Express›Core· 45 min read

Routing & Route Parameters

Map URLs and HTTP methods to handlers — the backbone of every API.

What you will learn

Handle GET and POST routes
Read route parameters and query strings
Send JSON responses

Routes = method + path + handler

A route is the rule that says "when a request comes in for this method and this path, run this function". Each route ties an HTTP method (get to read, post to create, put to update, delete to remove) and a path (the URL) to a handler function.

Here the same path /products has two routes — one for GET and one for POST — because the method makes them different:

GET and POST routes, sending JSON

app.get("/products", (req, res) => {
  res.json([{ id: 1, name: "Keyboard" }, { id: 2, name: "Mouse" }]);
});

app.post("/products", (req, res) => {
  res.json({ message: "Product created" });
});

Note: Output (visiting /products with a browser, which uses GET): [{"id":1,"name":"Keyboard"},{"id":2,"name":"Mouse"}] res.json(...) sends data back as JSON — the standard format APIs use. The GET route returns the list of products; the POST route (triggered when something sends data, not a plain browser visit) replies with a confirmation message. Same URL, different method, different handler.

Route parameters

Often the URL itself contains a value — like which user you want. A :param placeholder in the path captures that piece of the URL, and you read it from req.params. Separately, query strings (the ?key=value part of a URL, like ?sort=price) are read from req.query.

Route params and query strings

// GET /users/42  →  req.params.id is "42"
app.get("/users/:id", (req, res) => {
  res.json({ userId: req.params.id });
});

// GET /search?q=react  →  req.query.q is "react"
app.get("/search", (req, res) => {
  res.json({ searching: req.query.q });
});

Note: Output (visiting /users/42): {"userId":"42"} Output (visiting /search?q=react): {"searching":"react"} The :id in /users/:id is a wildcard: visit /users/42 and Express puts "42" in req.params.id. For /search, the part after ? is the query string, so ?q=react becomes req.query.q. (Both arrive as text strings, even when they look like numbers.)

Source	Where	Example
Route param	`req.params`	`/users/:id` → `req.params.id`
Query string	`req.query`	`?q=react` → `req.query.q`
Request body	`req.body`	JSON sent in a POST

Tip: Sending data: res.json(data) returns JSON (what APIs do), res.send(text) returns text/HTML, and res.status(404).json(...) sets the HTTP status code.

Q. For the route /users/:id, how do you read the id from the URL?

Answer: Route parameters defined with :id are read from req.params.id. Query strings use req.query; POST bodies use req.body.

✍️ Practice

Build a /users/:id route that returns the id as JSON.
Build a /greet?name=Asha route that reads the query and greets by name.

🏠 Homework

Build routes for a “books” resource: list all, and get one by id.