TypeScript Refactoring Patterns

Core Principles

1. Type Narrowing Over Type Assertions - Use type guards and discriminated unions instead of as casts
2. Const Assertions for Literals - Use as const for immutable literal types
3. Generic Constraints - Prefer extends constraints over any
4. Branded Types - Use branded types for domain-specific validation

Refactoring Patterns

Extract Discriminated Union

When you see multiple boolean flags, refactor to discriminated union:

// Before
interface User {
  isAdmin: boolean;
  isGuest: boolean;
  permissions?: string[];
}

// After
type User =
  | { role: 'admin'; permissions: string[] }
  | { role: 'guest' }
  | { role: 'member'; permissions: string[] };

Replace Conditional with Polymorphism

When you see switch statements on type, use the strategy pattern:

// Before
function process(item: Item) {
  switch (item.type) {
    case 'a': return processA(item);
    case 'b': return processB(item);
  }
}

// After
const processors: Record<ItemType, (item: Item) => Result> = {
  a: processA,
  b: processB,
};
const process = (item: Item) => processors[item.type](item);

Extract Type Guard

When narrowing types, create reusable type guards:

function isNonNullable<T>(value: T): value is NonNullable<T> {
  return value !== null && value !== undefined;
}

// Usage
const items = array.filter(isNonNullable);

Use Branded Types for Validation

Prevent primitive obsession with branded types:

type UserId = string & { readonly brand: unique symbol };
type Email = string & { readonly brand: unique symbol };

function createUserId(id: string): UserId {
  if (!isValidUuid(id)) throw new Error('Invalid user ID');
  return id as UserId;
}

Code Smell Detectors

Watch for these patterns and refactor:

• any types (replace with unknown + type guards)
• Non-null assertions ! (add proper checks)
• Type assertions as (use type guards)
• Optional chaining abuse ?.?.?. (restructure data)
• Index signatures without validation

Quick Wins

1. Enable strict: true in tsconfig
2. Use satisfies for type checking without widening
3. Prefer readonly arrays and objects
4. Use unknown for external data, validate at boundaries