Middleware

In Express.js, middleware refers to functions that have access to the request object (req), response object (res), and the next function in the application's request-response cycle. Middleware functions can perform a variety of tasks, such as

1. Modifying the request or response objects.

2. Ending the request-response cycle.

3. Calling the next middleware function in the stack.

Try running this code and see if the logs comes or not.

app.use(function(req, res, next) {
    console.log("request received");
    next();
})

app.get("/sum", function(req, res) {
    const a = parseInt(req.query.a);
    const b = parseInt(req.query.b);

    res.json({
        ans: a + b
    })
});

Modifying the request

app.use(function(req, res, next) {
    req.name = "harkirat"
    next();
})

app.get("/sum", function(req, res) {
    console.log(req.name);
    const a = parseInt(req.query.a);
    const b = parseInt(req.query.b);

    res.json({
        ans: a + b
    })
});

Ending the request/response cycle

app.use(function(req, res, next) {
    res.json({
        message: "You are not allowed"
    })
})

app.get("/sum", function(req, res) {
    console.log(req.name);
    const a = parseInt(req.query.a);
    const b = parseInt(req.query.b);

    res.json({
        ans: a + b
    })
});

Calling the next middleware function in the stack.

app.use(function(req, res, next) {
    console.log("request received");
    next();
})

app.get("/sum", function(req, res) {
    const a = parseInt(req.query.a);
    const b = parseInt(req.query.b);

    res.json({
        ans: a + b
    })
});

Route specific middlewares

Route-specific middleware in Express.js refers to middleware functions that are applied only to specific routes or route groups, rather than being used globally across the entire application

const express = require('express');
const app = express();

// Middleware function
function logRequest(req, res, next) {
  console.log(`Request made to: ${req.url}`);
  next();
}

// Apply middleware to a specific route
app.get('/special', logRequest, (req, res) => {
  res.send('This route uses route-specific middleware!');
});

app.get("/sum", function(req, res) {
    console.log(req.name);
    const a = parseInt(req.query.a);
    const b = parseInt(req.query.b);

    res.json({
        ans: a + b
    })
});

app.listen(3000, () => {
  console.log('Server running on port 3000');
});

Commonly used middlewares:

Through your journey of writing express servers , you’ll find some commonly available (on npm) middlewares that you might want to use

1. express.json

The express.json() middleware is a built-in middleware function in Express.js used to parse incoming request bodies that are formatted as JSON. This middleware is essential for handling JSON payloads sent by clients in POST or PUT requests.

const express = require('express');
const app = express();

// Use express.json() middleware to parse JSON bodies
app.use(express.json());

// Define a POST route to handle JSON data
app.post('/data', (req, res) => {
  // Access the parsed JSON data from req.body
  const data = req.body;
  console.log('Received data:', data);

  // Send a response
  res.send('Data received');
});

app.listen(3000, () => {
  console.log('Server running on port 3000');
});

cors - Cross origin resource sharing

Cross-Origin Resource Sharing (CORS) is a security feature implemented by web browsers that controls how resources on a web server can be requested from another domain. It's a crucial mechanism for managing cross-origin requests and ensuring secure interactions between different origins on the web.

Cross origin request from the browser

Same request from Postman