Route Parameters

Path parameters let URLs carry data. They land on req.params.

3 min read Level 1/5 #express#routing#params

What you'll learn

Use single and multiple params
Make params optional
Validate or coerce them

A route can capture parts of the URL with :name syntax. Captures land on req.params.

Single Parameter

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

// GET /users/42  →  { "id": "42" }

Note: params are always strings. Coerce if you need a number:

const id = Number(req.params.id);
if (!Number.isInteger(id)) {
  return res.status(400).json({ error: "id must be an integer" });
}

Multiple Parameters

app.get("/posts/:postId/comments/:commentId", (req, res) => {
  res.json(req.params);
  // { postId: '42', commentId: '7' }
});

Optional Parameters (Express 5)

In Express 5, :name? makes a parameter optional:

app.get("/posts/:year/:month?", (req, res) => {
  res.json(req.params);
  // GET /posts/2026/05  → { year: '2026', month: '05' }
  // GET /posts/2026      → { year: '2026' }
});

Wildcards / Splats

*splat captures the rest of the URL into req.params.splat:

app.get("/files/*splat", (req, res) => {
  res.json({ path: req.params.splat });
  // GET /files/2026/photos/01.jpg
  //   → { path: ['2026', 'photos', '01.jpg'] }
});

(Express 4 uses a bare * — different syntax.)

Constraining With Regex

Restrict what a parameter can match:

app.get("/users/:id(\\d+)", (req, res) => {
  // only matches when :id is digits
  res.json({ id: Number(req.params.id) });
});

Anything else falls through to the next route or 404.

`app.param()` — Run Code Per Param

app.param("id", (req, res, next, value) => {
  // runs whenever a route has :id
  req.userId = Number(value);
  if (!Number.isInteger(req.userId)) {
    return res.status(400).json({ error: "bad id" });
  }
  next();
});

app.get("/users/:id", (req, res) => {
  // req.userId is set, validated
});

DRYs up validation across every route that uses :id.

Query Strings →