使用Node原生test模块Mock时如何指定重载类型

我用Node.js原生node:test的mock库来Mock Nest.js的NestFactory.create重载函数，代码如下：

import { LoggerService } from "@nestjs/common";
import { NestFactory } from "@nestjs/core";
import { equal } from "node:assert/strict";
import { suite, test } from "node:test";

suite("Example", async () => {
  test("How to type the override", async ({ mock }) => {
    const create = mock.method(NestFactory, "create", async () => ({}));

    equal(
      create.mock.calls[0].arguments[1].logger instanceof LoggerService,
      true,
    );
  })
})

create存在两个重载定义：

create<T extends INestApplication = INestApplication>(module: IEntryNestModule, options?: NestApplicationOptions): Promise<T>;

create<T extends INestApplication = INestApplication>(module: IEntryNestModule, httpAdapter: AbstractHttpAdapter, options?: NestApplicationOptions): Promise<T>;

实际调用的是第一个重载（传入带logger属性的options对象），但TypeScript默认将create.mock.calls[0].arguments[1]的类型匹配为最后一个重载的AbstractHttpAdapter，导致报错：

Property 'logger' does not exist on type 'AbstractHttpAdapter<any, any, any>'.ts(2339)

已知可通过类型断言解决，但想了解是否有利用mock函数泛型的替代方案。

解决方案：给mock.method指定泛型匹配目标重载 #

可以通过给mock.method显式传入泛型参数，指定你实际使用的重载函数类型，让TypeScript正确推断调用参数的类型。

方式1：直接指定重载函数签名 #

import { LoggerService } from "@nestjs/common";
import { NestFactory, type NestApplicationOptions, type IEntryNestModule, type INestApplication } from "@nestjs/core";
import { equal } from "node:assert/strict";
import { suite, test } from "node:test";

suite("Example", async () => {
  test("How to type the override", async ({ mock }) => {
    // 显式指定泛型为第一个重载的函数类型
    const create = mock.method<
      (module: IEntryNestModule, options?: NestApplicationOptions) => Promise<INestApplication>
    >(NestFactory, "create", async () => ({}));

    // 此时arguments[1]会被正确推断为NestApplicationOptions | undefined
    equal(
      create.mock.calls[0].arguments[1]?.logger instanceof LoggerService,
      true,
    );
  })
})

方式2：提取重载类型别名简化写法 #

如果不想重复写函数签名，可以先从typeof NestFactory.create中提取第一个重载的类型：

import { LoggerService } from "@nestjs/common";
import { NestFactory, type NestApplicationOptions, type IEntryNestModule, type INestApplication } from "@nestjs/core";
import { equal } from "node:assert/strict";
import { suite, test } from "node:test";

// 从create的重载中提取第一个签名的类型
type CreateFirstOverload = typeof NestFactory.create extends {
  (module: IEntryNestModule, options?: NestApplicationOptions): Promise<INestApplication>;
  (...args: any[]): any;
} ? (module: IEntryNestModule, options?: NestApplicationOptions) => Promise<INestApplication> : never;

suite("Example", async () => {
  test("How to type the override", async ({ mock }) => {
    // 使用预定义的类型别名作为泛型
    const create = mock.method<CreateFirstOverload>(NestFactory, "create", async () => ({}));

    equal(
      create.mock.calls[0].arguments[1]?.logger instanceof LoggerService,
      true,
    );
  })
})

原理是mock.method的泛型参数允许你指定被Mock函数的具体类型签名，TypeScript会基于你指定的重载来推断Mock调用的参数类型，而非默认使用最后一个重载。

内容的提问来源于stack exchange，提问作者myol