1-17 express Middleware

express middleware

middleware , It refers to the intermediate process of business process

express Middleware concept

When a request arrives express After the server of , Multiple middleware can be called continuously , To preprocess this request

express Middleware format

express Middleware , It's essentially one function Processing function
Be careful ： In the formal parameter list of middleware functions , Must contain next Parameters . The routing processing function only contains req and res.

next Function function
next The key to realize the continuous call of multiple Middleware in a functional way , It means transferring the flow relationship to the next middleware or routing .

Define middleware functions

const ww = function(req,res,next){
    console.log(' Middleware function ')
    next()
}

Globally effective middleware

app.use((req,res,next)=>{
    const time = Date.now();
    req.time = time
    next()
})
// Global middleware can define multiple , It's done in order 

Partially effective middleware

 // Don't use app.use() Defined middleware , Middleware called local validation 
const mw = ((req,res,next)=>{
    console.log(' Local 1 middleware ')
    next()
})
const ms = ((req,res,next)=>{
    console.log(' Local 2 middleware ')
    next()
})
// You can also define multiple local middleware 
//[mw,ms]  It's OK to put it in an array 
app.get('/',mw,ms,(req,res)=>{
    res.send('666')
})
app.post('/',(req,res)=>{
    res.send('88')
})

Middleware considerations ：

1. Be sure to register middleware before routing
2. The request sent by the client , Multiple middleware processes can be called continuously
3. After executing the business code of middleware , Don't forget to call next() function
4. In order to prevent code logic confusion , call next() Don't write extra code after the function
5. When calling multiple Middleware in succession , Between multiple middleware , share req and res object

Classification of middleware
1 Application level middleware
adopt app,use() or app.get() or app.post(), Bound to the app Middleware on strength , It's called application level middleware

app.use((req,res,next)=>{
next()
})
app.get('/',nw1,(req,res)=>{
res.send('hellp')
})

2. Routing level middleware
The routing level middleware is bound to express.Router() Middleware on instance , It's called routing level middleware .

var app = express()
var router = express.Router()
// Routing level middleware 
router.use(function(){
next()
})

2. 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）

const express = require('express')
const app = express()
app.get('/',(req,res)=>{
 throw new Error(' An error occurred inside the server ')
 res.send('home page')
})
// Middleware with error level , Replenishment error , Prevent collapse 
app.use((err,req,res,next)=>{
    console.log(' Something went wrong '+err.message)
    res.send(`Error${err.message}`)
})

app.listen(80,()=>{
    console.log('server and at ')
})

Be careful ： Error level middleware must be registered after all routes .

express Built-in middleware

1. express.static() Fast hosting of static resources .
2. express.json() analysis JSON Request body data in format （ Compatibility , Only in 4.16.0+ Version available ）
3. express.urlencoded() analysis URL-encoded Request body data in format （ Compatibility , Only in 4.16.0+ Version available ）

app.use(express.static('./file'))
app.user(express.json())
app.user(express.urlencoded({extended:false}))

Third party middleware
Not express The official built-in , It's middleware developed by a third party . In the project , You can download and configure the third-party middle price on demand , So as to improve the development efficiency of the project .