Const Types Pattern (REQUIRED)
typescript
// ✅ ALWAYS: Create const object first, then extract type
const STATUS = {
ACTIVE: "active",
INACTIVE: "inactive",
PENDING: "pending",
} as const;
type Status = (typeof STATUS)[keyof typeof STATUS];
// ❌ NEVER: Direct union types
type Status = "active" | "inactive" | "pending";
Why? Single source of truth, runtime values, autocomplete, easier refactoring.
Flat Interfaces (REQUIRED)
typescript
// ✅ ALWAYS: One level depth, nested objects → dedicated interface
interface UserAddress {
street: string;
city: string;
}
interface User {
id: string;
name: string;
address: UserAddress; // Reference, not inline
}
// ❌ NEVER: Inline nested objects
interface User {
address: { street: string; city: string }; // NO!
}
Never Use any
typescript
// ✅ Use unknown for truly unknown types
function parse(input: unknown): User {
if (isUser(input)) return input;
throw new Error("Invalid input");
}
// ❌ NEVER
function parse(input: any): any { }
Utility Types
typescript
Pick<User, "id" | "name"> // Select fields Omit<User, "id"> // Exclude fields Partial<User> // All optional Required<User> // All required Readonly<User> // All readonly Record<string, User> // Object type Extract<Union, "a" | "b"> // Extract from union Exclude<Union, "a"> // Exclude from union NonNullable<T | null> // Remove null/undefined ReturnType<typeof fn> // Function return type Parameters<typeof fn> // Function params tuple