typescript-advanced-types

Define an EventMap type mapping event names to payload shapes, then use a generic class with mapped listener storage keyed by event name. The on and emit methods constrain callbacks and data to the payload type of the specific event key.

Use a conditional type with infer: type ReturnType<T> = T extends (...args: any[]) => infer R ? R : never. TypeScript also ships this as a built-in utility type, and the same infer pattern works for parameters and promise resolution types.

Mapped types iterate over an existing type's keys to transform properties, like Partial or Readonly. Conditional types select a type based on a check, like T extends string ? A : B, and distribute over unions when the checked type is a naked type parameter.

TypeScript has no built-in deep variants, but you can write recursive mapped types that check whether each property is an object and reapply the transformation. Functions and arrays need special handling to avoid breaking their structure.

Deeply nested conditional and recursive types force the compiler to evaluate many type instantiations, increasing check time. Mitigate this by caching computed types in aliases, limiting recursion depth, and preferring simpler type expressions where possible.

Use unknown for values whose type you cannot guarantee, since it forces narrowing through type guards before use. Any disables type checking entirely and should be avoided because it silently propagates unsafe assumptions through the codebase.