ExpressJS Middleware
ExpressJS-Middleware
- Express.js Middleware functions are functions that have access to the response object(res), request object(req), and the next function in the request-response cycle.
- It appears in middle between an initial request and the final intended route.
- It is used to perform tasks like body parsing for URL-encoded or JSON requests, cookie parsing or even building JavaScript modules on the fly.
Function or tasks of Middleware Function:
- Execute any code.
- Make changes to the request and the response objects.
- End the request-response cycle.
- Call the next middleware in the stack.
Example:
const express = require('express')
const app = express()
# function is middleware function
const myLogger = function (req, res, next) {
console.log('LOGGED')
next()
}
app.use(myLogger)
app.get('/', (req, res) => {
res.send('Hello World!')
})
app.listen(3000)
Types of Middleware
- Application-level middleware
- Router-level middleware
- Error-handling middleware
- Built-in middleware
- Third-party middleware
Application-level middleware
- Application-level middleware is bound to an instance of the app object by using app.use() and app.method() functions here method is the HTTP method.
Example:
const express = require('express')
const app = express()
app.use('/user/:id', (req, res, next) => {
console.log('Request Type:', req.method)
next()
})
Router-level middleware
- Router-level middleware is the same as application-level middleware except it is bound to an instance of express.Router().
Example:
const express = require('express')
const app = express()
const router=express.Router
router.use('/user/:id', (req, res, next) => {
console.log('Request Type:', req.method)
next()
})
Error-handling middleware
- Error-handling middleware is the same as other middleware except the middleware function takes four arguments instead of three.
Example:
const express = require('express')
const app = express()
app.use((err, req, res, next) => {
console.error(err.stack)
res.status(500).send('Something broke!')
})
Built-in middleware
- Built-in middleware functions are included with express.
- Built-in middleware functions present in Express are: express.static, express.json, and express.urlencoded.
- express.static serves static assets such as HTML files, images, and so on.
- express.json parse incoming requests with JSON payloads.
- express.urlencoded parse incoming requests with URL-encoded payloads.
Third-party middleware
- Install the Node.js module
npm install cookie-parser
const express = require('express')
const app = express()
const cookieParser = require('cookie-parser')
// load the cookie-parsing middleware
app.use(cookieParser())
