TypeScript strict patterns and best practices. Trigger: When writing TypeScript code - types, interfaces, generics.
// ✅ 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.
// ✅ 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
}
interface Admin extends User {
permissions: string[];
}
// ❌ NEVER: Inline nested objects
interface User {
address: { street: string; city: string }; // NO!
}
// ✅ ALWAYS: Use interface for object shapes and variable typings
interface User {
id: string;
name: string;
}
interface ApiResponse {
data: User[];
total: number;
}
// ✅ ONLY use type when interface cannot do the job:
// - Union/intersection types
type ID = string | number;
type AdminOrUser = Admin | User;
// - Mapped types
type ReadonlyUser = { readonly [K in keyof User]: User[K] };
// - Conditional types
type IsString<T> = T extends string ? true : false;
// - Const object type extraction (see Const Types Pattern)
type Status = (typeof STATUS)[keyof typeof STATUS];
// - Tuple types
type Pair = [string, number];
// ❌ NEVER: Use type for plain object shapes when interface works
type User = { // NO — use interface instead
id: string;
name: string;
};
Why? Interfaces are extendable (extends), give better error messages, support declaration merging, and signal clearly that the shape describes an object.
Reserve type for operations that interfaces cannot express.
any// ✅ Use unknown for truly unknown types
function parse(input: unknown): User {
if (isUser(input)) return input;
throw new Error("Invalid input");
}
// ✅ Use generics for flexible types
function first<T>(arr: T[]): T | undefined {
return arr[0];
}
// ❌ NEVER
function parse(input: any): any { }
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
function isUser(value: unknown): value is User {
return (
typeof value === "object" &&
value !== null &&
"id" in value &&
"name" in value
);
}
import type { User } from "./types";
import { createUser, type Config } from "./utils";
typescript, ts, types, interfaces, generics, strict mode, utility types