TypeScript strict patterns and best practices. Trigger: When implementing or refactoring TypeScript in .ts/.tsx (types, interfaces, generics, const maps, type guards, removing any, tightening unknown).
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// ✅ 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!
}
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
);
}
Do not model semantically coupled props as independent optionals — this allows invalid half-states that compile but break at runtime. Use discriminated unions with never to make invalid combinations impossible.
// ❌ BEFORE: Independent optionals — half-states allowed
interface PaginationProps {
onPageChange?: (page: number) => void;
pageSize?: number;
currentPage?: number;
}
// ✅ AFTER: Discriminated union — shape is all-or-nothing
type ControlledPagination = {
controlled: true;
currentPage: number;
pageSize: number;
onPageChange: (page: number) => void;
};
type UncontrolledPagination = {
controlled: false;
currentPage?: never;
pageSize?: never;
onPageChange?: never;
};
type PaginationProps = ControlledPagination | UncontrolledPagination;
Key rule: If two or more props are only meaningful together, they belong to the same discriminated union branch. Mixing them as independent optionals shifts correctness responsibility from the type system to runtime guards.
import type { User } from "./types";
import { createUser, type Config } from "./utils";
Create or update AgentSkills. Use when designing, structuring, or packaging skills with scripts, references, and assets.
Create or update AgentSkills. Use when designing, structuring, or packaging skills with scripts, references, and assets.
Set up and use 1Password CLI (op). Use when installing the CLI, enabling desktop app integration, signing in (single or multi-account), or reading/injecting/running secrets via op.
CLI to manage emails via IMAP/SMTP. Use `himalaya` to list, read, write, reply, forward, search, and organize emails from the terminal. Supports multiple accounts and message composition with MML (MIME Meta Language).
Create or update AgentSkills. Use when designing, structuring, or packaging skills with scripts, references, and assets.
Create or update AgentSkills. Use when designing, structuring, or packaging skills with scripts, references, and assets.
Set up and use 1Password CLI (op). Use when installing the CLI, enabling desktop app integration, signing in (single or multi-account), or reading/injecting/running secrets via op.
CLI to manage emails via IMAP/SMTP. Use `himalaya` to list, read, write, reply, forward, search, and organize emails from the terminal. Supports multiple accounts and message composition with MML (MIME Meta Language).