Express.js Lifecycle and Request Flow

When a client sends a request to an Express.js application, the request passes through several stages before a response is returned. This process is called the request lifecycle or request flow.

Understanding this flow is important because Express.js applications are built around handling requests and responses.

How Request Flow Works

The lifecycle usually follows these steps:

1. A client sends an HTTP request to the server;
2. Express.js receives the request;
3. Middleware functions process the request;
4. The matching route handler runs;
5. A response is sent back to the client.

Example:

const express = require('express');

const app = express();

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

// Route handler
app.get('/', (req, res) => {
  res.send('Home Page');
});

app.listen(3000);

In this example, the middleware runs first and logs a message. The next() function passes control to the next middleware or route handler. Then the route handler sends the final response.

Request and Response Objects

Express.js provides two important objects:

• req: contains information about the incoming request;
• res: used to send a response back to the client.

Example:

app.get('/users', (req, res) => {
  res.send('Users route');
});

Here:

• req contains request data such as headers, parameters, and query strings;
• res.send() sends data back to the browser or API client.

Middleware Flow

Middleware functions are executed in the order they are registered. Each middleware can:

• Modify the request;
• Run logic;
• End the response;
• Pass control using next().

If a middleware does not call next() or send a response, the request may remain unfinished.