TypeScript 通用 function 重载和/或类型保护

Question

I'm trying to have generic function to extract keys and values of enums and objects this way:

type Key = string | number | symbol;
type Dictionary<K extends Key, T> = Partial<Record<K, T>>;
type Obj = Dictionary<Key, unknown>; // stricter than object
type Enum = Dictionary<number, string>; // stricter than enum

function keys<K extends Key, T>(record: Record<K, T>): Array<K> {
  return Object.keys(record) as Array<K>;
}

function values<K extends Key, T>(record: Record<K, T>): Array<T> {
  return keys(record).map((key) => record[key]);
}

the problem is that Object.keys(anything) returns string[] whatever the keys of anything are, and even worse, when anything is an enum, it returns also the numeric values as strings (which is clearly wrong IMHO), so I wrote a specific function for this case:

function keys(record: Enum): Array<number> {
  return Object.keys(record)
    .map((x) => parseInt(x))
    .filter((x) => !isNaN(x));
}

now how can I merge this code in the generic keys function? with a type guard on the K in case it is a number I guess, but I tried so many ways without success...

BTW is there a "pure TypeScript" lib around that already does this?

Answer 1

It's not intuitive - but it's actually incorrect to assume keyof T from Object.keys - see https://stackoverflow.com/a/55012175/2310450 for a full explanation - but the TLDR is to bear in mind TS not performing object excess property checks and potential runtime prototype pollution.

If I understand your question correctly - I believe you can use Extract<keyof T, number> in your filter statement to pull out numerical keys from Enum.

function keys<E extends Enum>(record: E): Extract<keyof E, number>[] {
  return Object.keys(record)
    .map((x) => parseInt(x))
    .filter((x): x is Extract<keyof E, number> => !isNaN(x));
}

const res = keys(foo); // res: (0 | 1 | 124124)[]