Route Parameters, Query Strings & Request Body Parsing0%
Middleware Pipeline: Application-Level, Router-Level & Third-Party

Route Parameters, Query Strings & Request Body Parsing

Beginner12 min read•Updated: Jul 11, 2026
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Route Parameters, Query Strings & Request Body Parsing

Client applications transmit data to backend APIs through three primary channels: Route Parameters (identifying specific entities in the URL path), Query Strings (filtering, pagination, and sorting parameters), and the Request Body (structured JSON payloads).

1. Route Parameters (req.params)

Route parameters are named URL segments used to capture values at specified positions in the URL path. They are defined using a colon : prefix:

JavaScript
import express from 'express';
const app = express();
 
// Single parameter: /api/users/42
app.get('/api/users/:id', (req, res) => {
const userId = req.params.id; // "42" (string type)
res.json({ id: userId });
});
 
// Multiple parameters: /api/courses/react-101/reviews/99
app.get('/api/courses/:courseSlug/reviews/:reviewId', (req, res) => {
const { courseSlug, reviewId } = req.params;
res.json({ courseSlug, reviewId });
});

Note: All values in req.params are initially strings. Convert them with Number(req.params.id) or parseInt() when querying numeric database IDs.

2. Query Strings (req.query)

Query strings follow the ? character in a URL and are separated by &. They are commonly used for filtering, pagination, and searching:

Output
GET /api/products?category=electronics&minPrice=100&sort=asc&page=2

Express parses query strings automatically into the req.query object:

JavaScript
app.get('/api/products', (req, res) => {
const { category, minPrice, sort, page = 1, limit = 20 } = req.query;
 
console.log('Category filter:', category);
console.log('Minimum price:', minPrice);
console.log('Page number:', page);
 
res.json({
filters: { category, minPrice: Number(minPrice) },
pagination: { page: Number(page), limit: Number(limit) }
});
});

3. Request Body (req.body)

The request body carries large, structured payloads (JSON, form data) typically in POST, PUT, or PATCH requests. In Express, you MUST register body-parsing middleware before accessing req.body:

JavaScript
// Parse application/json
app.use(express.json({ limit: '1mb' }));
 
// Parse application/x-www-form-urlencoded (HTML form submissions)
app.use(express.urlencoded({ extended: true }));
 
app.post('/api/orders', (req, res) => {
const { items, shippingAddress, paymentToken } = req.body;
 
if (!items || items.length === 0) {
return res.status(400).json({ error: 'Order must contain at least one item' });
}
 
res.status(201).json({
orderId: 'ord_12345',
status: 'CONFIRMED',
totalItems: items.length
});
});
Interactive Quiz

Multiple Choice Questions

Question 1
Given the route definition app.get('/articles/:category/:id'), what will req.params contain when a client accesses /articles/tech/55?
Question 2
In an incoming URL /api/search?q=nodejs&sort=desc, how do you access the search term nodejs in an Express handler?
Question 3
What will req.body evaluate to if a client sends a JSON payload to an Express app that has NOT registered app.use(express.json())?
Question 4
What is the data type of req.params.id in the route /users/:id when the user requests /users/100?
Question 5
Why should you configure { limit: '1mb' } in express.json({ limit: '1mb' })?
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