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