Example of using Typescript keyof operator

getLast.ts

interface RecursiveArray<T> extends Array<T | RecursiveArray<T>> {}

function getLast<O extends object, K extends keyof O>(
  mapOrList: O | RecursiveArray<O>,
  key: K
): O[K] | null {
  if (!mapOrList) return null;

  if (Array.isArray(mapOrList)) {
    return getLast(mapOrList.reverse().find(m => !!getLast(m, key))!, key);
  }

  return mapOrList[key];
}
  
getLast({color: "green"}, "color" ) // "green"
  
getLast([{color: "red"}], "color"); // Argument of type '"color"' is not assignable to parameter of type 'never'