为什么在TypeScript中接口可以同时具有数字和字符串索引签名？

Question

According to the language specification it is possible to have an interface with both numeric and string index signature, so the following interface seems to be valid:

interface WeirdArray {
  [index: string]: string;
  [index: number]: string;
}

But I can't find a suitable example where such an interface can be used, can someone provide an example where such an interface makes sense?

Answer 1

Because the number indexer type has to be a subtype of the string indexer type, it's not terribly useful unless you had some crazy type where the string-named properties were of some base type and the number-named properties were a subtype of that type. I can't imagine a use for this.

In short, it's allowed because it's not disallowed .