如何使用Jest和SuperTest Mock Express中间件类方法？

你的verifyAuthenticated是类的箭头函数属性，这种属性在类实例化时就会被赋值给实例对象。而app.ts在启动时就导入了authMiddleware实例，并直接把verifyAuthenticated的引用绑定到了路由上。后续用jest.spyOn去mock这个属性时，路由里使用的还是最初加载的那个原函数引用，所以mock没有生效。

方案1：将箭头函数改为原型方法 #

把verifyAuthenticated从箭头函数属性改成类的原型方法，这样Jest可以正确mock原型上的方法，不管实例何时创建，调用的都是原型上的方法，mock会覆盖原实现。

修改AuthMiddleware.ts：

import { Request, Response, NextFunction } from "express";

class AuthMiddleware {
  constructor() {
    console.log("AuthMiddleware instance created");
  }

  // 移除箭头，改为原型方法
  verifyAuthenticated(request: Request, response: Response, next: NextFunction) {
    console.log("This is the real deal");
    next();
  };
}

export default new AuthMiddleware();

测试代码无需修改，运行后会打印This is the mock deal，mock生效。

方案2：先Mock再导入App #

如果不想修改类的方法类型，需要确保在mockverifyAuthenticated之后再导入app，避免app提前绑定原函数的引用。

修改测试文件：

import request from "supertest";
import authMiddleware from "../src/AuthMiddleware";
import { NextFunction, Request, Response } from "express";

describe("GET /", () => {
  test("Should return online.", async () => {
    // 先完成mock
    jest
      .spyOn(authMiddleware, "verifyAuthenticated")
      .mockImplementation((req: Request, res: Response, next: NextFunction) => {
        console.log("This is the mock deal");
        next();
      });

    // 延迟导入app，此时会使用mock后的方法
    const { default: app } = await import("../src/app");
    const agent = request(app);
    const response = await agent.get("/").send();

    expect(response.statusCode).toBe(200);
    expect(response.headers["content-type"]).toContain("application/json");
    expect(response.body.status).toBe("online");
  });
});

方案3：Mock整个AuthMiddleware模块 #

直接mock整个模块，让app导入的就是mock后的实例，从根源上替换原方法。

修改测试文件：

import request from "supertest";
import { NextFunction, Request, Response } from "express";

// 提前mock整个AuthMiddleware模块
jest.mock("../src/AuthMiddleware", () => {
  const mockVerify = (req: Request, res: Response, next: NextFunction) => {
    console.log("This is the mock deal");
    next();
  };
  return {
    default: {
      verifyAuthenticated: mockVerify
    }
  };
});

// 导入app，此时已使用mock后的模块
import app from "../src/app";

describe("GET /", () => {
  test("Should return online.", async () => {
    const agent = request(app);
    const response = await agent.get("/").send();

    expect(response.statusCode).toBe(200);
    expect(response.headers["content-type"]).toContain("application/json");
    expect(response.body.status).toBe("online");
  });
});

内容的提问来源于stack exchange，提问作者user16197389