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