TypeScript Strict Patterns Skill

Purpose

Ensure type-safe TypeScript development for CI/CD tooling, MCP servers, and build scripts used in the CIA platform. This skill covers strict compiler options, type guard patterns, and defensive coding practices for TypeScript components within a primarily Java/Maven project.

When to Use

• ✅ Writing MCP server tools in TypeScript
• ✅ Creating GitHub Actions custom actions
• ✅ Building CI/CD pipeline scripts and utilities
• ✅ Developing static site generators for political data
• ✅ Writing type-safe configuration parsers

Do NOT use for:

• ❌ Vaadin UI components (Java-based, use vaadin-component-design skill)
• ❌ Backend service logic (use Java/Spring patterns)

Strict Mode Configuration

Recommended tsconfig.json

{
  "compilerOptions": {
    "strict": true,
    "noUncheckedIndexedAccess": true,
    "noImplicitOverride": true,
    "noPropertyAccessFromIndexSignature": true,
    "exactOptionalPropertyTypes": true,
    "forceConsistentCasingInFileNames": true,
    "isolatedModules": true,
    "esModuleInterop": true,
    "moduleResolution": "node16",
    "module": "node16",
    "target": "ES2022",
    "declaration": true,
    "declarationMap": true,
    "sourceMap": true,
    "outDir": "./dist"
  }
}

What Strict Mode Enables

Flag	Purpose
strictNullChecks	Prevents null/undefined assignment errors
strictFunctionTypes	Enforces contravariant function parameter types
strictPropertyInitialization	Requires class properties to be initialized
noImplicitAny	Disallows implicit any types
noImplicitThis	Errors on this with implicit any type

Type Safety Patterns

Discriminated Unions for Political Data

interface Politician {
  readonly type: "politician";
  readonly personId: string;
  readonly firstName: string;
  readonly lastName: string;
  readonly party: SwedishParty;
}

interface Committee {
  readonly type: "committee";
  readonly committeeId: string;
  readonly name: string;
  readonly members: readonly string[];
}

type PoliticalEntity = Politician | Committee;

// Type guard
function isPolitician(entity: PoliticalEntity): entity is Politician {
  return entity.type === "politician";
}

Readonly Patterns

// Immutable data structures for political records
interface VotingRecord {
  readonly personId: string;
  readonly votes: ReadonlyArray<Vote>;
  readonly summary: Readonly<VotingSummary>;
}

// Readonly mapped type for API responses
type Immutable<T> = {
  readonly [K in keyof T]: T[K] extends object ? Immutable<T[K]> : T[K];
};

type ImmutableApiResponse = Immutable<RiksdagApiResponse>;

Type Guards for Runtime Validation

function isValidParty(value: unknown): value is SwedishParty {
  const validParties = ["S", "M", "SD", "C", "V", "KD", "L", "MP"] as const;
  return typeof value === "string" && validParties.includes(value as SwedishParty);
}

function assertNonNull<T>(value: T | null | undefined, name: string): asserts value is T {
  if (value === null || value === undefined) {
    throw new Error(`Expected non-null value for ${name}`);
  }
}

Branded Types for Domain Safety

type PersonId = string & { readonly __brand: "PersonId" };
type CommitteeId = string & { readonly __brand: "CommitteeId" };

function createPersonId(raw: string): PersonId {
  if (!/^[0-9a-f-]{36}$/.test(raw)) {
    throw new Error(`Invalid person ID format: ${raw}`);
  }
  return raw as PersonId;
}

Error Handling Patterns

// Result type for safe error handling
type Result<T, E = Error> =
  | { readonly success: true; readonly value: T }
  | { readonly success: false; readonly error: E };

async function fetchPoliticianData(id: PersonId): Promise<Result<Politician>> {
  try {
    const response = await fetch(`https://data.riksdagen.se/person/${id}`);
    if (!response.ok) {
      return { success: false, error: new Error(`HTTP ${response.status}`) };
    }
    const data: unknown = await response.json();
    const validated = validatePoliticianData(data);
    return { success: true, value: validated };
  } catch (error) {
    return { success: false, error: error instanceof Error ? error : new Error(String(error)) };
  }
}

CI/CD Integration

Package.json Scripts

{
  "scripts": {
    "build": "tsc --project tsconfig.json",
    "typecheck": "tsc --noEmit",
    "lint": "eslint src/ --ext .ts",
    "test": "vitest run"
  }
}

GitHub Actions Type Checking

- name: TypeScript Type Check
  run: npx tsc --noEmit --strict

Security Considerations

• Validate all external data at runtime — types are erased at runtime
• Use unknown over any for untyped data from APIs
• Sanitize string inputs before interpolation into commands or queries
• Never trust type assertions (as) without validation
• Audit dependencies with npm audit before adding packages