Express Application Initialization, Routing & Request/Response Objects0%
Route Parameters, Query Strings & Request Body Parsing

Express Application Initialization, Routing & Request/Response Objects

Beginner12 min read•Updated: Jul 11, 2026
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Express Application Initialization, Routing & Request/Response Objects

The core workflow of any Express application revolves around three pillars: initializing the application instance, registering routes mapped to HTTP verbs, and manipulating the Request (req) and Response (res) objects.

1. Application Initialization and Server Lifecycle

An Express application begins by invoking the top-level express() function exported by the module:

JavaScript
import express from 'express';
 
const app = express();
const PORT = process.env.PORT || 4000;
 
// Register routes here...
 
const server = app.listen(PORT, () => {
console.log(`Server listening on http://localhost:${PORT}`);
});
 
// Graceful Shutdown handling
process.on('SIGTERM', () => {
console.log('SIGTERM signal received: closing HTTP server');
server.close(() => {
console.log('HTTP server closed cleanly');
});
});

2. HTTP Verb Routing Methods

Express supports all standard HTTP verbs:

JavaScript
// GET: Retrieve resource
app.get('/api/products', (req, res) => {
res.json({ products: [] });
});
 
// POST: Create resource
app.post('/api/products', (req, res) => {
res.status(201).json({ message: 'Product created' });
});
 
// PUT: Replace entire resource
app.put('/api/products/:id', (req, res) => {
res.json({ message: `Product ${req.params.id} fully updated` });
});
 
// PATCH: Partial update of resource
app.patch('/api/products/:id', (req, res) => {
res.json({ message: `Product ${req.params.id} partially modified` });
});
 
// DELETE: Remove resource
app.delete('/api/products/:id', (req, res) => {
res.status(204).send(); // 204 No Content
});

3. The req and res Ecosystem

Express decorates the native Node.js request and response objects with convenient helper properties and methods:

Key req Properties

  • req.method: The HTTP verb (GET, POST, etc.)
  • req.path: The path part of the request URL (e.g., /api/users)
  • req.ip: Client IP address
  • req.headers: Key-value map of incoming HTTP headers
  • req.get(headerName): Case-insensitive header lookup (e.g., req.get('Authorization'))

Key res Methods

  • res.status(code): Sets the HTTP status code (chainable)
  • res.json(data): Sends JSON response with appropriate headers
  • res.send(body): Sends generic HTTP body (HTML, string, or Buffer)
  • res.sendStatus(code): Sets status code and sends its text representation (e.g., res.sendStatus(404) sends "Not Found")
  • res.redirect(url): Performs HTTP 302/301 redirection
  • res.set(field, value): Sets an outgoing HTTP header
Interactive Quiz

Multiple Choice Questions

Question 1
Which HTTP status code should be returned when a new resource is successfully created via a POST request?
Question 2
What is the fundamental difference between HTTP PUT and HTTP PATCH methods in RESTful API design?
Question 3
Which method on the Express res object sets the HTTP response code and returns the response in a chainable manner?
Question 4
How can you retrieve the value of the Authorization header safely regardless of whether the client sent it in upper or lowercase?
Question 5
What does res.status(204).send() communicate to the client?
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Route Parameters, Query Strings & Request Body Parsing

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