Middleware notes

1. Middleware concept

The so-called middleware refers to the intermediate processing link of business process

2. Global validation Middleware

Any request initiated by the client will trigger the middleware when it reaches the server
A global middleware can be defined by calling app.use (middleware function)

// Import required modules
  const express = require('express')
  const app = express()
  
  // //Define a simple middleware function
  // const kw = (req,res,next)=> {
  //     console.log('This is the simplest middleware ');
  //     //Transfer the flow relationship to the next middleware or routing
  //     next()
  // }
  
  // //Global validation Middleware
  // app.use(kw)
  
  // simplify
  // First Middleware
  app.use((req,res,next)=>{
      console.log('This is the simplest middleware');
      next()
  })
  // Second Middleware
  app.use((req,res,next)=>{
      console.log('This is the simplest middleware 3333');
      next()
  })
  
  app.get('/',(req,res)=>{
      console.log('Called/This route');
      res.send('fdff')
  })
  
  app.listen(3000,()=>{
      console.log('complete');
  })

3. Role of Middleware

Multiple middleware share one req and res. based on this feature, we can uniformly add custom attribute methods for req and res objects in the upstream middleware for use by the downstream middleware or router

4. Partially effective Middleware

Middleware that does not use app.use() definition is called local middleware

    const express = require('express')
    const app = express()
    
    // middleware 
    const mv1 = (req,res,next) => {
        console.log('This is the number of intermediate pieces');
        next()
    }
    
    app.get('/',(req,res)=>{
        res.send('ggjg')
    })
    
    app.get('/user',mv1,(req,res)=>{
        res.send('ffddf')
    })
    
    app.listen(3000,()=>{
        console.log('complete');
    })

5. Precautions for Middleware

Be sure to register the middleware before routing
The request sent by the client can be processed by calling multiple middleware continuously
After executing the business code of the middleware, don't forget to call the next() function
To prevent code logic confusion, do not write additional code after calling the next() function
When multiple middleware are called continuously, req and res objects are shared among multiple middleware

6. Middleware classification

Application level Middleware
- Middleware bound to app instances through app.use(), app.get(), or app.post(), is called application level middleware
Routing level Middleware
- The middleware bound to the express.Router() instance is called the routing level middleware

Error level Middleware

Note: the error level middleware must be registered after all routes

const express = require('express')
const app = express()
app.get('/',(req,res)=>{
 throw new Error('An internal error occurred on the server')
 res.send('fsaf')
})
// Error level middleware, placed at the end of all routes
app.use((err,req,res,next)=>{
 console.log('An error has occurred'+err.message);
 res.send(err.message)
})
app.listen(3000,()=>{
 console.log('complete');
})

Express built-in Middleware
- express.static is a built-in middleware for quickly hosting static resources, such as HTML files, pictures, CSS styles, etc. (no compatibility)
- express.json parses the request body data in JSON format (* * * compatible * *, only available in version 4.16.0 + `)
```
const express = require('express')
const app = express()
app.use(express.json())
app.post('/user',(req,res)=>{
 // req.body receive client request body
 res.send(req.body)
})
app.listen(3000,()=>{
 console.log('complete');
})
```
- express.urlencoded parses the request body data in URL encoded format (* * * compatible * *, only available in version 4.16.0 + ` with the same usage as above
Third party Middleware

7. Custom Middleware

Manually simulate a middleware like express.urlencoded

// Import express module
const express = require('express')
// Create a server instance of express
const app = express()
// 4. Import Node built-in module querystring
const qs = require('querystring')
// Middleware for parsing form data
app.use((req,res,next)=>{
    // Define the specific business logic of the middle price
// 1. Define a str string to store the request body data sent by the client
    let str = ''
// 2. Listen to the data event of req
    req.on('data',rem =>{
        str+=rem
    })
// 3. Listen for the end event of req
    req.on('end',()=>{
        // 5. Call the qs.parse() method to parse the query string into an object
        const arr = qs.parse(str)
        req.body=arr
        next()
    })
})
    app.post('/user',(req,res)=>{
        // 6. Mount the parsed data object as req.body attribute
        res.send(req.body)
    })
// Call the app.listen method, specify the port number and start the web server
app.listen(3000,()=>{
    console.log('complete');
})

Package on the above basis

The server

// Import express module
const express = require('express')
// Create a server instance of express
const app = express()
// 4. Import Node built-in module querystring
// Import middleware module
const arr = require('./Encapsulation Middleware')
// Global Middleware
app.use(arr)
app.post('/user', (req, res) => {
    // 6. Mount the parsed data object as req.body attribute
    res.send(req.body)
})
// Call the app.listen method, specify the port number and start the web server
app.listen(3000, () => {
    console.log('complete');
})

middleware

const qs = require('querystring')
// Middleware for parsing form data
module.exports=((req, res, next) => {
    // Define the specific business logic of the middle price
    // 1. Define a str string to store the request body data sent by the client
    let str = ''
    // 2. Listen to the data event of req
    req.on('data', rem => {
        str += rem
    })
    // 3. Listen for the end event of req
    req.on('end', () => {
        // 5. Call the qs.parse() method to parse the query string into an object
        const arr = qs.parse(str)
        req.body = arr
        next()
    })
})

Tags: node.js Middleware

Posted on Tue, 12 Oct 2021 21:42:59 -0400 by creative

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