Second parameter in express route handler - express

Question

I'm coming across code where there's an additional parameter for express route handlers beyond the path and the callback.
For example:
app.get('/path', authUser, (req,res) => {
...
}
where authUser is a function. Can anybody explain the role of such functions? To date I've only seen express routes with the two parameters.

Answer 1

These are middleware, which are functions that run before your route handler (your third function). You can have as many as these as possible. They basically modify/perform an action based on the request, or maybe manipulate the response.
So this is likely middleware that checks the request for an authenticated user, and will return a 401/403 if not authenticated, meaning that you can write your route handler under the assumption that you are authenticated.
For more info, check out this article

Answer 2

The family of methods such as app.get(), app.post(), app.use(), accept any number of request handlers as successive arguments:
app.get('/path', fn1, fn2, fn3, fn4);
These requests handlers can be used for a variety of purposes. Often times, they are what is generally referred to as middleware which prepares a request for further processing or in some cases blocks a request from further processing. But, they can also be normal request handlers too, they are not just limited to what most people call middleware.
In your specific case:
app.get('/path', authUser, (req,res) => {
...
}
We can guess by the name that authUser is checking to see if the user making the request has been properly authenticated and, if not, then an error status is probably sent as the response and the next request handler in the chain is not called. Or conversely, because authUser has already filtered out any unauthenticated users, the request handler here at the end of the chain can safely assume that the user is already authenticated. So, this particular use is a means of applying middleware to one specific route with no consequences for any other routes defined later.
But, I want to emphasize that this is a generic mechanism that is not limited to just what is classically described as middleware. It can also be used for request handlers that might execute conditionally based on other parameters. For example here's one such example where the first request handler looks the URL and decides to handle the whole request itself based on what it sees in the URL and, if not, passes it on to the next handler:
app.get('/book/:id', (req, res) => {
// check if id is purely numeric
if (/^\d+$/.test(req.params.id)) {
// this is a request for a book by numeric id
// look up the book numeric id in the database and return the meta
// data about this book
} else {
// not a book id, let next request handler have the request
next();
}
}, (req, res) => {
// must be a book title lookup
// look up the book in the title database and return the id
});