ddd-type-duplication-across-layers - SKILL.md Agent Skill

name: "ddd-type-duplication-across-layers" description: "When adding values to any union type in a domains/models/ file" version: "1.0.0" status: "approved" learned_from: "skills/learned/ddd-type-duplication-across-layers.md"

DDD Type Duplication Across Layers

Extracted: 2026-02-23 Context: Angular DDD codebase (gc.webapp) where domain types are duplicated in infrastructure API models

Problem

When adding a new value to a domain union type (e.g., CalculationBase), the build breaks because the same type is hardcoded separately in the infrastructure layer (API request/response models). The domain type compiles fine, but the adapter that maps domain models to API requests fails with Type 'X' is not assignable to type 'A' | 'B' | 'C'.

Solution

After modifying a domain union type, always search the entire codebase for other occurrences of the same union values:

grep -r "'Value1'.*'Value2'.*'Value3'" src/app/<domain>/

Common locations for duplicated types in this codebase:

domains/models/ - Domain type (primary)
infrastructure/api/request/ - API request types
infrastructure/api/response/ - API response types
domains/rules/*.spec.ts - Test files with hardcoded arrays

Example

// Domain model (updated)
export type CalculationBase = 'AmountExclTax' | 'Hectoliter' | 'Quantity' | 'Kilogram' | 'Liter';

// API request model (forgotten) - build break
export interface CreateTaxRequest {
  calculationBase: 'AmountExclTax' | 'Hectoliter' | 'Quantity'; // missing new values!
}

When to Use

When adding values to any union type in a domains/models/ file
When modifying enums or string literal types in a DDD-structured feature
Before running npm run build after type changes, proactively grep for all occurrences