Question about the next() function in expressjs - javascript

I'm having trouble wrapping my head around the concept of the next() function in express.js. I guess my first question would be is next() an express.js only function? My second question would be, in the example below what does next do? After the console function, it goes to the next function that is called after? I'm so confused.
var cb0 = function (req, res, next) {
console.log('CB0');
next();
}

With Express (and other similar systems), each request passes through a series of middleware functions (like your cb0). Each of those has a chance to do something with the request.
Since the thing a middleware function does may be asynchronous (for instance, reading a file, querying a database, etc.), Express can't just directly call the next bit of middleware after calling the previous one. So instead, it passes the middleware function a function, next, which that middleware uses to say "I'm done, run the next step." (In the Express version, you can also pass an argument to next, as Aikon Mogwai points out: If you pass it an Error, it triggers error handling for the route. If you pass it "route", it jumps to the next router, etc.).
So the concept of a next function isn't specific to Express, but the specific use in that example is.
Here's a much simplified example not using Express, but demonstrating the sort of thing it does with middleware functions when handling a request:
const app = {
middleware: [],
use(callback) {
this.middleware.push(callback);
}
};
app.use((req, res, next) => {
console.log("First handler synchronous part");
setTimeout(() => {
console.log("First handler async part finished");
next();
}, 800);
});
app.use((req, res, next) => {
console.log("Second handler is entirely synchronous");
next();
});
app.use((req, res, next) => {
console.log("Third handler synchronous part");
setTimeout(() => {
console.log("Third handler async part finished");
next();
}, 800);
});
// Code handling an incoming request
function handleRequest(req, app) {
// Copy the handlers
const middleware = app.middleware.slice();
// Create a "response"
const res = {};
// Call the handlers
let index = 0;
next();
function next() {
if (index < middleware.length) {
// Call the handler, have it call `next` when it's done
middleware[index++](req, res, next);
} else {
console.log("Request completed");
}
}
}
handleRequest({}, app);
It's probably worth mentioning that this manual-style of asynchronous middleware handling has been replaced with promises in Koa.js, which is a new(er) framework from the same people who did Express.js. With Koa, you make your callbacks async functions, and Koa's internals wait for the promise the async function returns to settle and then acts on the result of it setting (e.g., rejection or fulfillment, the value it fulfills with, etc.).

Next is used to pass control to the next middleware function. If not the request will be left hanging or open. Calling this function invokes the next middleware function in the app. The next() function is not a part of the Node.js or Express API but is the third argument that is passed to the middleware function.

The next() function requests the next middleware function in the application. The next() function is not a part of the Node.js or Express API, but it is the third case/argument which is passing to the middleware function. The next() function could be named anything, but by convention, it is always named "next". To avoid confusion, always use this convention.
For more info, you can visit the official tutorial of express

var express = require('express')
var app = express()
var CB0 = function (req, res, next) {
console.log('CB0')
next()
}
app.use(CB0)
app.get('/', function (req, res) {
res.send('Hello World!')
})
app.listen(3000)
Each and Every time app receives a request and prints the message "CB0" console in terminal window.
The middleware functions that are loaded first are also executed first.
The middleware function CB0 simply prints a message, then passes on the request to the next middleware function in the stack by calling the next() function.

Related

Can I call next callback twice?

I want to do some logging in the application. The flow I'm currently think is I pass a middleware function to my other middleware, then called the middleware function inside it.
I know it might be confusing, but this is the case I'm having now, I already have error logging middleware but want to invoke it later when on response finish
//middleware that called other middleware
module.exports = function(loggingMiddleware) {
return function (req, res, next) {
res.on("finish", () => {
loggingMiddleware(req, res, next) // will be called twice here
}
next() // to other middleware
}
}
My concern is, is it ok to call the next twice?

How does node.js next() middleware function work without arguments?

Middleware functions have a signature function (req, res, next), but in Express the next() call does not contain arguments. How is this so? See the following example from the
sample documentation
var express = require('express')
var app = express()
var myLogger = function (req, res, next) {
console.log('LOGGED')
next()
}
app.use(myLogger)
app.get('/', function (req, res) {
res.send('Hello World!')
})
app.listen(3000)
It certainly could be the case that a wrapping function is created that under-the-hood binds the arguments, allowing for a call with no additional parameters, but the documentation seems to indicate that the next parameter is called as-is, which does not make sense.
The docs describe the third argument, conventionally named next, as
Callback argument to the middleware function, called "next" by convention.
You can think of it similar to the conventional node.js callback-style argument provided to most async functions (without promises). When your middleware function is done doing its sync or async work, it should call next to indicate to the express router that it is done executing. This argument could be called done or callback, as we often see in other node.js libraries and examples, but is called next to provide a hint to the developer that the middleware chain will continue execution (other middleware may be called after this one).

How does this Node.js middleware work without arguments?

I'm using a function that someone else wrote for express and passport, which defines the middleware(?) as follows:
function isLoggedIn(req, res, next) {
if (req.isAuthenticated()){
return next();
}
else{
req.flash('error', 'You need to be logged in to access this page');
res.redirect('/login');
}
}
This function is used in the router as follows:
app.get('/page', isLoggedIn, function(req, res){
// ...
});
What I don't understand is, shouldn't the function be called with parameters req and res? Maybe the callback next is not necessary since it's the next argument of app.get, but how does the function access req and res? I would expect it to be called as follows:
app.get('/page', isLoggedIn(req, res), function(req, res){
// ...
});
How does it work without specifying the arguments?
Thanks,
Any functions that you pass to app.get() or app.use() are automatically called with req, res, next passed to them. That is how app.get() and app.use() are implemented.
To help you understand, this example:
app.get('/page', function(req, res){
console.log(req.params.foo);
});
is functionally the same as this:
app.get('/page', myHandler);
function myHandler(req, res) {
console.log(req.params.foo);
});
You do not want to do something like this:
app.get('/page', isLoggedIn(req, res), function(req, res){
// ...
});
because here you're attempting to execute isLoggedIn(req, res) (when req and res are not yet defined) and then pass it's returned value to app.get(). That is not what you want at all. You need to pass a function reference to app.get() and it will supply the parameters when it calls the function. Any time you put () after a function in Javascript, that means you want to execute it NOW. But, if you just pass the function's name, then that is a function reference which can be stored and called later as desired.
This code example is analogous to this:
var result = isLoggedIn(req, res);
app.get('/page', result, function(req, res){
// ...
});
Besides the fact that this would cause an error because req and res are not defined at program start time when this would execute, hopefully you can see that you don't want to execute isLoggedIn() now. Instead, you just want to pass a function reference so Express can call it later.
In this code
app.get('/page', isLoggedIn, function(req, res){
// ...
});
The app.get method is being called with three arguments:
the route to the page: /page
the middleware function
the request handler function
Basically, this code is telling the express framework that when a GET request is received for the /page path, then it should call two functions: first, the middleware function and second the handler function.
The important thing to note here is that it is the framework doing the work. The framework is going to call the middleware function, then it's going to call the handler function.
What I don't understand is, shouldn't the function be called with parameters req and res?
It will be called with these arguments, somewhere inside the get function. Suppose this is the simplified get, implemented as
// a super simple get that will expect a function
// and call the function with two arguments
function get( f ) {
var res = 1, req = 1;
f( res, req );
}
There are multiple ways of passing a function to get. For example, you pass an anonymous function
get( function( res, req ) {
return res + req;
}
You can also pass a named function defined elsewhere
function isLoggedIn( res, req ) {
return res + req;
}
get( isLoggedIn );
This however, is not what you'd expect:
get( isLoggedIn( res, req ) );
The problem here is that
isLoggedIn( res, req )
is no longer a function declaration. It is an invocation and the result of this invocation depends on what res and req are. With some luck, the invocation can even yield a number value, which means that get is no longer invoked with function as an argument but with the result of function invocation.
In short, to pass a named function to another function, you don't specify its arguments. The supposed syntax that would allow this doesn't even make sense because it would be indistinguishable from a syntax of actual function invocation (i.e. the value of the call).

Why don't I need to call next in a Sails.js controller function even if it has an (asynchronous?) database query in its body?

Example controller function:
getArticles: function(req, res) {
Articles.find({}).exec(function(err, articles) {
res.json(articles) // I guess this part is asynchronous
// next() here?
})
}
I am used to call next() at the end of an asynchronously executed block of code. That seems to be different in Sails.js. Can somebody explain this to me?
Because res.json() in this instance is the last thing you're asking the server to do. There is no next now.
The next() convention in this case is due to the fact that sails runs on top of express.js. Next() is the pattern in how express runs through its list of middleware. That is why you would use next() in your policies and such because they are middlewhere. res.json() however is at the end of the chain, so their is no need to call next().
If you're still confused, google express middleware next()
To use return? This example below shows why you would want to use return despite it not being needed. By using return we ensure that res.json('otherStuff') does not execute.
getArticles: function(req, res) {
Articles.find({}).exec(function(err, articles) {
if(true) return res.json(articles)
res.json('otherStuff')
})
}

Node.js Express - app.all("*", func) doesn't get called when visiting root domain

I'm trying to set a global function that is called on every page load, regardless of its location in my website. As per Express's API, I've used
app.all("*", doSomething);
to call the function doSomething on every page load, but it doesn't completely work. The function fires on every page load, except for page loads of the base domain (e.g. http://domain.com/pageA will call the function, but http://domain.com won't). Does anyone know what I'm doing wrong?
Thanks!
I bet that you placed
app.get('/', fn)
above
app.all("*", doSomething);
Remember that Express will execute middleware functions in the order they are registered until something sends a response
I know that's an old one, but still maybe useful to someone.
I think the problem could be:
app.use(express.static(path.join(__dirname, 'public')));
var router = express.Router();
router.use(function (req, res, next) {
console.log("middleware");
next();
});
router.get('/', function(req, res) {
console.log('root');
});
router.get('/anything', function(req, res) {
console.log('any other path');
});
where is middleware invoked on any path, but /
It happens because express.static by default serves public/index.html on /
To solve it add parameter to the static middleware:
app.use(express.static(path.join(__dirname, 'public'), {
index: false
}));
If you want to run some code on every request, you don't need to use the router.
Simply place a middleware above the router, and it will be called on every request:
app.use(function(req, res, next){
//whatever you put here will be executed
//on each request
next(); // BE SURE TO CALL next() !!
});
Hope this helps
Where is app.all('*') in the chain? If its after all the other routes, it might not be invoked.
app.post("/something",function(req,res,next){ ...dothings.... res.send(200); });
app.all('*',function(req,res) { ...this NEVER gets called. No next and res already sent });
Unless it was your intention to have it be last, in which case you have to be sure to call next() in the preceeding routes. For example:
app.post("/something",function(req,res,next){ ...dothings.... next();});
app.all('*',function(req,res) { ...this gets called });
Also, what's in doSomething? Are you sure its not getting called?
I also had this problem and I discovered that the number of arguments your doSomething function have might be a factor.
function doSomething(req, res, next) {
console.log('this will work');
}
whereas:
function doSomething(req, res, next, myOwnArgument) {
console.log('this will never work');
}

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