TypeScript Strict Mode

Core Rules

1. No any - ever. Use unknown if type is truly unknown
2. No type assertions (as Type) without justification
3. Prefer type over interface for data structures
4. Reserve interface for behavior contracts only

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Type vs Interface

type — for data structures

export type User = {
  readonly id: string;
  readonly email: string;
  readonly name: string;
  readonly roles: ReadonlyArray<string>;
};

Why type? Better for unions, intersections, mapped types. readonly signals immutability. More flexible composition with utility types.

interface — for behavior contracts

export interface UserRepository {
  findById(id: string): Promise<User | undefined>;
  save(user: User): Promise<void>;
}

Why interface? Signals "this must be implemented." Works with implements keyword. Conventional for dependency injection.

Schema Duplication

Define schemas once, import everywhere. Never duplicate the same validation logic across multiple files.

// ✅ Define once
export const CreateUserRequestSchema = z.object({
  email: z.string().email(),
  name: z.string().min(1),
});
export type CreateUserRequest = z.infer<typeof CreateUserRequestSchema>;

// Import and use wherever needed

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Strict Mode Configuration

tsconfig.json Settings

{
  "compilerOptions": {
    "strict": true,
    "noImplicitAny": true,
    "strictNullChecks": true,
    "noUnusedLocals": true,
    "noUnusedParameters": true,
    "noImplicitReturns": true,
    "noFallthroughCasesInSwitch": true,
    "noUncheckedIndexedAccess": true,
    "exactOptionalPropertyTypes": true,
    "noPropertyAccessFromIndexSignature": true,
    "forceConsistentCasingInFileNames": true,
    "allowUnusedLabels": false
  }
}

What Each Setting Does

Core strict flags:

• strict: true - Enables all strict type checking options
• noImplicitAny - Error on expressions/declarations with implied any type
• strictNullChecks - null and undefined have their own types (not assignable to everything)
• noUnusedLocals - Error on unused local variables
• noUnusedParameters - Error on unused function parameters
• noImplicitReturns - Error when not all code paths return a value
• noFallthroughCasesInSwitch - Error on fallthrough cases in switch statements

Additional safety flags (CRITICAL):

• noUncheckedIndexedAccess - Array/object access returns T | undefined (prevents runtime errors from assuming elements exist)
• exactOptionalPropertyTypes - Distinguishes property?: T from property: T | undefined (more precise types)
• noPropertyAccessFromIndexSignature - Requires bracket notation for index signature properties (forces awareness of dynamic access)
• forceConsistentCasingInFileNames - Prevents case sensitivity issues across operating systems
• allowUnusedLabels - Error on unused labels (catches accidental labels that do nothing)

Additional Rules

• No @ts-ignore without explicit comments explaining why
• These rules apply to test code as well as production code

Architectural Insight: noUnusedParameters Catches Design Issues

The noUnusedParameters rule can reveal architectural problems:

Example: A function with an unused parameter often indicates the parameter belongs in a different layer. Strict mode catches these design issues early.

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Immutability, Pure Functions, and Composition

For detailed patterns on immutability (readonly, ReadonlyArray), pure functions, composition, Result types, array methods, and factory functions, see the functional skill. These are the canonical patterns used across the codebase.

Key TypeScript-specific notes:

• Use readonly on all type properties and ReadonlyArray<T> for arrays
• The compiler enforces immutability when readonly is used — leverage this
• Factory functions (not classes) for object creation, supporting dependency injection

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Schema-First at Trust Boundaries

When Schemas ARE Required

• Data crosses trust boundary (external → internal)
• Type has validation rules (format, constraints)
• Shared data contract between systems
• Used in test factories (validate test data completeness)

// API responses, user input, external data
const UserSchema = z.object({
  id: z.string().uuid(),
  email: z.string().email(),
});
type User = z.infer<typeof UserSchema>;

// Validate at boundary
const user = UserSchema.parse(apiResponse);

When Schemas AREN'T Required

• Pure internal types (utilities, state)
• Result/Option types (no validation needed)
• TypeScript utility types (Partial<T>, Pick<T>, etc.)
• Behavior contracts (interfaces - structural, not validated)
• Component props (unless from URL/API)

// ✅ CORRECT - No schema needed
type Result<T, E> =
  | { success: true; data: T }
  | { success: false; error: E };

// ✅ CORRECT - Interface, no validation
interface UserService {
  createUser(user: User): void;
}

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Branded Types

For type-safe primitives:

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

// Type-safe at compile time
const processPayment = (userId: UserId, amount: PaymentAmount) => {
  // Implementation
};

// ❌ Can't pass raw string/number
processPayment('user-123', 100); // Error

// ✅ Must use branded type
const userId = 'user-123' as UserId;
const amount = 100 as PaymentAmount;
processPayment(userId, amount); // OK

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Summary Checklist

When writing TypeScript code, verify:

• No any types - using unknown where type is truly unknown
• No type assertions without justification
• Using type for data structures with readonly
• Using interface for behavior contracts
• Schemas defined once, not duplicated
• Strict mode enabled with all checks passing
• For immutability, pure functions, composition: see functional skill