Query string parsing in Express

Introduction

Query string parsing helps you get information from the URL after the question mark. This lets your app understand what the user wants.

When you want to read filters or search terms from a URL like /products?category=books

When you need to get page numbers or limits for showing lists, like /items?page=2&limit=10

When you want to customize responses based on user choices sent in the URL

When handling form submissions sent via GET method

When building APIs that accept parameters in the URL query

Syntax

Express

app.get('/path', (req, res) => {
  const value = req.query.key;
  // use value
  res.send(`Value is ${value}`);
});

req.query is an object containing all query parameters as strings.

If a parameter is missing, its value will be undefined.

Examples

Reads the term parameter from the query string like /search?term=books

Express

app.get('/search', (req, res) => {
  const term = req.query.term;
  res.send(`Search term: ${term}`);
});

Uses a default value if page is not provided in the URL.

Express

app.get('/items', (req, res) => {
  const page = req.query.page || 1;
  res.send(`Page number: ${page}`);
});

Extracts multiple query parameters at once using object destructuring.

Express

app.get('/filter', (req, res) => {
  const { category, sort } = req.query;
  res.send(`Category: ${category}, Sort: ${sort}`);
});

Sample Program

This Express app listens on port 3000. When you visit /products?category=books&page=2, it reads the query parameters and shows them in the response.

Express

import express from 'express';

const app = express();
const port = 3000;

app.get('/products', (req, res) => {
  const category = req.query.category || 'all';
  const page = Number(req.query.page) || 1;
  res.send(`Showing products in category: ${category}, page: ${page}`);
});

app.listen(port, () => {
  console.log(`Server running on http://localhost:${port}`);
});

Important Notes

Query parameters are always strings. Convert them to numbers or booleans if needed.

Use default values to avoid errors when parameters are missing.

Express automatically parses the query string into req.query.

Summary

Query string parsing lets you read extra info from URLs.

Use req.query in Express to get query parameters as an object.

Always handle missing or wrong types by setting defaults or converting values.