Middleware functions are functions that have access to the
request object (req), the response object (res), and the next middleware
function in the application’s request-response cycle. The next middleware
function is commonly denoted by a variable named next.
- As name suggests it comes in middle of something and that is request and response cycle.
- Middleware has access to request and response object.
- Middleware has access to next function of request-response life cycle.
Middleware
functions can perform the following tasks:
·
Execute any
code.
·
Make changes
to the request and the response objects.
·
End the
request-response cycle.
·
Call the next
middleware in the stack.
If the current
middleware function does not end the request-response cycle, it must call next() to
pass control to the next middleware function. Otherwise, the request will be
left hanging.
What is this next()?
A middleware is
basically a function that will the receive the Request and Response objects,
just like your route Handlers do. As a third argument you have another function
which you should call once your middleware code completed. This means you can
wait for asynchronous database or network operations to finish before
proceeding to the next step. This might look like the following:
If the current middleware function does not end the
request-response cycle, it must call
next() to
pass control to the next middleware function. Otherwise, the request will be
left hanging.
Types of express middleware
·
Application level middleware app.use
·
Router level middleware router.use
·
Built-in middleware express.static,express.json,express.urlencoded
·
Error handling middleware app.use(err,req,res,next)
·
Thirdparty middleware bodyparser,cookieparser
Example:
Logging Middleware (Application Level
Middleware )
const express = require('express');
const app = express();
// application level middleware
app.use((req, res, next) => {
console.log(`Logged ${req.url} ${req.method} -- ${new Date()}`);
next();
});
// users route
app.get('/users', (req, res) => {
res.json({
status: true,
});
});
// save route
app.post('/save', (req, res) => {
res.json({
status: true,
});
});
app.listen(3002, (req, res) => {
console.log('server running on port 3002');
});
Output
