Typescript generic to constrain to one key of a given interface/signature

one_of_type.ts

/**
 * A custom utility type to allow an object to be only one Type from the provided
 * Type.
 * @example
 * type OneOf = OneOfType<{userId:string, userEmail:string}>;
 * //allows {userId:string} || {userEmail:string}, but not both
 */
export type OneOfType<T> = ValueOf<OneOfByKey<T>>;
type OneOfByKey<T> = {[key in keyof T]: OneOnly<T, key>};
type OneOnly<Obj, Key extends keyof Obj> = {
  [key in Exclude<keyof Obj, Key>]+?: undefined;
} & Pick<Obj, Key>;
type ValueOf<Obj extends object> = Obj[keyof Obj];