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

---

Schema Organization

Organize Schemas by Usage

Common patterns:

• Centralized:

  src/schemas/

  for shared schemas
• Co-located: Near the modules that use them
• Layered: Separate by architectural layer (if using layered/hexagonal architecture)

Key principle: Avoid duplicating the same validation logic across multiple files.

Gotcha: Schema Duplication

Common anti-pattern:

Defining the same schema in multiple places:

• Validation logic duplicated across endpoints
• Same business rules defined in multiple adapters
• Type definitions not shared

Why This Is Wrong:

• ❌ Duplication creates multiple sources of truth
• ❌ Changes require updating multiple files
• ❌ Breaks DRY principle at the knowledge level
• ❌ Domain logic leaks into infrastructure code

Solution:

// ✅ CORRECT - Define schema once, import everywhere
// src/schemas/user-requests.ts
import { z } from 'zod';

export const CreateUserRequestSchema = z.object({
  email: z.string().email(),
  name: z.string().min(1),
});

export type CreateUserRequest = z.infer<typeof CreateUserRequestSchema>;

// Use in multiple places
import { CreateUserRequestSchema } from '../schemas/user-requests.js';

// Express endpoint
app.post('/users', (req, res) => {
  const result = CreateUserRequestSchema.safeParse(req.body);
  if (!result.success) {
    return res.status(400).json({ error: result.error });
  }
  // Use result.data (validated)
});

// GraphQL resolver
const createUser = (input: unknown) => {
  const validated = CreateUserRequestSchema.parse(input);
  return userService.create(validated);
};

Key Benefits:

• ✅ Single source of truth for validation
• ✅ Schema changes propagate everywhere automatically
• ✅ Type safety maintained across codebase
• ✅ DRY principle at knowledge level

Remember: If validation logic is duplicated, extract it into a shared schema.

---

Dependency Injection Pattern

Inject Dependencies, Don't Create Them

The Rule:

• Dependencies are always injected via parameters
• Never use

  new

  to create dependencies inside functions
• Factory functions accept dependencies as parameters

Why This Matters

Without dependency injection:

• ❌ Only one implementation possible
• ❌ Can't test with mocks (poor testability)
• ❌ Tight coupling to specific implementations
• ❌ Violates dependency inversion principle
• ❌ Can't swap implementations

With dependency injection:

• ✅ Any implementation works (in-memory, database, remote API)
• ✅ Fully testable (inject mocks for testing)
• ✅ Loose coupling
• ✅ Follows dependency inversion principle
• ✅ Runtime flexibility (configure implementation)

Example: Order Processor

❌ WRONG - Creating implementation internally

export const createOrderProcessor = ({
  paymentGateway,
}: {
  paymentGateway: PaymentGateway;
}): OrderProcessor => {
  // ❌ Hardcoded implementation!
  const orderRepository = new InMemoryOrderRepository();

  return {
    processOrder(order) {
      const payment = paymentGateway.charge(order.total);
      if (!payment.success) {
        return { success: false, error: payment.error };
      }

      orderRepository.save(order); // Using hardcoded repository
      return { success: true, data: order };
    },
  };
};

Why this is WRONG:

• Only ONE repository implementation possible (in-memory)
• Can't test with mock repository
• Can't swap to database repository or remote API
• Tight coupling to specific implementation

✅ CORRECT - Injecting all dependencies

export const createOrderProcessor = ({
  paymentGateway,  // ✅ Injected
  orderRepository, // ✅ Injected
}: {
  paymentGateway: PaymentGateway;
  orderRepository: OrderRepository;
}): OrderProcessor => {
  return {
    processOrder(order) {
      const payment = paymentGateway.charge(order.total);
      if (!payment.success) {
        return { success: false, error: payment.error };
      }

      orderRepository.save(order); // Delegate to injected dependency
      return { success: true, data: order };
    },
  };
};

Why this is CORRECT:

• ✅ Any OrderRepository implementation works (in-memory, PostgreSQL, MongoDB)
• ✅ Any PaymentGateway implementation works (Stripe, mock, testing)
• ✅ Easy to test (inject mocks)
• ✅ Loose coupling (depends on interfaces, not implementations)
• ✅ Runtime flexibility (choose implementation at startup)

---

Type vs Interface - Understanding WHY

The choice between

type

and

interface

is architectural, not stylistic.

Behavior Contracts → Use

interface

When to use: Interfaces define contracts that must be implemented.

Examples:

UserRepository

,

PaymentGateway

,

EmailService

,

CacheProvider

Why

interface

for behavior contracts?

1. Signals implementation contracts clearly

   • Interface communicates "this must be implemented elsewhere"
   • Type communicates "this is a data structure"

2. Better TypeScript errors when implementing

   • class X implements UserRepository

     gives clear errors
   • Types don't have

     implements

     keyword

3. Conventional for dependency injection

   • Standard pattern for dependency inversion
   • Clear separation between contract and implementation

4. Class-friendly for implementations

   • Many libraries use classes for services
   • Classes naturally implement interfaces

Example:

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

// Concrete implementation
export class PostgresUserRepository implements UserRepository {
  async findById(id: string): Promise<User | undefined> {
    // Implementation
  }
  // ... other methods
}

Data Structures → Use

type

When to use: Types define immutable data structures.

Examples:

User

,

Order

,

Config

,

ApiResponse

Why

type

for data?

1. Emphasizes immutability

   • Types with

     readonly

     signal "don't mutate this"
   • Functional programming alignment

2. Better for unions, intersections, mapped types

   • type Result<T, E> = Success<T> | Failure<E>

   • type Partial<T> = { [P in keyof T]?: T[P] }

3. Prevents accidental mutations

   • readonly

     properties enforce immutability at type level
   • Compiler catches mutation attempts

4. More flexible composition

   • Easier to compose with utility types
   • Better inference in complex scenarios

Example:

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

export type Order = {
  readonly id: string;
  readonly userId: string;
  readonly items: ReadonlyArray<OrderItem>;
  readonly total: number;
};

Architectural Pattern

This pattern supports clean architecture:

• Behavior contracts (

  interface

  ) = Boundaries between layers
• Data structures (

  type

  ) = Data flowing through the system
• Business logic depends on interfaces, not implementations
• Data is immutable (types with

  readonly

  )

---

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.

---

Immutability Patterns

Use

readonly

on All Data Structures

// ✅ CORRECT - Immutable data structure
type ApiRequest = {
  readonly method: 'GET' | 'POST' | 'PUT' | 'DELETE' | 'PATCH';
  readonly url: string;
  readonly headers?: {
    readonly [key: string]: string;
  };
  readonly body?: unknown;
};

// ❌ WRONG - Mutable data structure
type ApiRequest = {
  method: 'GET' | 'POST' | 'PUT' | 'DELETE' | 'PATCH';
  url: string;
  headers?: {
    [key: string]: string;
  };
  body?: unknown;
};

ReadonlyArray vs Array

// ✅ CORRECT - Immutable array
type ShoppingCart = {
  readonly id: string;
  readonly items: ReadonlyArray<CartItem>;
};

// ❌ WRONG - Mutable array
type ShoppingCart = {
  readonly id: string;
  readonly items: CartItem[];
};

Result Type Pattern for Error Handling

Prefer

Result<T, E>

types over exceptions for expected errors:

export type Result<T, E = Error> =
  | { readonly success: true; readonly data: T }
  | { readonly success: false; readonly error: E };

// Usage
export const findUser = (
  userId: string,
): Result<User> => {
  const user = database.findById(userId);
  if (!user) {
    return { success: false, error: new Error('User not found') };
  }

  return { success: true, data: user };
};

Why result types?

• Explicit error handling (type system enforces checking)
• No hidden control flow (unlike exceptions)
• Functional programming alignment
• Easier to test (no try/catch needed)

---

Factory Pattern for Object Creation

Use Factory Functions (Not Classes)

// ✅ CORRECT - Factory function
export const createOrderService = (
  orderRepository: OrderRepository,
  paymentGateway: PaymentGateway,
): OrderService => {
  return {
    async createOrder(order) {
      const validation = validateOrder(order);
      if (!validation.success) {
        return validation;
      }
      await orderRepository.save(order);
      return { success: true, data: order };
    },
    async processPayment(orderId, paymentInfo) {
      const order = await orderRepository.findById(orderId);
      if (!order) {
        return { success: false, error: new Error('Order not found') };
      }
      return paymentGateway.charge(order.total, paymentInfo);
    },
  };
};

// ❌ WRONG - Class-based creation
export class OrderService {
  constructor(
    private orderRepository: OrderRepository,
    private paymentGateway: PaymentGateway,
  ) {}

  async createOrder(order: Order) {
    // Implementation with `this`
  }
}

Why factory functions?

• Functional programming alignment
• No

  this

  context issues
• Easier to compose
• Natural dependency injection
• Simpler testing (no

  new

  keyword)

---

Location Guidance

Suggested File Organization

These are common patterns, not strict rules. Adapt to your project's needs.

Interfaces (Behavior Contracts)

• Common locations:

  src/interfaces/

  ,

  src/contracts/

  ,

  src/ports/

• Examples:

  UserRepository

  ,

  PaymentGateway

  ,

  EmailService

• Why: Behavior contracts that define boundaries between layers

Types (Data Structures)

• Common locations:

  src/types/

  ,

  src/models/

  , co-located with features
• Examples:

  User

  ,

  Order

  ,

  Config

• Why: Immutable data structures used throughout the system

Schemas (Validation)

• Common locations:

  src/schemas/

  ,

  src/validation/

  , co-located with features
• Examples:

  UserSchema

  ,

  OrderSchema

  ,

  ConfigSchema

• Why: Validation rules (consider avoiding duplication)

Business Logic

• Common locations:

  src/services/

  ,

  src/domain/

  ,

  src/use-cases/

• Examples:

  createUserService

  ,

  processOrder

  ,

  validatePayment

• Why: Core business logic (prefer framework-agnostic when possible)

Implementation Details

• Common locations:

  src/adapters/

  ,

  src/infrastructure/

  ,

  src/repositories/

• Examples:

  PostgresUserRepository

  ,

  StripePaymentGateway

  ,

  RedisCache

• Why: Framework-specific code, external integrations

Note: These are suggestions based on common patterns. Your project may use different conventions. The key principles are:

• Clear separation of concerns
• Minimal duplication of validation logic
• Dependencies point inward (toward business logic)

---

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;
}

---

Functional Programming Principles

These principles support immutability and type safety:

Pure Functions

• No side effects (don't mutate external state)
• Deterministic (same input → same output)
• Easier to reason about, test, and compose

// ✅ CORRECT - Pure function
const addItem = (
  items: ReadonlyArray<Item>,
  newItem: Item,
): ReadonlyArray<Item> => {
  return [...items, newItem]; // Returns new array
};

// ❌ WRONG - Impure function (mutates)
const addItem = (items: Item[], newItem: Item): void => {
  items.push(newItem); // Mutates input!
};

No Data Mutation

• Use spread operators for immutable updates
• Return new objects/arrays instead of modifying
• Let TypeScript's

  readonly

  enforce this

// ✅ CORRECT - Immutable update
const updateUser = (
  user: User,
  updates: Partial<User>,
): User => {
  return { ...user, ...updates }; // New object
};

// ❌ WRONG - Mutation
const updateUser = (user: User, updates: Partial<User>): void => {
  Object.assign(user, updates); // Mutates!
};

Composition Over Complex Logic

• Compose small functions into larger ones
• Each function does one thing well
• Easier to understand, test, and reuse

// ✅ CORRECT - Composed functions
const validate = (input: unknown) => UserSchema.parse(input);
const saveToDatabase = (user: User) => database.save(user);
const createUser = (input: unknown) => saveToDatabase(validate(input));

// ❌ WRONG - Complex monolithic function
const createUser = (input: unknown) => {
  if (typeof input !== 'object' || !input) throw new Error('Invalid');
  if (!('email' in input)) throw new Error('Missing email');
  // ... 50 more lines of validation and registration
};

Use Array Methods Over Loops

• Prefer

  map

  ,

  filter

  ,

  reduce

  for transformations
• Declarative (what, not how)
• Natural immutability (return new arrays)

// ✅ CORRECT - Functional array methods
const activeUsers = users.filter(u => u.active);
const userEmails = users.map(u => u.email);

// ❌ WRONG - Imperative loops
const activeUsers = [];
for (const u of users) {
  if (u.active) {
    activeUsers.push(u);
  }
}

---

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

---

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 (ports)
• Schemas defined in core, not duplicated in adapters
• Ports injected via parameters, never created internally
• Factory functions for object creation (not classes)

• readonly

  on all data structure properties
• Pure functions wherever possible (no mutations)
• Result types for expected errors (not exceptions)
• Strict mode enabled with all checks passing
• Artifacts in correct locations (ports/, types/, schemas/, domain/)