javascript - Typescript 对象索引器和与索引器类型不匹配的键

Question

Typescript 文档显示了以下示例:

interface NumberDictionary {
    [index: string]: number;
    length: number;    // ok, length is a number
    name: string;      // error, the type of 'name' is not a subtype of the indexer
}

以上示例的推荐解决方法是什么，例如我有一个对象，我知道它有一个 name: string 属性，它可以有任何其他可能的键，所有键都必须是数字？

Answer 1

问题是这样的类型本质上是不一致的。考虑以下代码:

let prop = Math.random() > 0.5 ? "name" : "other"
let dic: NumberDictionary;
let value = dic[prop] // typed as number but could end up as string at run-time

索引定义告诉我们 number 但我们可能会在运行时以 string 结束。

诚实的做法是让索引签名返回number |字符串。

interface NumberDictionary {
    [index: string]: number | string;
    length: number;
    name: string;
}
let prop = Math.random() > 0.5 ? "name" : "other"
let dic: NumberDictionary;
let value = dic[prop] // typed as number | string we need a type guard to tell teh difference

诚实的解决方案可能并不总是实用的，并且在充分意识到危险的情况下，您可以定义一个交集类型，让您摆脱不一致:

type NumberDictionary = {
  [index: string]: number;
} & {
  length: number;    
  name: string;
}

let prop = Math.random() > 0.5 ? "neverName" : "other"
let dic: NumberDictionary = {
  name: "",
  length: 1
} as NumberDictionary; // type assertion necessary, ts will still complain here about the inconsistentcy 
let value = dic[prop] // typed as number, hope everyone avoids passing in name