TypeScript Type Safety Enforcer

Enforce strict TypeScript type safety standards with zero tolerance for shortcuts.

Core Rule: NEVER Use "any" Type

This is an immediate fail. No exceptions.

// ❌ ABSOLUTELY FORBIDDEN
const data: any = getData();
function process(input: any): any { }
const items: any[] = [];

// ✅ ALWAYS DO THIS INSTEAD
const data: UserData = getData();
function process(input: UserInput): ProcessedResult { }
const items: string[] = [];

Type Safety Principles

1. Always Define Proper Types and Interfaces

Bad:

function createUser(data) {
  return {
    id: data.id,
    name: data.name,
    email: data.email
  };
}

Good:

interface UserInput {
  id: string;
  name: string;
  email: string;
}

interface User {
  id: string;
  name: string;
  email: string;
  createdAt: Date;
}

function createUser(data: UserInput): User {
  return {
    id: data.id,
    name: data.name,
    email: data.email,
    createdAt: new Date()
  };
}

2. Explicit Function Parameters and Return Types

Bad:

function calculate(a, b) {
  return a + b;
}

async function fetchUser(id) {
  const response = await api.get(`/users/${id}`);
  return response.data;
}

Good:

function calculate(a: number, b: number): number {
  return a + b;
}

async function fetchUser(id: string): Promise<User> {
  const response = await api.get<User>(`/users/${id}`);
  return response.data;
}

3. Use "unknown" Instead of "any"

When type is truly unknown, use unknown and narrow with type guards.

Bad:

function parseJSON(json: string): any {
  return JSON.parse(json);
}

Good:

function parseJSON(json: string): unknown {
  return JSON.parse(json);
}

// Then use type guard to narrow
function isUser(data: unknown): data is User {
  return (
    typeof data === 'object' &&
    data !== null &&
    'id' in data &&
    'name' in data &&
    'email' in data
  );
}

const data = parseJSON(jsonString);
if (isUser(data)) {
  // Now TypeScript knows data is User
  console.log(data.name);
}

4. Prefer Specific Types Over Broad Types

Bad:

type Status = string;
function setStatus(status: string) { }

Good:

type Status = 'active' | 'inactive' | 'pending';
function setStatus(status: Status) { }

// Or with enums
enum Status {
  Active = 'active',
  Inactive = 'inactive',
  Pending = 'pending'
}
function setStatus(status: Status) { }

5. Type Guards for Narrowing

Example:

// Type guard functions
function isString(value: unknown): value is string {
  return typeof value === 'string';
}

function isUser(value: unknown): value is User {
  return (
    typeof value === 'object' &&
    value !== null &&
    'id' in value &&
    typeof (value as any).id === 'string'
  );
}

// Usage
function process(input: unknown) {
  if (isString(input)) {
    // TypeScript knows input is string here
    console.log(input.toUpperCase());
  } else if (isUser(input)) {
    // TypeScript knows input is User here
    console.log(input.name);
  }
}

TypeScript Utility Types

Use built-in utility types instead of "any":

Partial

interface User {
  id: string;
  name: string;
  email: string;
}

// Make all properties optional
function updateUser(id: string, updates: Partial<User>) {
  // updates can have any combination of User properties
}

Required

interface UserInput {
  name?: string;
  email?: string;
}

// Make all properties required
function createUser(data: Required<UserInput>) {
  // data.name and data.email are guaranteed to exist
}

Pick<T, K>

interface User {
  id: string;
  name: string;
  email: string;
  password: string;
  createdAt: Date;
}

// Pick only specific properties
type PublicUser = Pick<User, 'id' | 'name' | 'email'>;

Omit<T, K>

// Exclude specific properties
type UserWithoutPassword = Omit<User, 'password'>;

Record<K, T>

// Create object type with specific keys
type UserRoles = Record<string, 'admin' | 'user' | 'guest'>;

const roles: UserRoles = {
  'user1': 'admin',
  'user2': 'user'
};

ReturnType

function getUser() {
  return { id: '1', name: 'John' };
}

// Get return type of function
type UserType = ReturnType<typeof getUser>;

Strict Type Checking Configuration

Ensure tsconfig.json has strict mode enabled:

{
  "compilerOptions": {
    "strict": true,
    "noImplicitAny": true,
    "strictNullChecks": true,
    "strictFunctionTypes": true,
    "strictBindCallApply": true,
    "strictPropertyInitialization": true,
    "noImplicitThis": true,
    "alwaysStrict": true,
    "noUnusedLocals": true,
    "noUnusedParameters": true,
    "noImplicitReturns": true,
    "noFallthroughCasesInSwitch": true
  }
}

Common Patterns

API Response Typing

Bad:

async function fetchData(url: string) {
  const response = await fetch(url);
  return response.json();
}

Good:

interface ApiResponse<T> {
  data: T;
  status: number;
  message: string;
}

async function fetchData<T>(url: string): Promise<ApiResponse<T>> {
  const response = await fetch(url);
  return response.json();
}

// Usage
const userResponse = await fetchData<User>('/api/user');
console.log(userResponse.data.name); // Fully typed

Event Handlers

Bad:

function handleClick(e) {
  console.log(e.target.value);
}

Good:

function handleClick(e: React.MouseEvent<HTMLButtonElement>) {
  console.log(e.currentTarget.value);
}

function handleChange(e: React.ChangeEvent<HTMLInputElement>) {
  console.log(e.target.value);
}

State Management

Bad:

const [data, setData] = useState(null);

Good:

const [data, setData] = useState<User | null>(null);

// Or with initial value
const [users, setUsers] = useState<User[]>([]);

Props in React

Bad:

function UserCard(props) {
  return <div>{props.name}</div>;
}

Good:

interface UserCardProps {
  user: User;
  onClick?: (id: string) => void;
  className?: string;
}

function UserCard({ user, onClick, className }: UserCardProps) {
  return (
    <div className={className} onClick={() => onClick?.(user.id)}>
      {user.name}
    </div>
  );
}

Error Handling with Types

Bad:

try {
  const data = await fetchUser();
} catch (error) {
  console.log(error.message);
}

Good:

try {
  const data = await fetchUser();
} catch (error) {
  if (error instanceof Error) {
    console.log(error.message);
  } else {
    console.log('Unknown error:', error);
  }
}

// Or create custom error types
class ApiError extends Error {
  constructor(
    message: string,
    public statusCode: number,
    public response?: unknown
  ) {
    super(message);
    this.name = 'ApiError';
  }
}

try {
  const data = await fetchUser();
} catch (error) {
  if (error instanceof ApiError) {
    console.log(`API Error ${error.statusCode}: ${error.message}`);
  } else if (error instanceof Error) {
    console.log('Error:', error.message);
  }
}

Validation Checklist

Before committing TypeScript code:

• No "any" types anywhere
• All function parameters have explicit types
• All function return types are explicit
• Interfaces defined for all objects
• Proper null/undefined handling with ? or union types
• Type guards used when narrowing unknown types
• Utility types used instead of "any"
• Strict mode enabled in tsconfig.json
• No implicit any errors
• All React props properly typed

When to Use Each Approach

Use Interfaces for:

• Object shapes
• API contracts
• Props definitions
• Extending types

interface User {
  id: string;
  name: string;
}

interface AdminUser extends User {
  permissions: string[];
}

Use Types for:

• Unions
• Intersections
• Primitives
• Tuples

type Status = 'active' | 'inactive';
type ID = string | number;
type Point = [number, number];
type UserWithTimestamp = User & { timestamp: Date };

Use Enums for:

• Fixed set of constants
• When you need reverse mapping

enum UserRole {
  Admin = 'admin',
  User = 'user',
  Guest = 'guest'
}

Rejection Criteria

Reject pull requests that contain:

• ❌ Any usage of any type without absolute justification
• ❌ Implicit any from missing type annotations
• ❌ Functions without return type declarations
• ❌ Untyped parameters
• ❌ Disabled TypeScript checks with @ts-ignore or @ts-nocheck
• ❌ Type assertions with as any

Only accept any if:

• Working with truly dynamic third-party library with no types
• Legacy code migration (temporary, must be documented)
• Extremely complex generic type that's impossible to express (rare)

Even then, prefer:

// Instead of any
const data: unknown = thirdPartyLib.getData();

// Or create minimal interface
interface ThirdPartyData {
  [key: string]: unknown;
}

Summary: Type Safety is Non-Negotiable

Golden Rules:

1. Never use "any" - use "unknown" and type guards
2. Always type function parameters and returns explicitly
3. Define interfaces for all data structures
4. Use utility types (Partial, Pick, Omit, etc.)
5. Enable strict mode in tsconfig.json
6. Type guards for narrowing unknown types
7. Specific types over broad types
8. Reject code with "any" types

Remember: Type safety catches bugs at compile time, not runtime. It's not optional—it's essential for maintainable code.