如何修改TypeScript泛型函数签名实现服务返回类型参数推断？

我正在创建一个createService函数，第一个参数是“服务”数组，第二个参数需要自动推断这些服务的返回类型。示例如下：

const serviceOne = createService([], () => Promise.resolve({} as Service1));
const serviceTwo = createService([], () => Promise.resolve({} as Service2));

const serviceThree = createService(
  [serviceOne, serviceTwo],
  (one, two) => Promise.resolve({}), // one应被推断为Service1类型，two应被推断为Service2类型
);

目前我把参数类型设为any，函数签名如下：

function createService<T extends Service<any>[], U>(deps: T, factory: (...args: any[]) => Promise<U>) {
  return {} as Service<U>;
}

type Service<U> = {
  instance: U
};

需要修改函数签名实现上述类型推断。

你可以借助TypeScript的元组类型映射和条件类型，提取依赖数组中每个服务的instance类型，让工厂函数的参数自动匹配对应类型。修改后的代码如下：

type Service<U> = {
  instance: U;
};

// 工具类型：从Service数组中提取每个元素的instance类型，组成对应顺序的元组
type ExtractServiceInstances<T extends Service<any>[]> = {
  [K in keyof T]: T[K] extends Service<infer U> ? U : never;
};

function createService<
  T extends Service<any>[], 
  U
>(
  deps: T, 
  factory: (...args: ExtractServiceInstances<T>) => Promise<U>
): Service<U> {
  return {} as Service<U>;
}

关键说明 #

1. ExtractServiceInstances工具类型：遍历依赖数组的每个元素，通过infer关键字提取出每个Service的instance类型，最终生成与依赖数组顺序一致的元组类型。
2. 工厂函数参数类型绑定：将工厂函数的args参数类型指定为ExtractServiceInstances<T>，TypeScript会根据传入的deps数组，自动推断出工厂函数每个参数的具体类型。
3. 泛型约束保持：T依然约束为Service<any>[]，确保传入的依赖都是合法的服务实例；返回值保持Service<U>，保证服务实例的类型一致性。

验证示例 #

修改后类型推断会正常工作：

type Service1 = { id: number; name: string };
type Service2 = { log: (msg: string) => void };

const serviceOne = createService([], () => Promise.resolve({ id: 1, name: "test" } as Service1));
const serviceTwo = createService([], () => Promise.resolve({ log: (msg) => console.log(msg) } as Service2));

const serviceThree = createService(
  [serviceOne, serviceTwo],
  (one, two) => {
    // one 自动推断为 Service1 类型
    console.log(one.id);
    // two 自动推断为 Service2 类型
    two.log("hello");
    return Promise.resolve({ combined: true });
  }
);

内容的提问来源于stack exchange，提问作者Andy Jessop