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typescript-advanced-patterns

Advanced TypeScript — type-level programming, library/DSL APIs, declaration merging, modern language features at depth (decorators, using, const T, NoInfer, variance), and feature implementation patterns built on advanced types. Trigger on tasks involving recursive conditional types, infer patterns, mapped-type key remapping, variadic tuples, fluent builders with phantom state, schema-first inference (Zod/Valibot), end-to-end-typed API clients, finite state machines, module augmentation, and library-publishing concerns. Trigger even when the user does not say "advanced" — if the work involves type-level algorithms, library-author API design, or going beyond surface-level uses of TS 5.x features, this is the skill. Assumes the reader has absorbed the `typescript-refactor` skill — this one extends those patterns at depth, never restates them.

skills/.experimental/typescript-advanced-patterns/SKILL.md

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# TypeScript Advanced Patterns Best Practices

Type-level programming, library-author idioms, and feature-implementation patterns that go beyond surface uses of TypeScript 5.x. Contains 40 rules across 5 categories, prioritised by impact on consumer codebases.

## When to Apply

Reference these guidelines when:

- Designing a public library or DSL surface (fluent builders, event emitters, route parsers, query builders, schema-derived clients)
- Writing type-level algorithms (recursive conditionals, accumulator pattern, key remapping, variadic tuples, type-level string/number ops, type-level tests)
- Using TS 5.x features in non-trivial ways (Stage 3 decorators, `using` composition, `const T` for overload disambiguation, `NoInfer` for anchor-vs-constrained parameters, variance annotations, the bivariance hole)
- Encoding workflow state, transitions, and capabilities at the type level so illegal states and missing checks are compile errors
- Integrating with the declaration & module system (module augmentation, declaration merging, ambient asset modules, library type publishing via `exports`/`typesVersions`)

## Boundary with neighbouring skills