TypeScript Development Guidelines

Compiler Configuration

Strict Mode Settings

Always enable strict mode in tsconfig.json:

{
  "compilerOptions": {
    "strict": true,
    "noUncheckedIndexedAccess": true,
    "noImplicitReturns": true,
    "noFallthroughCasesInSwitch": true,
    "exactOptionalPropertyTypes": true,
    "noPropertyAccessFromIndexSignature": true
  }
}

Path Aliases

Configure path aliases for clean imports:

{
  "compilerOptions": {
    "baseUrl": ".",
    "paths": {
      "@/*": ["src/*"],
      "~types": ["src/types"],
      "~components/*": ["src/components/*"],
      "~features/*": ["src/features/*"]
    }
  }
}

Type Patterns

Prefer unknown Over any

// Avoid
function parse(input: any): object { ... }

// Prefer
function parse(input: unknown): object {
  if (typeof input !== 'string') {
    throw new Error('Expected string input');
  }
  return JSON.parse(input);
}

Use Type Guards Instead of Assertions

// Avoid
const user = data as User;

// Prefer
function isUser(data: unknown): data is User {
  return (
    typeof data === 'object' &&
    data !== null &&
    'id' in data &&
    'name' in data
  );
}

if (isUser(data)) {
  console.log(data.name); // Type-safe access
}

Discriminated Unions for State

type AsyncState<T> =
  | { status: 'idle' }
  | { status: 'loading' }
  | { status: 'success'; data: T }
  | { status: 'error'; error: Error };

function handleState<T>(state: AsyncState<T>) {
  switch (state.status) {
    case 'idle':
      return null;
    case 'loading':
      return <Spinner />;
    case 'success':
      return <Data value={state.data} />;
    case 'error':
      return <ErrorMessage error={state.error} />;
  }
}

Generic Constraints

// Constrain generics to specific shapes
function getProperty<T, K extends keyof T>(obj: T, key: K): T[K] {
  return obj[key];
}

// Constrain to types with specific methods
function sortItems<T extends { compareTo(other: T): number }>(items: T[]): T[] {
  return [...items].sort((a, b) => a.compareTo(b));
}

Utility Types

Mapped Types

// Make all properties readonly
type Immutable<T> = {
  readonly [K in keyof T]: T[K] extends object ? Immutable<T[K]> : T[K];
};

// Make specific properties optional
type PartialBy<T, K extends keyof T> = Omit<T, K> & Partial<Pick<T, K>>;

// Extract function parameter types
type FirstParam<T> = T extends (first: infer P, ...args: any[]) => any ? P : never;

Conditional Types

// Extract array element type
type ArrayElement<T> = T extends (infer E)[] ? E : never;

// Extract promise result type
type Awaited<T> = T extends Promise<infer R> ? Awaited<R> : T;

// Filter union types
type ExtractStrings<T> = T extends string ? T : never;

React + TypeScript Patterns

Component Definition

// Define props interface separately
interface ButtonProps {
  variant: 'primary' | 'secondary';
  size?: 'sm' | 'md' | 'lg';
  disabled?: boolean;
  onClick: () => void;
  children: React.ReactNode;
}

// Function component (React 19: React.FC discouraged)
function Button({ variant, size = 'md', disabled, onClick, children }: ButtonProps) {
  return (
    <button
      className={`btn btn-${variant} btn-${size}`}
      disabled={disabled}
      onClick={onClick}
    >
      {children}
    </button>
  );
}

Refs (React 19+)

// React 19: ref is a regular prop, forwardRef not required
interface InputProps {
  ref?: React.Ref<HTMLInputElement>;
  label: string;
  value: string;
  onChange: (value: string) => void;
}

function Input({ ref, label, value, onChange }: InputProps) {
  return (
    <label>
      {label}
      <input ref={ref} value={value} onChange={(e) => onChange(e.target.value)} />
    </label>
  );
}

Hooks with Types

// useState with explicit type
const [user, setUser] = useState<User | null>(null);

// useRef with DOM element
const inputRef = useRef<HTMLInputElement>(null);

// useCallback with typed parameters
const handleSubmit = useCallback((data: FormData) => {
  submitForm(data);
}, [submitForm]);

// useMemo for derived state
const sortedItems = useMemo(() =>
  [...items].sort((a, b) => a.name.localeCompare(b.name)),
  [items]
);

Event Handlers

// Form events
const handleChange = (e: React.ChangeEvent<HTMLInputElement>) => {
  setValue(e.target.value);
};

const handleSubmit = (e: React.FormEvent<HTMLFormElement>) => {
  e.preventDefault();
  // process form
};

// Mouse/keyboard events
const handleClick = (e: React.MouseEvent<HTMLButtonElement>) => {
  console.log(e.clientX, e.clientY);
};

const handleKeyDown = (e: React.KeyboardEvent<HTMLInputElement>) => {
  if (e.key === 'Enter') {
    submit();
  }
};

Context with Types

interface ThemeContextValue {
  theme: 'light' | 'dark';
  toggle: () => void;
}

const ThemeContext = createContext<ThemeContextValue | null>(null);

function useTheme() {
  const context = useContext(ThemeContext);
  if (!context) {
    throw new Error('useTheme must be used within ThemeProvider');
  }
  return context;
}

Code Organization

Feature-Based Structure

src/
├── features/
│   └── auth/
│       ├── api/              # API calls
│       ├── components/       # Feature components
│       ├── hooks/           # Feature hooks
│       ├── types/           # Feature types
│       └── index.ts         # Public exports
├── components/              # Shared components
├── hooks/                   # Shared hooks
├── types/                   # Global types
├── utils/                   # Utility functions
└── lib/                     # Third-party integrations

Barrel Exports

// features/auth/index.ts
export { LoginForm } from './components/LoginForm';
export { useAuth } from './hooks/useAuth';
export type { User, AuthState } from './types';

Type-Only Imports

// Prefer type-only imports for types
import type { User, AuthState } from './types';
import { validateUser } from './utils';

Validation with Zod

import { z } from 'zod';

// Define schema
const UserSchema = z.object({
  id: z.string().uuid(),
  email: z.string().email(),
  name: z.string().min(1).max(100),
  role: z.enum(['admin', 'user', 'guest']),
  createdAt: z.coerce.date(),
});

// Infer TypeScript type from schema
type User = z.infer<typeof UserSchema>;

// Validate at runtime
function parseUser(data: unknown): User {
  return UserSchema.parse(data);
}

// Safe parse (doesn't throw)
function tryParseUser(data: unknown): User | null {
  const result = UserSchema.safeParse(data);
  return result.success ? result.data : null;
}

Testing Patterns

Type-Safe Mocks

import { vi, type MockedFunction } from 'vitest';

// Mock with proper types
const mockFetch = vi.fn() as MockedFunction<typeof fetch>;

// Factory for test data
function createUser(overrides: Partial<User> = {}): User {
  return {
    id: crypto.randomUUID(),
    name: 'Test User',
    email: 'test@example.com',
    ...overrides,
  };
}

Testing React Components

import { render, screen } from '@testing-library/react';
import userEvent from '@testing-library/user-event';

test('submits form with valid data', async () => {
  const user = userEvent.setup();
  const onSubmit = vi.fn();

  render(<LoginForm onSubmit={onSubmit} />);

  await user.type(screen.getByLabelText('Email'), 'test@example.com');
  await user.type(screen.getByLabelText('Password'), 'password123');
  await user.click(screen.getByRole('button', { name: 'Login' }));

  expect(onSubmit).toHaveBeenCalledWith({
    email: 'test@example.com',
    password: 'password123',
  });
});

Common Pitfalls

Avoid Object Index Signatures Without Guards

// Dangerous: assumes key exists
const value = obj[key]; // type is T | undefined with noUncheckedIndexedAccess

// Safe: check before access
if (key in obj) {
  const value = obj[key];
}

Handle Promise Rejections

// Avoid: unhandled rejection
async function fetchData() {
  const response = await fetch('/api/data');
  return response.json();
}

// Prefer: explicit error handling
async function fetchData(): Promise<Result<Data, Error>> {
  try {
    const response = await fetch('/api/data');
    if (!response.ok) {
      return { success: false, error: new Error(`HTTP ${response.status}`) };
    }
    return { success: true, data: await response.json() };
  } catch (error) {
    return { success: false, error: error instanceof Error ? error : new Error(String(error)) };
  }
}

Avoid Enums, Use Const Objects

// Avoid: enums have quirks
enum Status {
  Active = 'active',
  Inactive = 'inactive',
}

// Prefer: const object with as const
const Status = {
  Active: 'active',
  Inactive: 'inactive',
} as const;

type Status = typeof Status[keyof typeof Status]; // 'active' | 'inactive'

Type Narrowing in Callbacks

// Problem: TypeScript doesn't narrow in callbacks
function process(value: string | null) {
  if (value === null) return;

  // value is string here, but...
  setTimeout(() => {
    // TypeScript can't guarantee value is still string
    console.log(value.toUpperCase()); // Error without storing in const
  }, 100);
}

// Solution: store narrowed value
function process(value: string | null) {
  if (value === null) return;
  const safeValue = value; // captured as string

  setTimeout(() => {
    console.log(safeValue.toUpperCase()); // OK
  }, 100);
}

ESLint Configuration

// eslint.config.js (flat config)
import tseslint from 'typescript-eslint';

export default tseslint.config(
  ...tseslint.configs.strictTypeChecked,
  {
    rules: {
      '@typescript-eslint/no-explicit-any': 'error',
      '@typescript-eslint/no-unused-vars': ['error', { argsIgnorePattern: '^_' }],
      '@typescript-eslint/prefer-nullish-coalescing': 'error',
      '@typescript-eslint/strict-boolean-expressions': 'error',
    },
  }
);

See Also

• Reference: advanced-types.md - Deep dive into conditional and mapped types
• Reference: react-patterns.md - React 19 patterns with TypeScript
• Reference: testing.md - Comprehensive testing strategies