HTTP Request and Response Cycle in Express.js

The HTTP request and response cycle is the process through which a client communicates with a server. In Express.js, every client request is received by the server, processed, and a response is sent back to the client. Understanding this cycle is essential for building web applications and APIs.

• It defines how a client and server communicate over HTTP.
• Every request is processed before a response is returned.
• Express.js uses the req and res objects to handle this communication.

Request Object

The Request Object (req) represents the HTTP request sent by the client to the Express server. It contains information about the request, such as the URL, route parameters, query parameters, request body, headers, and cookies.

Syntax:

app.get('/', (req, res) => {
    // Access request data using req
});

Request Object Properties

Response Object

The Response Object (res) is passed as the second parameter to the route handler. It is used to send responses such as HTML pages, JSON data, files, images, or status codes back to the client.

Response Object Properties

Methods to Send Request to Server

1. Client Sends a Request

The cycle starts when a clients - such as browser , mobile app or API testing tool(like postman)- sends an HTTP request to the server.

This request includes:

• HTTP method (e.g., GET, PUT, POST, DELETE)
• URL/EndPoint (e.g., /users, /products/1)
• Headers (e.g., content-type, authorization)
• Optional Data(like form-data, or JSON in the request body)

2. Express Receives the Request

Express.js listens for incoming requests on the specified routes and HTTP methods. When a matching route is found, it passes the request to the corresponding route handler.

filename: app.js

const express = require('express');
const app = express();
app.get('/', (req, res) => {
    res.send('App created successfully');
});
app.listen(3000, () => {
    console.log('Server running on port 3000');
});

Console Output:

Browser Output:

3. Middleware Processing

Before reaching the router handler, the request can pass through one or more middleware functions. Middleware can modify the req (request) or perform actions like authentication , logging, or parsing data.

filename: app.js

app.use((req, res, next) => {
    console.log("Request received");
    next();
});

4. Route Handler Executes

The matched route executes its callback function, where you can access request data using the req object and send a response using the res object.

filename: express.js

const express = require('express');
const app= express();
app.get('/user',(req,res)=>{
    res.send('Data added Successfully!')
}).listen(8080,()=>{
    console.log('User Data Saved!')
})

To run the file use node <filename>

Output:

Note: the output will run on localhost:8080/user, where /user will the user endpoint

5. Server Sends a Response

Using the res object , Express sends the response back to the client. You can send plain text, JSON , HTML or status code.

filename: app.js

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

6. Cycle Completes

Once the response is sent , the cycle ends. The client receives the result, and may act on it or display it to the user.

Status Code

When a server responds to client request in Express.js , it returns more than just a data - it also checks an HTTP Status code. These codes are 3- digit numbers that indicates the result of the request. it helps the client to understand whether the request is successful, failed or requires further action. To use appropriate status code improves communication between client and server, enhances debugging and ensures better API design.

Common HTTP Status Code and their Usage in Express.js