I'm confused about how next() works in Node.js and Express middleware.
There have been some other questions about middleware works, for example here, but I'm looking for a different answer.
The main question bugging me is, who is providing the next() function?
For example, in my standard generated express app, I'm given this code:
// catch 404 and forward to error handler
app.use(function(req, res, next) {
var err = new Error('Not Found');
err.status = 404;
next(err);
});
But who calls this function and what is the nature of the next() that it provides? How do I know what the next() will look like? I have no idea what next() does.
An Express application is essentially a series of middleware calls.
Middleware is a function with access to the request object (req), the
response object (res), and the next middleware in line in the
request-response cycle of an Express application, commonly denoted by
a variable named next.
As written above, Expressjs provides the next callback function. Next is just a way of calling the next middleware in the flow.
For many examples see Express's Using middleware
Related
I am curious and really willing to learn and understand the whole concept behind the request and response circle of all backend
my question is I have a node js express framework
app is started
app.use('/api',router)
require('./routers/user')(router)
require('./routers/uretim')(router)
require('./routers/durus')(router)
require('./routers/rapor')(router)```
all my functions are called and executed and waiting for a request
since the order of this app is first use
app.use('/api',router)
then the router is called at this particular point router function has nothing attached to it,
so does it mean as we started our application we have created the routes with the executed functions below the app.use
main question
user enter ..../api
our back end was hit
what is the first process, or function that happens within our backend?
So the USE is used to load middleware, what you have is to use the router middleware on the /get url..
A better example of what is happening here is to define some action middleware on that url :
app.use("/api", (req, res, next) => {
// For example, a GET request to `/api` will print "GET /api"
console.log(`${req.method} ${req.url}`);
next();
});
When you start your server, if you hit your /api url, it will log the request info in the console and continue.
However the other URL's will not.. So you can tailor specific middleware for certain endpoints.
As for your routing itself, you will need to define the routes and their controller logic. Something like :
// HTTP GET Request Definition
app.get('/api',(req, res) => {
res.send("Hey, you hit API");
} );
This will happen after the middleware has been actioned.
Express
I am currently building an API thanks to Express.js, and I still can not believe how amazing it is. I figured out how to use middlewares, handling requests and responses, how to go to the next middleware ... But there is something that triggers me a lot, which is next().
I know what next() is, I just can't figure out what kind of parameters can next() take, and what they will do. At first I thought it was to pass data to the next middleware, turned out I was wrong, there is req.locals for this.
Can someone enlighten me on this ?
You have three choices for calling next():
1. Continue routing. If you just want to continue routing to the next route handler in the chain that matches this route, then you just call it with no parameters:
next();
This is most often used in middleware:
app.use((req, res, next) => {
// set an item in the session and then continue routing
req.session.xxx = "foo";
next();
});
2. Abort routing with an error to your centralized error handler. If you want to set an error and have routing skip to your generalized error handler, then you can pass an error:
next(new Error("timeout contacting database"));
This will then invoke your general error handling route and pass it that specific error where your general error handling code can decide what to do. This allows you to centralize your error handling response code in one place. Examples of using next(err) are here.
You can see the doc for generalized error handling in Express for examples on how that works.
3. Skip route handlers in this router, continue routing in other routers. If you want to skip all route handlers in the current router, but continue with any other routes that might be registered, then you can do this:
next('route');
From the doc:
You can provide multiple callback functions that behave just like middleware, except that these callbacks can invoke next('route') to bypass the remaining route callback(s). You can use this mechanism to impose pre-conditions on a route, then pass control to subsequent routes if there is no reason to proceed with the current route.
If you repeatedly search for "next(" on this page of the doc, you will find lots of examples. There's an example of this usage here in the doc.
FYI, Express 5 may add some new uses for next() because it may return a promise that lets you know when routing is now completely done.
In my server.js file, where I may have many lines of code that use app.use, do these lines app.use execute every time a path is hit? I am a little confused on how this works when using the express router. If I have something like
var example1 = require('./js/exampleRoutes.js)
var example2 = require('./js/example2Routes.js)
app.use(express.static(process.cwd() + '/public'));
app.use(bodyParser.urlencoded({ extended: false }));
app.use(methodOverride("_method"));
app.use(example1, example2);
app.listen(PORT);
Do all four lines of app.use execute everytime a path is hit, and depending on the path hit, it will redirect the program to either example1 routes or example2 routes?
exampleRoutes.js
router.get('/', function(req, res){
....
});
example2Routes.js
router.post('/create', function(req, res){
....
});
Does express app.use execute every time a path is heard on server.js
It depends upon whether you're using a path with any of your app.use() statements and it depends upon whether all the app.use() statements pass control onto the next handler or whether some of them purposely terminate the chain and handle the final response themselves.
All app.use() statements are registered as middleware handlers for all requests that hit that app object. If it has a specific path on it, then it will only be called if it matches that path. If it does not have a path (as in your examples), then it will be considered for every request.
When a given middleware handler is called, it is passed three arguments as in this example from the Express doc:
app.use(function (req, res, next) {
console.log('Time: %d', Date.now());
next();
});
Note, that last argument next. When a middleware function gets called, it has three main choices for how it's going to work:
It can do something and then let the request continue on to other handlers.
It can do something and then trigger an error (for example authentication failed or a require session cookie is missing).
It can do something and then send the final response (res.send(xxxx)) and not pass control on to other handlers.
The mechanism for passing control on to other handlers is by calling next() when you are done with your processing and ready for other handlers to get their crack at this request. If each middleware handler calls next() for each request they see, then ALL the app.use() handlers will get called for every single request. In the example above from the Express doc, you can see that this is a very simple middleware handler. All it does is log the time of the request and then it calls next() to pass control onto the next handler in the chain.
If one of the middleware handler does not call next(), then for that request, none of the handlers that come after it in the chain will get called for that request.
On a separate note, you can limit middleware to certain url paths only. Rather than calling
app.use(example1)
you can include a path as an argument such as:
app.use('/api', example1);
Then, this middleware will only be called for paths that start with /api.
Or, you can similarly use Routers to create branches of middleware. For example, if you have some middleware handlers that you only want to be called for requests that start with /api, then you can create a router for the /api prefix on the URL and attach the middleware handlers to that router instead of to the app object and then the middleware handlers on that router will only be called for requests that start with /api and are thus directed to that router.
// create api router
var router = express.Router();
// register middleware that is called only when requests go through this router
router.use(example1);
// register normal get handler
router.get('/whatever', function(req, res) {...});
// hook api router into the app
app.use('/api', router);
The express.js website confused me, because there are two contradictory statements:
You define error-handling middleware last, after other app.use() and routes calls
– /guide/error-handling.html
and
All you need to do is add a middleware function at the very bottom of the stack (below all other functions) to handle a 404 response
– /starter/faq.html
It seems to be telling me that both of these middleware layers should be the last ones in the stack...? Erm, so what, I am meant to add two app.use middleware layers last? That is not possible...
Anyway, obviously one of these pages forgot to mention the exception of the other, so I'd like to know: which middleware should I register last? My error-handler or my 404-handler?
I'd like these two functions, but I'm not sure which order they need to go in for express to work properly:
app.use(function(req, res, next) {
res.status(404).send('Sorry cant find that!');
});
app.use(function(err, req, res, next) {
console.error(err.stack);
res.status(500).send('Something broke!');
});
Also I'm wondering, do I need to call next from within either handler?
The order doesn't really matter as your last middleware only gets called, when in some middleware before next(err) has been called. Your 404 middleware will always get called if nothing else has handled (and finished) the request.
And for your second question: no, you don't want to call next()in either of those as you finish the request with res.send().
Edit: I would leave the order like that though as the 404 middleware is not really an error handler.
I am a little confused how I can interact with the socket when I need to pass in data from my routes file. I have the standard setup using node.js, express, mongodb etc.
I'll run my socket.io logic inside server.js, and I include the routes.js inside that file.
var routes = require('./app/routes');
In this case I need to run some io.sockets inside my routes.js file.
My question is what is a good way to do this? I could run that specific route inside server.js but that gets messy. If I were to use sockets in my routes.js do I need to re-require
var io = require('socket.io').listen(server);
That means I need to create a new http server object, and I don't like that since I am requiring this file in server.js, this seems like a counter-intuitive move to me.
So how do I make a request to the server and pass for example a user name?
Example:
app.get('/profile', isLoggedIn, function(req, res) {
res.render('profile', { user : req.user });
io.sockets.on('connection', function (socket) {
console.log(socket);
console.log(req.user); // Here I have access to the user and can pass that around in my sockets
});
});
I am also using route middleware, so I'd like to keep that in my routes.js, IDK I am just confused what I should do, hopefully someone understands my angle here?
Edit:
What about passing something into the routes?
routes.initialize(app, passport);
You can get the req and res objects directly in the socket by invoking socket.request (a Getter function which is available via __proto__). See docs.
I was then able to find req.user in socket.request.client.user.