Route Parameters and Query Strings

Static routes like /about only get you so far. Real applications need URLs that carry data: /users/42, /products/laptops/8, or /search?q=express&page=2. Express gives you two tools for this: route parameters for identifying resources and query strings for optional modifiers. This lesson covers both, plus validation basics.

Route Parameters

A route parameter is a named URL segment that starts with a colon. Whatever the client puts in that position is captured into req.params:

app.get('/users/:id', (req, res) => {
  res.send('User ID is ' + req.params.id);
});

Requesting /users/42 responds with 'User ID is 42', and /users/alice responds with 'User ID is alice'. The parameter matches a single segment, so /users/42/posts does not match this route.

Multiple Parameters

You can capture several segments at once. Each name becomes a property on req.params:

app.get('/products/:category/:productId', (req, res) => {
  const { category, productId } = req.params;
  res.json({ category, productId });
});

A request to /products/laptops/8 yields { category: 'laptops', productId: '8' }. Note that captured values are always strings, even when they look like numbers. Convert with Number() before doing math or database lookups by numeric ID.

A Realistic Lookup Example

Here is a pattern you will use constantly: find an item by ID and return 404 when it does not exist.

const users = [
  { id: 1, name: 'Aisha' },
  { id: 2, name: 'Marco' },
];

app.get('/api/users/:id', (req, res) => {
  const id = Number(req.params.id);
  const user = users.find(u => u.id === id);
  if (!user) {
    return res.status(404).json({ error: 'User not found' });
  }
  res.json(user);
});

The early return is important. Without it, execution would continue and try to send a second response.

Query Strings

Everything after the ? in a URL is the query string: /search?q=express&page=2. Express parses it into req.query automatically:

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

Query values are also strings and, crucially, always optional. Your handler must supply sensible defaults, as the example does with an empty search and page 1.

Params or Query: Which One?

Use a route parameter when the value identifies the resource itself: /users/42 means user number 42; without the ID the URL is meaningless. Use the query string for optional instructions about how to return data: sorting, filtering, pagination, or search terms. A good mental test: if removing the value should produce a 404, it belongs in the path; if removing it should just change the output, it belongs in the query.

Validating Input

Never trust URL input. A quick guard prevents confusing errors deeper in your code:

app.get('/api/orders/:id', (req, res) => {
  const id = Number(req.params.id);
  if (!Number.isInteger(id) || id <= 0) {
    return res.status(400).json({ error: 'id must be a positive integer' });
  }
  res.json({ orderId: id });
});

Later in the series we will move validation into reusable middleware.

Key Takeaways

:name in a path captures a URL segment into req.params.name.
req.query holds parsed query string values; both params and query are strings.
Use params for identity, query strings for options like filters and pagination.
Return 404 for missing resources and 400 for malformed input, with early returns.
Always provide defaults for optional query values.

Next lesson: Middleware Explained, the concept that makes Express truly powerful.