Generics — why a type parameter instead of any?

Generics keep the relationship between types instead of throwing it away with any.

function first<T>(items: readonly T[]): T | undefined {
  return items[0];
}
const name = first(['Anna', 'Ola']); // string | undefined

interface ApiResult<T> {
  data: T;
  requestId: string;
}

To be precise: T is a type parameter; the compiler often infers it. For an operation that needs id, you can constrain T with extends { id: string }.

Pitfall: a fetchJson<T> function doesn't prove the network response has type T. Validating the data is still a separate step.