type ValueOf<T> = T[keyof T];

// Creates a new object populated with the results of calling a provided
// function on every item in the object.
export const mapObject = <T extends object, U>(
  obj: T,
  fn: (v: ValueOf<T>) => U,
) => Object.fromEntries(Object.entries(obj).map(([k, v]) => [k, fn(v)]));

// High-order function that returns a user-defined type guard
export const hasField = <T, K extends keyof T>(key: K) => {
  return (item: T): item is T & Required<Pick<T, K>> => {
    return typeof item[key] !== "undefined";
  };
};