Advanced TypeScript Patterns

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**Conditional & Mapped Types:**
- Use conditional types (`T extends U ? X : Y`) to derive types from runtime shapes
- `infer` keyword extracts sub-types: `type ReturnType<T> = T extends (...args: any[]) => infer R ? R : never`
- Mapped types transform object shapes: `type Readonly<T> = { readonly [K in keyof T]: T[K] }`
- Combine with template literal types for string manipulation: `type EventName<T> = \`on\${Capitalize<T & string>}\``

**Template Literal Types:**
- Enforce string formats at the type level: `type CSSColor = \`#\${string}\` | \`rgb(\${string})\``
- Generate exhaustive event union types: `type Event = \`\${Entity}:\${Action}\``
- Use with `Extract<>` and `Exclude<>` to filter string unions by pattern
- Power autocomplete in IDE while keeping type safety for known-prefix strings

**Discriminated Unions:**
- Always use a literal `kind`, `type`, or `status` discriminant field for union variants
- Narrowing with `switch (action.type)` gives exhaustive checking when combined with `never` default
- Prefer discriminated unions over optional fields for mutually exclusive state shapes
- Use `satisfies` operator to validate objects against a type without widening

**Type Guards & Assertions:**
- Write user-defined type guards: `function isApiError(e: unknown): e is ApiError { return ... }`
- Use `asserts` signature for throwing guards: `function assert(cond: unknown): asserts cond { ... }`
- Prefer type guards over type assertions (`as`) — guards are checked at runtime, assertions are not
- Validate external data with Zod and use `z.infer<typeof schema>` to derive the TypeScript type

**Branded & Nominal Types:**
- Create branded types to prevent mixing semantically different primitives: `type UserId = string & { readonly __brand: "UserId" }`
- Use a `brand<T, B>()` helper function to safely create branded values with runtime identity
- Apply to currency amounts, IDs, sanitized strings, and unit-specific numbers
- Branded types catch bugs like passing an `OrderId` where a `UserId` is expected

**Module Augmentation:**
- Extend third-party types without forking: `declare module "express" { interface Request { user?: AuthUser } }`
- Augment global interfaces for app-wide type additions: `declare global { interface Window { analytics: Analytics } }`
- Use `interface` merging (not `type` aliases) for augmentation — only interfaces can be merged
- Keep augmentation files co-located with the module they extend (`express.d.ts` next to middleware)

**Utility Patterns:**
- `DeepPartial<T>`, `DeepReadonly<T>`, `NonNullable<T>` — build a shared `types/utils.ts` library
- `ValueOf<T>` for extracting union of object values: `type ValueOf<T> = T[keyof T]`
- `Awaited<T>` (built-in) to unwrap Promise return types in async chains
- Use `const` assertions (`as const`) to narrow literal types and create readonly tuples

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