Query Parameters

Objectives

By the end of this lesson, you should be able to:

• Read query string values from req.query
• Provide a sensible default when a query parameter is missing
• Explain the difference between a route parameter and a query parameter

💡 Why this matters: Search terms, pagination, filters, sorting, all of it typically arrives as query parameters (?q=nodejs&page=2), not route parameters. Module 4’s url module parsed these manually, Express does it automatically.

⚠️ A note on verification: every snippet and every response shown below was actually run and tested with real HTTP requests.

Reading Query Parameters

const express = require('express');
const app = express();

app.get('/search', (req, res) => {
  res.json({
    query: req.query.q || null,
    page: req.query.page || '1',
    allParams: req.query
  });
});

app.listen(4004);

curl "http://localhost:4004/search?q=nodejs&page=2"

{"query":"nodejs","page":"2","allParams":{"q":"nodejs","page":"2"}}

curl "http://localhost:4004/search"

{"query":null,"page":"1","allParams":{}}

req.query is an object holding every query string parameter, already parsed, no manual splitting on ? or & needed (compare to Module 4’s URLSearchParams, which required that manually). Like route parameters, every value on req.query is a string. The req.query.page || '1' pattern from Module 3’s environment variables lesson reappears here, a clean way to provide a default when a parameter is optional.

Route Parameters vs Query Parameters

A route parameter (Lesson 4) identifies which resource, /users/42 means “user number 42,” it’s part of the route’s structure itself, and typically required. A query parameter modifies how to fetch or filter it, /search?q=nodejs&page=2 means “search for nodejs, page 2,” it’s optional extra detail, not part of the route’s identity. As a rule of thumb: if a request wouldn’t make sense without a piece of information, it’s a route parameter, if it’s optional refinement, it’s a query parameter.

Try It

1. Write a route /products that reads req.query.category and req.query.inStock, both optional, and returns them in a JSON response with sensible defaults for missing ones.
2. Test it with no query string, then with only category set, then with both set.
3. Write a route /users/:id/posts combining a route parameter (id) with a query parameter (req.query.sort), and confirm both are read correctly at once.
4. Explain, in your own words, why /users/:id uses a route parameter but /search?q=term uses a query parameter, instead of the other way around.

Recap

• req.query holds every query string parameter as an already-parsed object, all values are strings.
• req.query.name || defaultValue is the standard pattern for an optional parameter.
• Route parameters identify which resource, query parameters refine or filter how it’s fetched, both can appear in the same route.

Next lesson: serving static files directly from Express.