Express router.param() Method
What you’ll learn
- How to register parameter middleware with
router.param(). - How to validate params before route handlers run.
- How to preload records and attach them to
req. - How to avoid hanging requests and duplicate logic.
Syntax
javascript
router.param(name, function (req, res, next, value) {
// preprocess value
next();
});1
Validate route parameter once
javascript
router.param('userId', function (req, res, next, userId) {
if (!/^\d+$/.test(userId)) return res.status(400).send('Invalid userId');
next();
});2
Preload model by param value
javascript
router.param('postId', function (req, res, next, postId) {
var post = posts.find(function (p) { return String(p.id) === postId; });
if (!post) return res.status(404).send('Post not found');
req.post = post;
next();
});3
Use preloaded object in route
javascript
router.get('/posts/:postId', function (req, res) {
res.json(req.post);
});⚠️ Common pitfalls
- Forgetting
next()after successful validation/preload. - Writing heavy business logic inside
router.param()instead of parameter concerns. - Assuming it runs for unrelated param names.
❓ FAQ
It registers callback logic for a named route parameter on a router instance.
It runs for matched routes that include the configured parameter name, before route handlers.
Yes. It is commonly used to validate parameter format and return errors early.
Yes. You can fetch records by param value and attach them to req for downstream handlers.
router.param() is scoped to one router module, while app.param() applies at app level.
Did you know?
router.param() runs when a named route parameter is present, making it ideal for validation and preloading records.
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