Extract fields of a specific type from a Record in TypeScript

FieldsOfType.ts

type AllowedFieldsWithType<TObj, TKey> = {
  [K in keyof TObj]: TObj[K] extends TKey ? K : never;
};

export type FieldsOfType<T, K> = AllowedFieldsWithType<T, K>[keyof T];

// Usage Example:
//
// const obj = {
//  foo: 'bar',
//  bar: 123,
//  baz: 'hello!',
//  boom: false,
// }
//
// type MyRecord = typeof obj;
// type StringFields = FieldsOfType<MyRecord, string>; # => "foo" | "baz"
// type OtherFields = FieldsOfType<MyRecord, number | boolean>; # => "bar" | "boom"