Typescript nobbery

type_craze.ts

type KeyType = "boo" | "poo";

function filterByType<T extends { key: KeyType }, K extends KeyType>(
  children: T[],
  type: K
): (T & { key: K })[] {
  return children.filter((child) => child.key === type) as (T & { key: K })[];
}

const boos = filterByType(
  [
    { key: "boo", value: "boo" },
    { key: "poo", value: "poo" },
  ],
  "boo"
);

boos[0].key;