TypeScript如何提取类型中指定值类型对应的属性键

核心是通过映射类型遍历泛型T的所有属性，筛选出值类型符合要求的属性键，再组合为数组类型即可。

核心类型定义 #

// 通用工具类型：提取类型T中所有值类型匹配V的属性键
type KeysOfType<T, V> = {
  [K in keyof T]: T[K] extends V ? K : never
}[keyof T];

// 提取值为number的属性键数组类型
type KeysOfNumbers<T> = Array<KeysOfType<T, number>>;
// 提取值为boolean的属性键数组类型
type KeysOfBooleans<T> = Array<KeysOfType<T, boolean>>;

注意事项 #

• 原代码存在字段大小写不匹配问题：CSV表头为IsRead、IsAD，但Mail类型中定义为小写开头的isRead、isAD，运行时会因字段名不一致导致布尔转换失效，需要统一两边命名。
• TypeScript 对Array.includes的参数有严格类型校验，context.column.toString()返回通用string类型，会和字面量类型的属性键产生类型冲突，只需在调用includes时做简单类型断言即可，不影响运行逻辑。

完整修正代码 #

import {CastingFunction, parse} from 'csv-parse/browser/esm/sync';

const input =
  'ID,Type,From,Title,Content,Date,IsRead,IsAD\r\n1,0,Mars,My car glass was broken,How much DOGE to fix this.....,423042301654134900000,false,false';

// 字段名和CSV表头对齐
type Mail = {
  ID: string;
  Type: number;
  From: string;
  Title: string;
  Content: string;
  Date: number;
  IsRead: boolean;
  IsAD: boolean;
};

type KeysOfType<T, V> = {
  [K in keyof T]: T[K] extends V ? K : never
}[keyof T];

type KeysOfNumbers<T> = Array<KeysOfType<T, number>>;
type KeysOfBooleans<T> = Array<KeysOfType<T, boolean>>;

const castNumberAndBoolean =
  <T>(
    keysOfNumbers: KeysOfNumbers<T>,
    keysOfBooleans: KeysOfBooleans<T>,
  ): CastingFunction =>
  (value, context) => {
    const column = context.column.toString();
    if (keysOfNumbers.includes(column as KeysOfType<T, number>)) {
      return Number(value);
    }
    if (keysOfBooleans.includes(column as KeysOfType<T, boolean>)) {
      return value === 'true';
    }
    return value;
  };

parse(input, {
  columns: true,
  cast: castNumberAndBoolean<Mail>(['Type', 'Date'], ['IsRead', 'IsAD']),
});

修改完成后，TypeScript 会自动校验传入的字段名是否合法：如果将非number类型的字段（如类型为string的ID）传入第一个参数，或是将非boolean类型的字段传入第二个参数，编译器会直接抛出类型错误，从根源上避免传参错误。

内容的提问来源于stack exchange，提问作者張政鈞