Classification of middleware

In order to facilitate our understanding and memory of the use of middleware ,Express The official use of common middleware , Divided into 5 Categories: , Namely ：
① Application level middleware
② Routing level middleware
③ Error level middleware
④ Express Built-in middleware
⑤ Third party middleware

Application level middleware

adopt app.use() or app.get() or app.post() , Bound to the app Middleware on instance , It's called application level middleware , The code example is as follows ：

Routing level middleware

Bound to the express.Router() Middleware on instance , It's called routing level middleware . There is no difference between its usage and application level middleware . It's just , Application level middleware is bound to app For instance , The routing level middleware is bound to router For instance , The code example is as follows ：

Error level middleware

The role of error level middleware ： It's designed to catch unexpected errors in the entire project , So as to prevent the abnormal collapse of the project .

Format ： Error level middleware function In the processing function , There has to be 4 Parameters , The order of formal parameters is from front to back , Namely (err, req, res, next).

Be careful ： Error level middleware , Must register after all routes ！
Because in the order from front to back , If you put the error level Middleware in front , When an error occurs , There is no processing code when the program is executed later .

Express Built-in middleware

since Express 4.16.0 Version start ,Express Built in 3 A common middleware , Greatly improved Express Project development efficiency and experience ：
① express.static Built in middleware for fast hosting static resources , for example ： HTML file 、 picture 、CSS Patterns, etc （ No compatibility ）
② express.json analysis JSON Request body data in format （ Compatibility , Only in 4.16.0+ Available in version ）
③ express.urlencoded analysis URL-encoded Request body data in format （ Compatibility , Only in 4.16.0+ Available in version ）

Examples of use ：

express.json Use example of

//  Import  express  modular 
const express = require('express')
//  establish  express  Server instance of 
const app = express()

//  Be careful ： Except for error level middleware , Other middleware , Must be configured before routing 
//  adopt  express.json()  This middleware , Parse... In the form  JSON  Formatted data 
app.use(express.json())

app.post('/user', (req, res) => {
    
  //  On the server , have access to  req.body  This attribute , To receive the request body data sent by the client 
  //  By default , If you don't configure middleware to parse form data , be  req.body  Default equal to  undefined
  console.log(req.body)
  res.send('ok')
})

//  call  app.listen  Method , Specify the port number and start web The server 
app.listen(80, function () {
    
  console.log('Express server running at http://127.0.0.1')
})

Be careful ： On the server side , Can pass req,body To get JSON Form data and url-encoded Formatted data
By default , If you don't configure middleware to parse form data , be req.body Default equal to undefined

express.urlencoded Examples of use

//  Import  express  modular 
const express = require('express')
//  establish  express  Server instance of 
const app = express()

//  Be careful ： Except for error level middleware , Other middleware , Must be configured before routing 
//  adopt  express.urlencoded()  This middleware , Parsing   In the form  url-encoded  Formatted data 
app.use(express.urlencoded({
     extended: false }))

app.post('/book', (req, res) => {
    
  //  On the server , have access to  req.body  This attribute , To receive the request body data sent by the client 
  //  By default , If you don't configure middleware to parse form data , be  req.body  Default equal to  undefined
  //  On the server side , Can pass  req,body  To get  JSON  Form data and  url-encoded  Formatted data 
  console.log(req.body)
  res.send('ok')
})

//  call  app.listen  Method , Specify the port number and start web The server 
app.listen(80, function () {
    
  console.log('Express server running at http://127.0.0.1')
})

Third party middleware

Not Express The official built-in , It's middleware developed by a third party , It's called third-party middleware . In the project , You can download and configure third-party middleware as needed , So as to improve the development efficiency of the project .

for example ： stay [email protected] In the previous version , Regular use body-parser This third-party middleware , To parse the request body data . The procedure is as follows ：

• function npm install body-parser Install middleware
• Use require Import Middleware
• call app.use() Register and use middleware

Be careful ：Express Built in express.urlencoded middleware , Is based on body-parser This third-party middleware is further encapsulated .
（ This explains why the registration syntax of the two is so similar ）

Use the code ：

//  Import  express  modular 
const express = require('express')
//  establish  express  Server instance of 
const app = express()

// 1.  Import middleware for parsing form data  body-parser
const parser = require('body-parser')
// 2.  Use  app.use()  Register middleware 
app.use(parser.urlencoded({
     extended: false }))
// app.use(express.urlencoded({ extended: false }))

app.post('/user', (req, res) => {
    
  //  If there is no middleware configured to parse form data , be  req.body  Default equal to  undefined
  console.log(req.body)
  res.send('ok')
})

//  call  app.listen  Method , Specify the port number and start web The server 
app.listen(80, function () {
    
  console.log('Express server running at http://127.0.0.1')
})