`app.param()`

app.param(name, fn) wires a handler that runs whenever a route has the named param. Great for fetch-and-validate.

3 min read Level 2/5 #express#routing#params

What you'll learn

Register a param handler
Replace boilerplate "find by id" code
Know when this trick shines vs hurts

app.param(name, fn) registers a function that runs before any route with a :name parameter. Use it to validate or hydrate.

Without It — Repetition

app.get("/users/:id", async (req, res) => {
  const id = Number(req.params.id);
  if (!Number.isInteger(id)) return res.status(400).end();
  const user = await db.users.findById(id);
  if (!user) return res.status(404).end();
  res.json(user);
});

app.patch("/users/:id", async (req, res) => {
  const id = Number(req.params.id);
  if (!Number.isInteger(id)) return res.status(400).end();
  const user = await db.users.findById(id);
  if (!user) return res.status(404).end();
  // ... update
});

The same validate-and-fetch dance, repeated in every handler.

With It — DRY

app.param("id", async (req, res, next, value) => {
  const id = Number(value);
  if (!Number.isInteger(id)) {
    return res.status(400).json({ error: "id must be integer" });
  }
  const user = await db.users.findById(id);
  if (!user) {
    return res.status(404).json({ error: "user not found" });
  }
  req.user = user;
  next();
});

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

app.patch("/users/:id", async (req, res) => {
  Object.assign(req.user, req.body);
  await db.users.save(req.user);
  res.json(req.user);
});

On a Router

Router#param works the same way:

const userRouter = Router();

userRouter.param("id", loadUser);

userRouter.get("/:id",   getUser);
userRouter.patch("/:id", updateUser);

app.use("/api/users", userRouter);

The param handler is scoped to that router — it doesn’t fire on other routes that happen to use :id.

Caveats

Param handlers only fire when the parameter is present in the matched route.
They fire once per request, even if the route has the param multiple times.
Async errors must call next(err) (or throw, in Express 5).

When To Use

app.param is a sharp tool. Good for:

“Load this resource by ID” patterns
Validation that applies across many routes

Bad for:

Logic that should be visible at each call site
Routes where the param means different things

Used sparingly, it removes a lot of boilerplate. Overused, it hides what a route does.

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