implementation-functional-patterns

TypeScript Functional Patterns

Implementation reference for the functional shapes that supersede the Gang of Four catalog in idiomatic TypeScript. Sibling to implementation-design-patterns: 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
• Replacing a Command class with a closure stored in a queue (when undo/serialization is not required)
• Replacing an Adapter / Facade class that exists only to forward calls or hide subsystem orchestration
• Replacing a Proxy class with native JS Proxy (for transparent interception) or an HOF wrapper (for selective wrapping)
• Replacing a Flyweight factory class with a Map/WeakMap cache + factory function
• Replacing a State / Visitor / Composite class hierarchy with a discriminated union + exhaustive match function
• Replacing a Mediator / Observer class with an event emitter or reactive signal
• Reviewing TSX where lambdas appear inline in JSX, hook deps, or memo'd child props — placement controls identity
• Recognizing imperative for loops with mutated accumulators that would read more honestly as reduce, Object.groupBy, or flatMap