typescript-advanced-types

Define an event map interface mapping event names to payload types, then use generics with mapped types so `on` and `emit` methods infer the correct payload per event. The compiler rejects mismatched payloads at compile time.

Use a conditional type with the `infer` keyword: `type ReturnType<T> = T extends (...args: any[]) => infer R ? R : never`. This extracts the return type from any function type for reuse in other type definitions.

Mapped types iterate over an object's keys to transform properties, like making them readonly or optional. Conditional types select a type based on a condition using `extends`, enabling logic like extracting types or distributing over unions.

Use `unknown` instead of `any` because it forces type checking before use. `any` disables all type checking and defeats TypeScript's purpose, while `unknown` requires type guards or assertions before operations.

Deeply nested conditional types force the compiler to evaluate many type branches recursively, increasing check time. Cache complex type computations, limit recursion depth, and prefer simpler types where possible.

Use discriminated unions when modeling states like loading, success, or error, where a shared literal field (such as `status` or `type`) lets switch statements narrow types automatically. This gives exhaustive checking and safe property access per variant.