, an `AbstractFactory` returning families of products, or a class with `clone()` / Memento save-restore
- Refactoring a Singleton class with `private constructor` + `static getInstance`
- Refactoring a mutable Builder class for a configuration object with many optional fields
- Refactoring a Strategy / Template Method / Bridge class hierarchy where variation is a single method
- Replacing a Chain of Responsibility class chain or Decorator wrapper-class stack
- Replacing a custom Iterator class with stream methods, generators, or lazy iterator helpers