---
name: typescript-refactor
description: TypeScript and TSX refactoring and modernization guidelines from a principal specialist perspective, current to TypeScript 6.0 and React 19. This skill should be used when refactoring, reviewing, or modernizing TypeScript or React/TSX code for type safety, compiler performance, and idiomatic patterns. Triggers on tasks involving type architecture, narrowing, generics, discriminated unions, error handling, React component and hook typing, or migration to modern TypeScript features (satisfies, using, const type parameters, inferred type predicates, isolatedDeclarations, erasable syntax, import attributes).
---

# TypeScript Refactor Best Practices

Comprehensive TypeScript and TSX refactoring and modernization guide designed for AI agents and LLMs. Contains 47 rules across 9 categories, prioritized by impact to guide automated refactoring, code review, and code generation. Current to TypeScript 6.0 and React 19.

## When to Apply

Reference these guidelines when:
- Refactoring TypeScript or React/TSX code for type safety and maintainability
- Designing type architectures (discriminated unions, branded types, generics)
- Narrowing types to eliminate unsafe `as` casts
- Typing React components and hooks (props, refs, events, state) in `.tsx` files
- Adopting modern TypeScript 5.x–6.0 features (`satisfies`, `using`, const type parameters, inferred type predicates, erasable syntax, `with` import attributes)
- Optimizing compiler performance in large codebases (`isolatedDeclarations`, project references)
- Implementing type-safe error handling patterns
- Reviewing code for TypeScript quirks and pitfalls

## Rule Categories by Priority

| Priority | Category | Impact | Prefix |
|----------|----------|--------|--------|
| 1 | Type Architecture | CRITICAL | `arch-` |
| 2 | Type Narrowing & Guards | CRITICAL | `narrow-` |
| 3 | Modern TypeScript | HIGH | `modern-` |
| 4 | React & TSX | HIGH | `tsx-` |
| 5 | Generic Patterns | HIGH | `generic-` |
| 6 | Compiler Performance | MEDIUM-HIGH | `compile-` |
| 7 | Error Safety | MEDIUM | `error-` |
| 8 | Runtime Patterns | MEDIUM | `perf-` |
| 9 | Quirks & Pitfalls | LOW-MEDIUM | `quirk-` |

## Quick Reference

### 1. Type Architecture (CRITICAL)

- [`arch-discriminated-unions`](references/arch-discriminated-unions.md) — Use discriminated unions over string enums for exhaustive pattern matching
- [`arch-branded-types`](references/arch-branded-types.md) — Use branded types for domain identifiers to prevent value mix-ups
- [`arch-satisfies-over-annotation`](references/arch-satisfies-over-annotation.md) — Use `satisfies` for config objects to preserve literal types
- [`arch-interfaces-over-intersections`](references/arch-interfaces-over-intersections.md) — Extend interfaces instead of intersecting types for better error messages
- [`arch-const-assertion`](references/arch-const-assertion.md) — Use `as const` for immutable literal inference
- [`arch-readonly-by-default`](references/arch-readonly-by-default.md) — Default to readonly types for function parameters and return values
- [`arch-avoid-partial-abuse`](references/arch-avoid-partial-abuse.md) — Avoid `Partial<T>` abuse for builder patterns

### 2. Type Narrowing & Guards (CRITICAL)

- [`narrow-custom-type-guards`](references/narrow-custom-type-guards.md) — Replace `as` with runtime-checked guards; TS 5.5+ infers the predicate
- [`narrow-assertion-functions`](references/narrow-assertion-functions.md) — Use assertion functions for precondition checks
- [`narrow-exhaustive-switch`](references/narrow-exhaustive-switch.md) — Enforce exhaustive switch with `never`
- [`narrow-in-operator`](references/narrow-in-operator.md) — Narrow with the `in` operator for interface unions
- [`narrow-eliminate-as-casts`](references/narrow-eliminate-as-casts.md) — Eliminate `as` casts with proper narrowing chains

### 3. Modern TypeScript (HIGH)

- [`modern-using-keyword`](references/modern-using-keyword.md) — Use the `using` keyword for resource cleanup
- [`modern-const-type-parameters`](references/modern-const-type-parameters.md) — Use const type parameters for literal inference
- [`modern-template-literal-types`](references/modern-template-literal-types.md) — Use template literal types for string patterns
- [`modern-noinfer-utility`](references/modern-noinfer-utility.md) — Use `NoInfer` to control type parameter inference
- [`modern-verbatim-module-syntax`](references/modern-verbatim-module-syntax.md) — Enable `verbatimModuleSyntax` for explicit import types
- [`modern-erasable-syntax`](references/modern-erasable-syntax.md) — Prefer erasable syntax over enums and namespaces for type-stripping
- [`modern-import-attributes`](references/modern-import-attributes.md) — Use `with` import attributes instead of deprecated `assert`

### 4. React & TSX (HIGH)

- [`tsx-avoid-react-fc`](references/tsx-avoid-react-fc.md) — Type props directly instead of `React.FC`
- [`tsx-ref-as-prop`](references/tsx-ref-as-prop.md) — Pass `ref` as a prop instead of `forwardRef` (React 19)
- [`tsx-extend-native-props`](references/tsx-extend-native-props.md) — Extend native element props with `ComponentPropsWithRef` instead of redeclaring them
- [`tsx-discriminated-props`](references/tsx-discriminated-props.md) — Model mutually-exclusive props as discriminated unions
- [`tsx-event-handler-types`](references/tsx-event-handler-types.md) — Type event handlers with React synthetic event types
- [`tsx-hook-typing`](references/tsx-hook-typing.md) — Type `useState`/`useRef` for nullable and mutable state

### 5. Generic Patterns (HIGH)

- [`generic-constrain-dont-overconstrain`](references/generic-constrain-dont-overconstrain.md) — Constrain generics minimally
- [`generic-avoid-distributive-surprises`](references/generic-avoid-distributive-surprises.md) — Control distributive conditional types
- [`generic-mapped-type-utilities`](references/generic-mapped-type-utilities.md) — Build custom mapped types for repeated transformations
- [`generic-return-type-inference`](references/generic-return-type-inference.md) — Preserve return type inference in generic functions

### 6. Compiler Performance (MEDIUM-HIGH)

- [`compile-explicit-return-types`](references/compile-explicit-return-types.md) — Add explicit return types to exported functions
- [`compile-avoid-deep-recursion`](references/compile-avoid-deep-recursion.md) — Avoid deeply recursive type definitions
- [`compile-project-references`](references/compile-project-references.md) — Use project references for monorepo builds
- [`compile-base-types-over-unions`](references/compile-base-types-over-unions.md) — Use base types instead of large union types
- [`compile-isolated-declarations`](references/compile-isolated-declarations.md) — Enable `isolatedDeclarations` for parallel declaration emit

### 7. Error Safety (MEDIUM)

- [`error-result-type`](references/error-result-type.md) — Use Result types instead of thrown exceptions
- [`error-exhaustive-error-handling`](references/error-exhaustive-error-handling.md) — Use exhaustive checks for typed error variants
- [`error-typed-catch`](references/error-typed-catch.md) — Type catch clause variables as `unknown`
- [`error-discriminated-error-unions`](references/error-discriminated-error-unions.md) — Model domain errors as discriminated unions

### 8. Runtime Patterns (MEDIUM)

- [`perf-union-literals-over-enums`](references/perf-union-literals-over-enums.md) — Use union literals instead of enums — enums are non-erasable
- [`perf-avoid-delete-operator`](references/perf-avoid-delete-operator.md) — Avoid the `delete` operator on objects
- [`perf-object-freeze-const`](references/perf-object-freeze-const.md) — Use `Object.freeze` with `as const` for true immutability
- [`perf-object-keys-narrowing`](references/perf-object-keys-narrowing.md) — Avoid `Object.keys` type widening
- [`perf-map-set-over-object`](references/perf-map-set-over-object.md) — Use `Map` and `Set` over plain objects for dynamic collections

### 9. Quirks & Pitfalls (LOW-MEDIUM)

- [`quirk-excess-property-checks`](references/quirk-excess-property-checks.md) — Understand excess property checks on object literals
- [`quirk-empty-object-type`](references/quirk-empty-object-type.md) — Avoid the `{}` type — it means non-nullish
- [`quirk-structural-typing-escapes`](references/quirk-structural-typing-escapes.md) — Guard against structural typing escape hatches
- [`quirk-variance-annotations`](references/quirk-variance-annotations.md) — Use variance annotations to document generic intent (not for speed)

## How to Use

Read individual reference files for detailed explanations and code examples:

- [Section definitions](references/_sections.md) — Category structure and impact levels
- [Rule template](assets/templates/_template.md) — Template for adding new rules

## Reference Files

| File | Description |
|------|-------------|
| [references/_sections.md](references/_sections.md) | Category definitions and ordering |
| [assets/templates/_template.md](assets/templates/_template.md) | Template for new rules |
| [metadata.json](metadata.json) | Version and reference information |