European-Parliament-MCP-Server: Skill for Claude Code

TypeScript Strict Patterns Skill

Context

This skill applies when:

• Writing TypeScript code with strict mode enabled
• Defining types and interfaces for European Parliament data
• Creating branded types for IDs to prevent mixing
• Implementing discriminated unions for type-safe variants
• Converting Zod schemas to TypeScript types
• Using utility types (Pick, Omit, Partial, Required)
• Handling nullable types safely
• Implementing generic type patterns
• Ensuring type safety across API boundaries

TypeScript strict mode is enabled in this project with strictNullChecks, noImplicitAny, and noUncheckedIndexedAccess. All code must comply with these strict rules.

Rules

1. Never Use any: Use unknown for truly dynamic types, then narrow with type guards
2. Always Define Return Types: Explicitly type all function return values
3. Use Branded Types for IDs: Prevent mixing different ID types (MEP_ID vs DocumentID)
4. Leverage z.infer<>: Derive TypeScript types from Zod schemas
5. Handle Nulls Explicitly: Check for null/undefined before accessing properties
6. Use Discriminated Unions: Type-safe variants with discriminator field
7. Prefer interface for Objects: Use type for unions, aliases, mapped types
8. Use Utility Types: Leverage Pick, Omit, Partial, Required, Record
9. Type All Parameters: Never rely on implicit parameter types
10. Use as const: For literal types and readonly values

Examples

✅ Good Pattern: Branded Types

import { z } from 'zod';

// Branded type for MEP IDs (prevents mixing with other IDs)
const MEP_ID = z.number().int().positive().brand<'MEP_ID'>();
type MEP_ID = z.infer<typeof MEP_ID>;

// Branded type for Document IDs
const DocumentIDSchema = z.string()
  .regex(/^EP-\d{8}-\d{5}$/)
  .brand<'DocumentID'>();
type DocumentID = z.infer<typeof DocumentIDSchema>;

// Type safety: Can't mix IDs
function getMEP(id: MEP_ID): Promise<MEP> { /* ... */ }
function getDocument(id: DocumentID): Promise<Document> { /* ... */ }

const mepId: MEP_ID = MEP_ID.parse(12345);
const docId: DocumentID = DocumentIDSchema.parse('EP-20240101-00001');

await getMEP(mepId);     // ✅ Works
await getDocument(docId); // ✅ Works
// await getMEP(docId);    // ❌ Type error: DocumentID not assignable to MEP_ID