typescript-advanced-types

Define an event map interface mapping event names to payload types, then use generics with keyof constraints so on() and emit() only accept valid events with correctly typed data. The compiler rejects mismatched payloads at compile time.

Use conditional types with the infer keyword, such as T extends (...args: infer P) => infer R. Built-in utilities ReturnType and Parameters already provide this, but custom infer patterns let you extract promise values, array elements, and nested structures.

Mapped types iterate over an object's keys to transform properties, like Partial or Readonly. Conditional types select a type based on a check using extends, enabling logic like filtering union members or inferring inner types.

Use type guards (value is T) or assertion functions whenever narrowing unknown or union values at runtime, because assertions bypass compiler checks and can hide errors. Guards keep runtime behavior and static types in sync.

Deeply nested conditional or recursive types slow compilation and can hit recursion depth limits. Type-level safety also does not replace runtime validation, so external data still needs schema checks at system boundaries.