Router-level middleware order using Express.js

Question

I have the following routers:

/* GET /users/username */
router.get("/:username", function(req, res, next) {
  User.findOne({
    username: req.params.username
  }).exec(function(err, user) {
    if (err) return next(error);
    res.json(user);
  });
});

/* GET /users/id */
router.get("/:id", function(req, res, next) {
  User.findById(req.params.id, function(err, user) {
    if (err) return next(err);
    res.json(user);
  });
});

In the above order, /* GET /users/id */ will return null, and if I swap the order the /* GET /users/username */ will result in this error:

Cast to ObjectId failed for value "Guest" at path "_id" for model "User"

Basically, the second middleware is being ignored. However, both are needed for different behaviors. I'm using AngularJS $resource to interact with RESTful server-side data sources.

1. Why can't I use both simultaneously? Why is the second middleware ignored?
   
2. In which order should these two routers be to both working?

Answer 1

The second middleware is ignored, because the url pattern is the same... How do you want the router make the differenciation between an 'username' or an 'id';

You've got to check if it's an id first per exemple.

router.get("/:username", function(req, res, next) {
  const username = req.params.username;
  if (!isNaN(parseInt(username)) {     // <------ Check if it's an ID
     next();
  } else {
  User.findOne({
    username: req.params.username
  }).exec(function(err, user) {
    if (err) return next(error);
    res.json(user);
  });
   }
});

/* GET /users/id */
router.get("/:id", function(req, res, next) {
  User.findById(req.params.id, function(err, user) {
    if (err) return next(err);
    res.json(user);
  });
});