Express Middleware
What is Express Middleware ?
Middleware is a function which takes request object , either it returns response to the client or passes the control to the next middleware.
In expressJS all the route handler functions are middleware, because it takes request and terminates the request processing pipeline bu sending request to the client.
const express = require('express');
const app = express();
app.listen(2000,()=>{
console.log("Server is listening PORT 2000")
})
app.get('/',(req,res)=>{
res.send("Sending response to client")
})Inbuilt json() middleware
const express = require('express');
const app = express();
app.listen(2000,()=>{
console.log("Server is listening PORT 2000")
})
app.use(express.json());
app.get('/',(req,res)=>{
res.send("Sending response to client")
})
express.json() returns a middleware function , and the job of the function is to read the request and if there is json in the body then it will parse the json into json object.
firstname property and the firstname is "SAHEB" then it will set the last name "SUTRADHAR" and pass control to the next middleware function , if the request body does not have firstname property and if the name is not "SAHEB" then it will terminate the process there by sending error response to the client .const express = require('express');
const app = express();
app.listen(2000,()=>{
console.log("Server is listening PORT 2000")
})
app.use(express.json());
app.use((req,res,next)=>{
if(req && req.body.firstname && (req.body.firstname == "SAHEB")){
req.body.lastname="SUTRADHAR";
next()
}
else{
res.status(401).send('Name not matched')
}
});
app.post('/',(req,res)=>{
res.send(req.body)
})
Create a custom middleware
Middleware function are normal functions which has three arguments function(req , res , next){} , it should perform al least one operation either send response to client or pass control to next middleware , otherwise the request process operation will get stuck .
Once we create a middleware function we need to add or register it to the request process pipeline, there are two ways to register it.
if we use app.use(mifflewareFN) then it will get applied for all the apis , We can add a middleware to a particular api follow the below example
const express = require('express');
const app = express();
app.listen(2000,()=>{
console.log("Server is listening PORT 2000")
})
app.use(express.json())
let nameCheckMiddleware=(req,res,next)=>{
if(req && req.body && req.body.firstname && (req.body.firstname == "SAHEB")){
req.body.lastname="SUTRADHAR";
next()
}
else{
res.status(401).send('Name not matched')
}
}
app.post('/',[nameCheckMiddleware],(req,res)=>{
res.send(req.body)
})
app.post('/',[middleware1, middleware1],(req,res)=>{}) in all http method we can set one or more than middleware functions.Builtin middleware functions
In express we have many builtin middleware function , check for more Expressjs Built-in Middleware Functions
express.json() we have already seen that why we use this middleware function.
express.urlencoded() This builtin middleware function we use to parse the incoming request with query string and converts it into json object .
Example :
const express = require('express');
const app = express();
app.listen(2000,()=>{
console.log("Server is listening PORT 2000")
})
app.use(express.urlencoded({extended:true}))
app.post('/',[nameCheckMiddleware],(req,res)=>{
res.send(req.body)
})
We need to pass an object with extended property as parameter of urlencoaded function else in the console you will see this warning message body-parser deprecated undefined extended: provide extended option app.js:7:17
I hope this article is helpful for you , i am very glad to help you thanks for reading
