Advanced TypeScript Patterns with Claude Code: Utility Types, Discriminated Unions, and Type Guards

Using any is the fast path to eliminating everything TypeScript is supposed to give you. Type-inferred code is self-documenting and catches bugs at refactor time — not in production at 2am. Claude Code, guided by CLAUDE.md rules, generates advanced type designs from day one. Here's the exact setup and the patterns it produces. Step 1: Lock Down Claude Code with CLAUDE.md ## TypeScript Strict Rules - No `any` — use `unknown` + type guards instead - No `as T` assertions — implement type guard functions - No `// @ts-ignore` or `// @ts-expect-error` - External API responses: Zod parse, not type assertion - State modeled as Discriminated Union - Template Literal Types for event names and map keys - Use `satisfies` for type-checked config objects - tsconfig: `"strict": true, "noUncheckedIndexedAccess": true` Write it once. Every Claude Code session follows it automatically. Pattern 1: Discriminated Union for API State Instead of isLoading: boolean + data: T | null + error: string | null scat