Routing in Express

1. Route params and route declaration order

• /users/:id matches any value at that path position and binds it to req.params.id as a string. For example, on GET /users/5, req.params.id === '5'.
• Express matches routes in the exact order of declaration, from top to bottom, executing the first matching route without prioritizing "more specific" routes automatically.
• Common bug: declaring /users/:id before /users/me causes GET /users/me to be swallowed by /users/:id (mistakenly invoking the get-user-by-id handler with id = 'me'), meaning /users/me is never reached.
• Fix: specific routes must be declared before generalized parameterized routes:

app.get('/users/me', getCurrentUser);
app.get('/users/:id', getUserById);

2. Query string

• req.query contains the parsed query string as an object. For example, on GET /orders?status=pending&limit=10, req.query.status === 'pending' and req.query.limit === '10'.
• All values in req.query are strings, even if they look like numbers. Express does not infer data types because query strings are fundamentally plain text in the URL.
• To treat them as numbers or booleans, you must explicitly convert them (Number(req.query.limit), parseInt(), etc.). Common mistake: comparing req.query.limit === 10 always evaluates to false due to comparing a string with a number.

3. Wildcard

• Routes using /files/* match any subpath following the prefix, regardless of folder depth (/files/images/logo.png, /files/docs/a/b/c.pdf).
• Common use case: serving static files with nested directory structures whose depths are unknown in advance.
• Another use case: catch-all 404 handler route, placed at the very bottom of the entire route list (applying the principle that "generalized routes go after specific routes"):

app.use('/api', apiRoutes);
app.use('/*', (req, res) => {
  res.status(404).json({ error: 'Not found' });
});

4. Router() to split routes across multiple files

• As a project grows, managing multiple resources with complex logic is easier to read, debug, and update by splitting routes across files based on resources.
• express.Router() creates an isolated "mini Express app" to define dedicated routes and middleware, attached to the main app via app.use(prefix, router):

// routes/users.js
const router = require('express').Router();
router.get('/', getUsersHandler);
router.get('/:id', getUserByIdHandler);
router.post('/', createUserHandler);
module.exports = router;

// app.js
const usersRouter = require('./routes/users');
app.use('/users', usersRouter);

• Inside the sub-route file, there is no need to repeat the prefix (/users) on every route, because app.use('/users', usersRouter) attaches that prefix automatically; the sub-file only defines relative routes (/, /:id).