给定一个已有的键，如何在有序的typescript字符串枚举中获得下一个或上一个键？

enum OrderedStringEnum {
    A = 'A',
    B = 'B',
    C = 'C',
    D = 'D',
}

type NextKey<K extends keyof typeof OrderedStringEnum> = 
    K extends typeof OrderedStringEnum.A ? typeof OrderedStringEnum.B : 
    K extends typeof OrderedStringEnum.D ? never : 
    keyof typeof OrderedStringEnum extends infer T ? T extends K ? never : T[number] : never;

type PrevKey<K extends keyof typeof OrderedStringEnum> = 
    K extends typeof OrderedStringEnum.D ? typeof OrderedStringEnum.C : 
    K extends typeof OrderedStringEnum.A ? never : 
    keyof typeof OrderedStringEnum extends infer T ? T extends K ? never : T[number] : never;

const nextKey = <K extends keyof typeof OrderedStringEnum>(key: K): NextKey<K> => {
    const keys = Object.keys(OrderedStringEnum) as (keyof typeof OrderedStringEnum)[];
    const index = keys.indexOf(key);
    return (index === -1 || index === keys.length - 1) ? null : (OrderedStringEnum as any)[keys[index + 1]];
};

const prevKey = <K extends keyof typeof OrderedStringEnum>(key: K): PrevKey<K> => {
    const keys = Object.keys(OrderedStringEnum) as (keyof typeof OrderedStringEnum)[];
    const index = keys.indexOf(key);
    return (index <= 0) ? null : (OrderedStringEnum as any)[keys[index - 1]];
};

console.log(nextKey(OrderedStringEnum.A)); // 输出: B
console.log(prevKey(OrderedStringEnum.C)); // 输出: B
console.log(nextKey(OrderedStringEnum.D)); // 输出: null
console.log(prevKey(OrderedStringEnum.A)); // 输出: null