typescript-advanced-types

Define an event map type 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 a conditional type with infer: type ReturnType<T> = T extends (...args: any[]) => infer R ? R : never. This extracts the return type R from any function type passed as the generic parameter.

Mapped types iterate over an existing type's keys to transform properties, like Partial or Readonly. Conditional types select a type based on a condition using extends, enabling logic like extracting types or distributing over unions.

Use unknown for values whose type is not yet known, since it forces type narrowing through guards before use. Unlike any, unknown preserves type safety and prevents accidental unsafe operations on the value.

Each nested conditional forces the compiler to evaluate more type branches, and recursive types multiply this cost exponentially. Cache intermediate results in named type aliases and limit recursion depth to keep compilation fast.