request and response

Node.jsJunior~2 min read

1. What Request and Response are

In Express.js:

req (request) is the object representing the incoming HTTP request from the client;

res (response) is the object the server uses to send a response back to the client.

When a client makes a request (say, GET /users?id=10), Express automatically creates these two objects and passes them into your route handler.

Example:

javascript

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

app.get('/hello', (req, res) => {
  console.log(req.method); // "GET"
  console.log(req.url);    // "/hello"
  res.send('Hello, Express!');
});

app.listen(3000);

Client → server:

javascript

GET /hello

The server responds:

javascript

Hello, Express!

2. The `req` (Request) object

req is the object representing the HTTP request that came in from the client.

It holds everything the client sent: the URL, parameters, body, headers, and so on.

`req`'s main properties

Property	Description	Example
`req.method`	The request's HTTP method	`'GET'`, `'POST'`, `'PUT'`, `'DELETE'`
`req.url`	The full request URL	`'/users?name=tim'`
`req.path`	Just the path, no query	`'/users'`
`req.query`	The query-parameters object	`{ name: 'tim' }`
`req.params`	The route parameters	`/users/:id` → `{ id: '123' }`
`req.body`	The request body (for POST/PUT)	`{ name: 'John' }` (with `express.json()`)
`req.headers`	The HTTP headers	`{ 'content-type': 'application/json' }`
`req.cookies`	Cookies (with cookie-parser attached)	`{ token: 'abc123' }`
`req.ip`	The client's IP address	`'127.0.0.1'`
`req.protocol`	`'http'` or `'https'`	`'http'`
`req.get(headerName)`	Get a specific header	`req.get('User-Agent')`

An example request with parameters:

javascript

app.get('/user/:id', (req, res) => {
  console.log(req.params.id); // path: /user/42 → "42"
  console.log(req.query);     // query: ?name=Tim → { name: 'Tim' }
  res.send('User found');
});

3. The `res` (Response) object

res is the object Express uses to send a response back to the client.

You can:

return text, JSON, HTML;
set the status, headers, cookies;
end the response.

`res`'s main methods

Method	Purpose	Example
`res.send(data)`	Send text, HTML, or JSON	`res.send('Hello!')`
`res.json(obj)`	Send a JSON object	`res.json({ ok: true })`
`res.status(code)`	Set the response status	`res.status(404).send('Not Found')`
`res.set(header, value)`	Set an HTTP header	`res.set('X-Powered-By', 'Express')`
`res.redirect(url)`	Redirect the client	`res.redirect('/login')`
`res.download(file)`	Send a file for download	`res.download('report.pdf')`
`res.sendFile(path)`	Send a file as the response	`res.sendFile(__dirname + '/index.html')`
`res.end()`	End the response with no body	`res.end()`

An example with JSON and a status

javascript

app.post('/api/users', (req, res) => {
  const user = req.body;
  console.log(user);
  res.status(201).json({
    message: 'User created',
    data: user
  });
});

Response:

javascript

{
  "message": "User created",
  "data": { "name": "Tim", "age": 25 }
}

4. How Express ties req and res together

Every time a client makes an HTTP request:

javascript

Client → [Express App] → Middleware → Route Handler → res.send()

Express creates a pair of objects:

req, to carry the request's data;
res, to send back the response.

They only live for the duration of handling one request.

5. An example using both objects

javascript

app.post('/feedback', (req, res) => {
  const { name, message } = req.body;
  console.log(`From ${name}: ${message}`);

  res.status(200).send(`Thank you, ${name}, your message has been received!`);
});

6. Summary

req (Request) is the incoming-request object: it holds the method, path, parameters, body, headers, and other client data.

res (Response) is the response object: used to send data to the client and set the status and headers.

Together they form the foundation of the client-server interaction in Express.js.

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