I see it is used in various ways: next(), next('route'), next(error)...
Where can I find the official documentation for the next() method?
I don't find any documentation summing up it's use cases on the express api docs...
UPDATE:
From what I coroborated, next works like this:
next() : sends req to the next middleware function of the current route
next('route'): sends req to next matching route
and less obvious...
next(anythingElse) sends req to the next error handling middleware, where err will be equal to anythingElse
You can view the "callback" argument under app.use() at http://expressjs.com/en/4x/api.html#app.use
There are also middleware callback function examples as well (http://expressjs.com/en/4x/api.html#middleware-callback-function-examples)
You are looking at the wrong page in the express documentation. The next callback is just to initiate/continue the request-response cycle. The actual guide is located under "Using middleware" title of Guides section on the Expressjs website.
Middleware functions are functions that have access to the request
object (req), the response object (res), and the next middleware
function in the application’s request-response cycle. The next
middleware function is commonly denoted by a variable named next.
Middleware functions can perform the following tasks:
Execute any code.
Make changes to the request and the response objects.
End the request-response cycle.
Call the next middleware function in the stack.
If the current middleware function does not end the request-response
cycle, it must call next() to pass control to the next middleware
function. Otherwise, the request will be left hanging.
Related
In node JS Express, we can write middlewares to intercept requests to either
Call the next middleware in the chain by invoking next
End the chain by calling res.send or similar functions provided by res
That means, everytime we want to end a request and send a response in a particular middleware, we have to add (at least) the below snippet.
res.send();
Are there ways to write a response frame middleware like this:
responseFrame = (res,req,responseData) => {
res.send(responseData);
}
and insinde route.js, use this middleware on all path
app.use(responseFrame);
Then, we simply have to end any middleware with next(), as long as we define the correct routes, Express will take care of sending the response (if the next middleware is the responseFrame)
You can use res.locals for that.
https://expressjs.com/en/api.html#res.locals
I am familiar with Express but new to Restify. Restify's document has many examples calling next() after res.send() as below:
server.get('/echo/:name', function (req, res, next) {
res.send(req.params);
return next();
});
This looks like a recommended pattern by some Restify experts as well:
The consequences of not calling next() in restify
What's the real use case of doing this? After you call res.send(), is there anything a handler next in the chain can do?
After I did some more research, I think I found an answer.
Only practical use case of calling next() after res.send() is when you want to install 'post-render' type middlewares to catch all transactions after route handers finish their job.
In other cases, it simply adds unnecessary overhead for every and each request because it scans the rest of routes to find a next match.
Most middlewares are 'pre-render' type, which don't need next() call anyway. Even for post-render type middlewares, depending on voluntary calls of next() is simply too risky. You'd rather want to use more error-proof method like https://github.com/jshttp/on-finished instead.
Calling next() after res.send() is a bad practice causing unnecessary overhead in most cases. It should be used only when it is absolutely needed and you know what you are doing.
In express and connect, is it bad to use "next" in middleware if I do not need it? Are there any possible negative outcomes? Assume there is no middleware which will be called after this middleware, and therefore the next will not call anything. I know it is bad for modularity, as if you want to add a callback for another middleware it may be accidentally triggered by the next in this middleware. However, in this case next is bad for modularity anyway, as middleware often interact in unexpected ways.
As an example of an unneeded next, consider the sample MEAN.JS stack, constructed by the guys who originally came up with the stack's name. It seems to have some next callbacks which do not ever get called. Many are in the users controller, including the signin function:
exports.signin = function(req, res, next) {
passport.authenticate('local', function(err, user, info) {
if (err || !user) {
res.status(400).send(info);
} else {
// Remove sensitive data before login
user.password = undefined;
user.salt = undefined;
req.login(user, function(err) {
if (err) {
res.status(400).send(err);
} else {
res.json(user);
}
});
}
})(req, res, next);
};
This function has a next callback defined. This next callback is then used by the passport.authenticate() custom middleware function as a parameter. However, this parameter is never used in the function itself. I have tried taking out the next definition from the function definition, as well as the custom passport middleware, and the route seems to still work. However, perhaps passport uses it in its authenticate() function, and leaving it out did not cause any trouble here but it may cause trouble in some cases.
I was recently looking at passport's tutorials on http://passportjs.org, and I came across a function in the section on custom callbacks on the authenticate page that looks almost exactly like the signin function in MEAN.JS. One difference was that it actually had some next callbacks (for error handling), so the next parameter was actually useful. Is it possible that the MEAN.JS app took a lot of code from passportjs.org's guide and changed it over time, but left in some vestigial remnants that do not do anything but were causing no harm? Or does the next parameter actually do something in passport.authenticate() that is not immediately obvious? Regardless of why this happened, does an extra next parameter in connect middleware cause any bad side effects if it is not used?
When writing middleware, the next parameter is optional. It's purpose is so that the next middleware in the chain will be called. If you want the current middleware to be the last one called for a given request, not executing the next parameter will accomplish that. This is fine for code that you write for yourself, but it's typically better to always execute the next parameter in middleware that may be used elsewhere because you don't know what else they could be adding.
For example, maybe you wanted to add some kind of logging that happens after a request is completed. If your middleware that runs before the logging middleware doesn't execute next, it won't be logged.
http://expressjs.com/api.html#middleware
Not executing next will simply not start the next middleware. There are no other side effects of not executing it other than those caused by not moving to the next middleware (for example, if the response hasn't ended yet, not calling next will result in a timeout.)
I recently noticed an undocumented next function on the expressjs response object.
Is that function the same as next in the middleware function(req, res, next) ?
Is its use discouraged because it is undocumented?
Yes, it is the same function as can be seen here in the source code.
Yes, I would say it's use is discouraged because it is undocumented and also unnecessary since each middleware function will get next passed in as the 3rd argument.
The docs read:
The app.VERB() methods provide the routing functionality in Express,
where VERB is one of the HTTP verbs, such as app.post(). Multiple
callbacks may be give, all are treated equally, and behave just like
middleware, with the one exception that these callbacks may invoke
next('route') to bypass the remaining route callback(s). This
mechanism can be used to perform pre-conditions on a route then pass
control to subsequent routes when there is no reason to proceed with
the route matched.
What do they mean by "bypass the remaining route callbacks?"? I know that next() will pass control to the next matching route. But... what function will get control with next('route')...?
I hated it when I answer my own question 5 minutes later.
next('route') is when using route middleware. So if you have:
app.get('/forum/:fid', middleware1, middleware2, function(){
// ...
})
the function middleware1() has a chance to call next() to pass control to middleware2, or next('route') to pass control to the next matching route altogether.
The given answer explains the main gist of it. Sadly, it is much less intuitive than you might think, with a lot of special cases when it is used in combination with parameters. Just check out some of the test cases in the app.param test file. Just to raise two examples:
app .param(name, fn) should defer all the param routes implies that if next("route") is called from a param handler, it would skip all following routes that refer to that param handler's own parameter name. In that test case, it skips all routes refering to the id parameter.
app .param(name, fn) should call when values differ when using "next" suggests that the previous rule has yet another exception: don't skip if the value of the parameter changes between routes.
...and there is more...
I'm sure there is a use-case for this kind of next('route') lever, but, I agree with previous comments in that it certainly makes things complicated and non-intuitive.