So in Express, You have a simple block of code like this:
app.get('/', function(req, res){
res.send('Hello World');
});
On request of '/', do this function of code sending in req and res objects.
Some other blocks of code, seem to pass a 3rd argument in being next()
Pulled from the Express documentation
app.get('/users/:id?', function(req, res, next){
var id = req.params.id;
if (id) {
// do something
} else {
next();
}
});
I can't understand what the point of next() is or why its being used. In that example, if id doesn't exist, what is next actually doing?
It passes control to the next matching route. In the example you give, for instance, you might look up the user in the database if an id was given, and assign it to req.user.
Below, you could have a route like:
app.get('/users', function(req, res) {
// check for and maybe do something with req.user
});
Since /users/123 will match the route in your example first, that will first check and find user 123; then /users can do something with the result of that.
Route middleware (note: link is to 2.x documentation, but this is tested as working on 3.x) is a more flexible and powerful tool, though, in my opinion, since it doesn't rely on a particular URI scheme or route ordering. I'd be inclined to model the example shown like this, assuming a Users model with an async findOne():
function loadUser(req, res, next) {
if (req.params.userId) {
Users.findOne({ id: req.params.userId }, function(err, user) {
if (err) {
return next(new Error("Couldn't find user: " + err));
}
req.user = user;
next();
});
} else {
next();
}
}
// ...
app.get('/user/:userId', loadUser, function(req, res) {
// do something with req.user
});
app.get('/users/:userId?', loadUser, function(req, res) {
// if req.user was set, it's because userId was specified (and we found the user).
});
// Pretend there's a "loadItem()" which operates similarly, but with itemId.
app.get('/item/:itemId/addTo/:userId', loadItem, loadUser, function(req, res) {
req.user.items.append(req.item.name);
});
Being able to control flow like this is pretty handy. You might want to have certain pages only be available to users with an admin flag:
/**
* Only allows the page to be accessed if the user is an admin.
* Requires use of `loadUser` middleware.
*/
function requireAdmin(req, res, next) {
if (!req.user || !req.user.admin) {
return next(new Error("Permission denied."));
}
next();
}
app.get('/top/secret', loadUser, requireAdmin, function(req, res) {
res.send('blahblahblah');
});
Hope this gave you some inspiration!