编写中间件

概述

中间件函数是在应用程序的请求-响应周期中可以访问 请求对象 (req)、响应对象 (res) 和 next 函数的函数。next 函数是 Express 路由中的一个函数，当被调用时，它会在当前中间件之后执行中间件。

中间件函数可以执行以下任务：

• 执行任何代码。
• 更改请求和响应对象。
• 结束请求-响应周期。
• 调用堆栈中的下一个中间件。

如果当前中间件函数没有结束请求-响应循环，它必须调用 next() 将控制权传递给下一个中间件函数。否则，请求将被挂起。

下图显示了中间件函数调用的元素：

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示例

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const express = require('express')
const app = express()

app.get('/', (req, res) => {
  res.send('Hello World!')
})

app.listen(3000)

中间件函数 myLogger

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const myLogger = function (req, res, next) {
  console.log('LOGGED')
  next()
}

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const express = require('express')
const app = express()

const myLogger = function (req, res, next) {
  console.log('LOGGED')
  next()
}

app.use(myLogger)

app.get('/', (req, res) => {
  res.send('Hello World!')
})

app.listen(3000)

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中间件函数 requestTime

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const requestTime = function (req, res, next) {
  req.requestTime = Date.now()
  next()
}

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const express = require('express')
const app = express()

const requestTime = function (req, res, next) {
  req.requestTime = Date.now()
  next()
}

app.use(requestTime)

app.get('/', (req, res) => {
  let responseText = 'Hello World!<br>'
  responseText += `<small>Requested at: ${req.requestTime}</small>`
  res.send(responseText)
})

app.listen(3000)

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中间件函数 validateCookies

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async function cookieValidator (cookies) {
  try {
    await externallyValidateCookie(cookies.testCookie)
  } catch {
    throw new Error('Invalid cookies')
  }
}

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const express = require('express')
const cookieParser = require('cookie-parser')
const cookieValidator = require('./cookieValidator')

const app = express()

async function validateCookies (req, res, next) {
  await cookieValidator(req.cookies)
  next()
}

app.use(cookieParser())

app.use(validateCookies)

// error handler
app.use((err, req, res, next) => {
  res.status(400).send(err.message)
})

app.listen(3000)

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可配置的中间件

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module.exports = function (options) {
  return function (req, res, next) {
    // Implement the middleware function based on the options object
    next()
  }
}

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const mw = require('./my-middleware.js')

app.use(mw({ option1: '1', option2: '2' }))

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