nodejs middleware execution continues after return - node.js

Question

I have the following middleware:
const mongoose = require('mongoose');
module.exports = function(req, res, next) {
const keys = Object.keys(req.params);
keys.forEach(elem => {
if (
(elem.includes('id') || elem.includes('Id')) &&
!mongoose.Types.ObjectId.isValid(req.params[elem])
)
return res
.status(400)
.json({ msg: `id: ${req.params[elem]} is invalid` });
});
next();
};
it is called in a get request:
// #route GET api/movies/:id
// #desc Get a movie with specified id from db
// #access Public
router.get('/:id', checkId, async (req, res) => {
const movie = await Movie.findById(req.params.id);
res.json(movie);
});
When I make a request in postman with an invalid id (ex: 1234) I recieve the proper response being 400 with msg: 'id 1234 is invalid' yet the execution still passes to the reqest callback code and an error is thrown as I try to access db with invalid id.
so the question is why does the middleware still allow the execution of next() even if it already returned with the 400?

Answer 1

You need to tell the router that there was a problem by calling next("some-error"). For example, you could do this:
module.exports = function(req, res, next) {
const keys = Object.keys(req.params);
keys.forEach(elem => {
if (
(elem.includes('id') || elem.includes('Id')) &&
!mongoose.Types.ObjectId.isValid(req.params[elem])
) {
res
.status(400)
.json({ msg: `id: ${req.params[elem]} is invalid` });
return next("invalidinput");
}
});
next();
};
Or you could be more generic by setting the result outside in the router if you like, like this:
In your middleware:
module.exports = function(req, res, next) {
const keys = Object.keys(req.params);
keys.forEach(elem => {
if (
(elem.includes('id') || elem.includes('Id')) &&
!mongoose.Types.ObjectId.isValid(req.params[elem])
) {
// === Report the error and let the router handle it
return next({
type: "invalidinput",
msg: `id: ${req.params[elem]} is invalid`
);
}
});
next();
};
Then at the bottom in your router:
// handle any errors
router.use(err, req, res, next) => {
if (err) {
if (err.type === "invalidinput") {
return req.status(400).json({msg: err.msg});
}
else {
return res.status(500).json({msg: "Internal error."});
}
}
return next();
}

Answer 2

another possible solution here is to convert the forEach to a classical for loop, thu making this middleware run synschronously
module.exports = function(req, res, next) {
const keys = Object.keys(req.params);
for (let i = 0; i < keys.length; i++) {
if (
(keys[i].includes('id') || keys[i].includes('Id')) &&
!mongoose.Types.ObjectId.isValid(req.params[keys[i]])
)
return res
.status(400)
.json({ msg: `id: ${req.params[keys[i]]} is invalid` });
}
next();
};