我是Nodejs的新手，我正在udemy上学习Nodejs课程，我在多次重新运行集成测试时遇到了一些listen EADDRINUSE: address already in use :::4000的问题。第一次成功了，但是后来我在下面的行中得到了上面提到的错误

const server = app.listen(port, () => {winston.info(`Listening on port ${port}`)});

我正在粘贴我的index.js和两个测试文件，如果有人能指出我，这将对我有很大的帮助。

index.js

const Joi = require("@hapi/joi");
    Joi.objectId = require("joi-objectid")(Joi);
    const winston = require("winston");
    const express = require("express");
    const app = express();

    require("./startup/logging")();
    require("./startup/config")();
    require("./startup/dbconnectivity")();
    require("./startup/routes")(app);
    const port = process.env.port || 4000;
    const server = app.listen(port, () => {winston.info(`Listening on port ${port}`)});
    // exporting server object to be used in integration tests.
    module.exports = server;

**Integration test file for Genre**

const request = require("supertest");
let server;
const {Genere} = require("../../models/genere");
const {User} = require("../../models/user");

describe("/api/genere", () => {
    beforeEach(() => {
        console.log("Before each Genre");
        server = require("../../index");
    });
    afterEach(async () => {
        console.log("After each Genre");
        await Genere.deleteMany({});
        await server.close();
    });

    describe("/GET", () => {
        it("should return list of generes", async() => {
            await Genere.insertMany([
                {name: "genre1"},
                {name: "genre2"},
                {name: "genre3"}
            ]);
            const res = await request(server).get("/api/geners");
            expect(res.status).toBe(200);
            console.log("response body is : " + res.body);
            expect(res.body.length).toBe(3);
            expect(res.body.map(g => g.name)).toContain("genre1");
        });
    });

    describe("/GET/:id", () => {
        it("should return genre with id", async() => {
            const genre = new Genere({name: "genre1"});
            await genre.save();
            const res = await request(server).get("/api/geners/"+ genre.id);
            expect(res.status).toBe(200);
            expect(res.body.name).toBe("genre1");
        });

        it("should return error with invalid id", async() => {
            const genre = new Genere({name: "genre1"});
            await genre.save();
            const res = await request(server).get("/api/geners/1");
            expect(res.status).toBe(404);
            expect(res.text).toMatch(/Invalid/);

        });
    });

    describe("/POST", () => {
        it("should return 401 if not authorized", async() => {
            const genere = new Genere({name: "genere1"});
            const res = await request(server).post("/api/geners").send(genere);
            expect(res.status).toBe(401);
        });

        it("should return 400 if the name is less than 4 chars", async() => {
            const res = await createRequestWithGenre({name: "g1"});
            expect(res.status).toBe(400);
        });

        it("should return 400 if the name is greater than 25 chars", async() => {
            const genreName = Array(26).fill("a").join("");
            const res = await createRequestWithGenre({name: genreName})
            expect(res.status).toBe(400);
        });

        it("should return 201 with gener object if proper object is sent", async() => {
            const res = await createRequestWithGenre({name: "genre1"})
            expect(res.status).toBe(201);
            expect(res.body).toHaveProperty("_id");
            expect(res.body).toHaveProperty("name", "genre1");

            const genre = await Genere.find({ name: "genre1"});
            expect(genre).not.toBe(null);
        });

        async function createRequestWithGenre(genre) {
            const token = new User().generateAuthToken();
            return await request(server)
            .post("/api/geners")
            .set("x-auth-token", token)
            .send(genre);
        }
    });
});

当我添加另一个文件进行集成测试，如下面的一个，我开始得到这个文件代码后提到的错误。

const {User} = require("../../models/user");
 const {Genere} = require("../../models/genere");
 const request = require("supertest");
let token;

 describe("middleware", () => {

        beforeEach(() => {
            console.log("Before each Middleware");
            token = new User().generateAuthToken();
            server = require("../../index");
        });

        afterEach(async () => {
            console.log("After each Middleware");
            await Genere.deleteMany({});
            await server.close();
        });

        const exec = async() => {
            return await request(server)
            .post("/api/geners")
            .set("x-auth-token", token)
            .send({name: "gener1"});
        }

         it("should return 400 if invalid JWT token is sent", async() => {
            token = "invalid_token";
            const res = await exec();
            expect(res.status).toBe(400); 
            expect(res.text).toBe("Invalid auth token");
        });
  });

控制台错误

middleware
    ✕ should return 400 if invalid JWT token is sent (510ms)

  ● middleware › should return 400 if invalid JWT token is sent

    listen EADDRINUSE: address already in use :::4000

      10 | require("./startup/routes")(app);
      11 | const port = process.env.port || 4000;
    > 12 | const server = app.listen(port, () => {winston.info(`Listening on port ${port}`)});
         |                    ^
      13 | // exporting server object to be used in integration tests.
      14 | module.exports = server;

      at Function.listen (node_modules/express/lib/application.js:618:24)
      at Object.<anonymous> (index.js:12:20)
      at Object.beforeEach (tests/integration/middleware.test.js:11:22)

如果有人能帮助我为什么它在多次运行时失败，那么它将真正有助于我理解为什么我们需要每次打开和关闭服务器对象。