typescript-refactor

name	typescript-refactor
description	TypeScript refactoring and modernization guidelines from a principal specialist perspective. This skill should be used when refactoring, reviewing, or modernizing TypeScript code to ensure type safety, compiler performance, and idiomatic patterns. Triggers on tasks involving TypeScript type architecture, narrowing, generics, error handling, or migration to modern TypeScript features.

TypeScript Refactor Best Practices

Comprehensive TypeScript refactoring and modernization guide designed for AI agents and LLMs. Contains 43 rules across 8 categories, prioritized by impact to guide automated refactoring, code review, and code generation.

When to Apply

Reference these guidelines when:

• Refactoring TypeScript code for type safety and maintainability
• Designing type architectures (discriminated unions, branded types, generics)
• Narrowing types to eliminate unsafe as casts
• Adopting modern TypeScript 4.x-5.x features (satisfies, using, const type parameters)
• Optimizing compiler performance in large codebases
• 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	Generic Patterns	HIGH	generic-
5	Compiler Performance	MEDIUM-HIGH	compile-
6	Error Safety	MEDIUM	error-
7	Runtime Patterns	MEDIUM	perf-
8	Quirks & Pitfalls	LOW-MEDIUM	quirk-

Quick Reference

1. Type Architecture (CRITICAL)

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

2. Type Narrowing & Guards (CRITICAL)

• narrow-custom-type-guards — Write custom type guards instead of type assertions
• narrow-assertion-functions — Use assertion functions for precondition checks
• narrow-exhaustive-switch — Enforce exhaustive switch with never
• narrow-in-operator — Narrow with the in operator for interface unions
• narrow-eliminate-as-casts — Eliminate as casts with proper narrowing chains
• narrow-typeof-chains — Use typeof narrowing before property access

3. Modern TypeScript (HIGH)

• modern-using-keyword — Use the using keyword for resource cleanup
• modern-const-type-parameters — Use const type parameters for literal inference
• modern-template-literal-types — Use template literal types for string patterns
• modern-noinfer-utility — Use NoInfer to control type parameter inference
• modern-accessor-keyword — Use accessor for auto-generated getters and setters
• modern-verbatim-module-syntax — Enable verbatimModuleSyntax for explicit import types

4. Generic Patterns (HIGH)

• generic-infer-over-annotate — Let TypeScript infer instead of explicit annotation
• generic-constrain-dont-overconstrain — Constrain generics minimally
• generic-avoid-distributive-surprises — Control distributive conditional types
• generic-mapped-type-utilities — Build custom mapped types for repeated transformations
• generic-return-type-inference — Preserve return type inference in generic functions

5. Compiler Performance (MEDIUM-HIGH)

• compile-explicit-return-types — Add explicit return types to exported functions
• compile-avoid-deep-recursion — Avoid deeply recursive type definitions
• compile-project-references — Use project references for monorepo builds
• compile-base-types-over-unions — Use base types instead of large union types

6. Error Safety (MEDIUM)

• error-result-type — Use Result types instead of thrown exceptions
• error-exhaustive-error-handling — Use exhaustive checks for typed error variants
• error-typed-catch — Type catch clause variables as unknown
• error-never-for-unreachable — Use never to mark unreachable code paths
• error-discriminated-error-unions — Model domain errors as discriminated unions

7. Runtime Patterns (MEDIUM)

• perf-union-literals-over-enums — Use union literals instead of enums
• perf-avoid-delete-operator — Avoid the delete operator on objects
• perf-object-freeze-const — Use Object.freeze with as const for true immutability
• perf-object-keys-narrowing — Avoid Object.keys type widening
• perf-map-set-over-object — Use Map and Set over plain objects for dynamic collections

8. Quirks & Pitfalls (LOW-MEDIUM)

• quirk-excess-property-checks — Understand excess property checks on object literals
• quirk-empty-object-type — Avoid the {} type — it means non-nullish
• quirk-type-widening-let — Prevent type widening with let declarations
• quirk-variance-annotations — Use variance annotations for generic interfaces
• quirk-structural-typing-escapes — Guard against structural typing escape hatches

How to Use

Read individual reference files for detailed explanations and code examples:

• Section definitions — Category structure and impact levels
• Rule template — Template for adding new rules

Reference Files

File	Description
references/_sections.md	Category definitions and ordering
assets/templates/_template.md	Template for new rules
metadata.json	Version and reference information