HTTP request and response cycle

Every interaction between a client and an Express server follows the same pattern: a request comes in, gets processed, and a response goes out. Express represents these two sides with the req and res objects.

The req object

req carries everything about the incoming request — URL, params, query string, body, headers, cookies.

app.get("/", (req, res) => {
    // read data from req here
});

Frequently used req properties:

Property	Description
req.app	Reference to the Express app instance.
req.body	Data submitted by the client (needs a body-parsing middleware).
req.cookies	Cookies sent by the client (needs cookie-parser).
req.ip	The client's IP address.
req.path	The path portion of the URL.
req.route	The route currently matched.
req.params	Named route parameters.
req.query	Query-string parameters.
req.files	Uploaded files (needs an upload middleware).
req.is()	Checks the request's content type against a given MIME type.

The res object

res is how you send data back — HTML, JSON, files, status codes.

Frequently used res methods:

Method	Description
res.app	Reference to the app instance.
res.append()	Appends a value to a response header.
res.cookie()	Sets a cookie.
res.get()	Reads a response header's current value.
res.end()	Ends the response.
res.json()	Sends a JSON response.
res.links()	Sets Link response headers.
res.render()	Renders a view template.
res.location()	Sets the Location header.
res.send()	Sends a response of any type.
res.set()	Sets a response header.
res.status()	Sets the HTTP status code.

Walking through a request

1. A client sends a request — say, a mobile app requesting /products/42.
2. Express matches the URL and method to a registered route.

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

app.get("/", (req, res) => {
    res.send("Order Service is live");
});

app.listen(4600, () => console.log("Listening on 4600"));

3. Any middleware registered before the route runs first.

app.use((req, res, next) => {
    console.log("Incoming request logged");
    next();
});

4. The matched route handler runs and can read req, then build a response with res.

app.get("/order/:id", (req, res) => {
    res.send(`Order #${req.params.id} details`);
});

5. Express sends the response back to the client.

app.get("/status", (req, res) => {
    res.status(200).json({ message: "OK" });
});

6. The cycle ends. If the client sends another request, the whole flow repeats.

Status codes at a glance

Code	Meaning	Example
200 OK	Success	res.status(200).send('OK')
201 Created	Resource created	res.status(201).json({ message: 'Created' })
204 No Content	Success, empty body	res.status(204).send()
400 Bad Request	Client error	res.status(400).send('Bad Request')
401 Unauthorized	Needs authentication	res.status(401).json({ error: 'Unauthorized' })
403 Forbidden	Access denied	res.status(403).send('Forbidden')
404 Not Found	Resource missing	res.status(404).send('Not Found')
500 Internal Server Error	Server-side failure	res.status(500).send('Server Error')

Choosing the right status code makes APIs easier to consume, debug, and monitor.