第一步创建文件并初始化

yarn init

第二步安装依赖 

yarn add body-parser express nodemon

body-parser 用于获取 body 参数， nodemon 用于热更新
第三步创建功能文件

comon/index.js

const fs = require('fs');
// promisify 异步处理
const { promisify } = require('util');
const path = require('path');
const readFile = promisify(fs.readFile)

// 获取 json 数据
module.exports.getFileData = async (fileName) => {
  const filePath = path.join(__dirname, `../json/${fileName}.json`)
  const data = await readFile(filePath, 'utf-8')
  return JSON.parse(data)
}

json/userData.json

[
  {
    "userName": "Fengchengzhi",
    "sex": "男",
    "age": 25,
    "mobile": 123456,
    "mailbox": "xxxxx",
    "password": "123456"
  },
  {
    "userName": "root",
    "sex": "男",
    "age": 25,
    "mobile": 123456,
    "mailbox": "xxxxx",
    "password": "123456"
  }
]

router/index.js

const bodyParser = require('body-parser');
// create application/json parser 用于获取 body 参数
const jsonParser = bodyParser.json()

// create application/x-www-form-urlencoded parser
// const urlencodedParser = bodyParser.urlencoded({ extended: false })

module.exports = {
  jsonParser
}

router/login.js

const express = require('express');
// 用于获取 body 参数
const { jsonParser } =  require('./index');
// 获取 json 数据
const { getFileData } = require('../common');

const router = express.Router();

// 登录
router.post('/api/login', jsonParser, async(req, res) => {
  global.isLogin = true
  // 获取所有用户信息
  const allUserInfo = await getFileData('userData')
  const { userName, password } = req.body
  const info = allUserInfo.find(item => item.userName == userName)
  if (!info) {
    res.json({
      code: 200,
      data: null,
      message: '用户不存在！'
    })
  } else {
    if (info.password !== password) {
      res.json({
        code: 200,
        data: null,
        message: '密码错误！'
      })
    } else {
      res.json({
        code: 200,
        data: info,
        message: '登录成功！'
      })
    }
  }
})

// 退出登陆
router.post('/api/loginOut', jsonParser, (req, res) => {
  global.isLogin = false
  res.json({
    code: 200,
    data: null,
    message: '退出登录成功！'
  })
})


module.exports = router;

index.js

const express = require('express');
const app = express();
const login = require('./router/login');

// 登录状态
global.isLogin = false

// 登录拦截
app.all('*', (req, res, next) => {
  if (req.url !== '/api/login') {
    if (!global.isLogin) {
      return res.json({
        code: 304,
        data: null,
        message: '未登录！'
      })
    }
  }
  next()
})

// router
app.use('/', login)

app.listen(5000, () => {
  console.log('服务启动')
})

package.json

{
  "name": "node-api",
  "version": "1.0.0",
  "main": "index.js",
  "license": "MIT",
  "dependencies": {
    "body-parser": "^1.19.2",
    "express": "^4.17.3",
    "nodemon": "^2.0.15"
  },
  "scripts": {
    "start": "nodemon index.js"
  }
}

启动 yarn start

测试结果 （注意这里的请求是做过跨域处理的）