代码之家 › 专栏 › 技术社区 › A. L

如何将只读字符串[]泛型转换为已知对象

typescript

A. L · 技术社区 · 3 年前

我正在尝试转换一个泛型 readonly string[] 转换为具有已知关键点的对象。但我遇到了一个错误:

Element implicitly has an 'any' type because expression of type 'string' can't be used to index type 'Record<T[number], number>'.
  No index signature with a parameter of type 'string' was found on type 'Record<T[number], number>'.

代码:

class ArrayToObject<T extends readonly string[]> {
  constructor (private keys : T) {}

  GET_OBJECT () {
    const all = {} as Record<T[number], number>
    for (const index in this.keys) {
      all[this.keys[index]] = index
    }
    return all;
  }
}

我基本上想从泛型中得到的东西与此类似:

const a = ["a", "b"] as const
const o : Record<typeof a[number], number> = {
  a : 1,
  b : 2
}

playground

1 回复 | 直到 3 年前

Cheng Dicky 3 年前

这是因为类型系统只能知道 T extends readonly string[] 但是它不能正确地推断其元素的类型。将T定义为元素类型可以帮助类型系统推断它。

此外 for...in 语句将产生一个字符串键值。您需要将其转换为数字或使用 Array.keys() 或 Array.entries() 以获取数字索引。

class ArrayToObject<T extends string> {
  constructor (private keys : readonly T[]) {}
  GET_OBJECT () {
    const all = {} as Record<T, number>
    for (const [index, value] of this.keys.entries()) {
      all[value] = index // + 1 if you want 1-based
    }
    return all;
  }
}

playground

Mark Hanna 3 年前

您可以使用从只读字符串中获取字符串并集 type inference ,如下所示:

const a = ["a", "b"] as const;

type StringFromReadonlyArray<T extends readonly string[]> = T extends readonly (infer U)[] ? U : string;

const tooMany: Record<StringFromReadonlyArray<typeof a>, number> = {
  a: 1,
  b: 2,
  c: 3, // <-- Error: Type '{ a: number; b: number; c: number; }' is not assignable to type 'Record<"a" | "b", number>'.
};

const notEnough: Record<StringFromReadonlyArray<typeof a>, number> = { // <-- Error: Property 'b' is missing in type '{ a: number; }' but required in type 'Record<"a" | "b", number>'.
  a: 1,
};

const justRight: Record<StringFromReadonlyArray<typeof a>, number> = {
  a: 1,
  b: 2,
};

TypeScript Playground

那个 StringFromReadonlyArray 如果可能的话,实用程序类型会从只读数组中提取字符串并集,否则它将解析为泛型 string 类型