Critical Patterns
Const Types Pattern (REQUIRED)
// ✅ 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";
Flat Interfaces (REQUIRED)
// ✅ 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 (REQUIRED)
// ✅ 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 { }
Decision Tree
Need a set of constants? → Use const object + typeof Need optional fields? → Use Partial<T> Need to pick fields? → Use Pick<T, K> Need to exclude fields? → Use Omit<T, K> Need runtime type check? → Create type guard function Need to extract type? → Use ReturnType<typeof fn>
Code Examples
Type Guards
function isUser(value: unknown): value is User { return ( typeof value === "object" && value !== null && "id" in value && "name" in value ); }
Utility Types
Pick<User, "id" | "name"> // Select fields Omit<User, "id"> // Exclude fields Partial<User> // All optional Required<User> // All required Record<string, User> // Object type NonNullable<T | null> // Remove null/undefined ReturnType<typeof fn> // Function return type
Import Types
import type { User } from "./types"; import { createUser, type Config } from "./utils";
Commands
npx tsc --noEmit # Type check without emit npx tsc --init # Initialize tsconfig.json npx tsc -w # Watch mode
Resources
Specialized TypeScript documentation:
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Best Practices: best-practices.md
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Type Safety: type-safety.md
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NestJS Integration: nestjs.md
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Node.js Patterns: node.md