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Until now I've defined my get and post handlers with just (req, res) as arguments, with the assumption being that I put these handlers last in the chain of middleware, and make sure that I handle any responses and error handling properly within these handlers... hence it doesn't matter that I don't make any reference to next.
Is this a valid and sensible approach, or is it good practice always to call next() even if (at present) there is nothing coming afterwards? For example, perhaps in the future you might want to do some handling after these routes... or maybe there's a reason I haven't yet come across why it's good practice to always call next().
For example, there is the following simple example in the express routing guide:
app.get('/example/b', function (req, res, next) {
console.log('the response will be sent by the next function ...')
next()
}, function (req, res) {
res.send('Hello from B!')
})
Of course, I appreciate that this is a very simple example to illustrate that handlers can be chained, and is not intended to provide a complete framework for a get handler, but would it be better to define and use next even in the second handler, as follows?
app.get('/example/b', function (req, res, next) {
console.log('the response will be sent by the next function ...')
next()
}, function (req, res, next) {
res.send('Hello from B!')
next()
})
Or is it actually common practice to assume that a handler function that sends a response back to the client should not call next()... i.e. the assumption should be that the chain will end at the handler that actually sends the response?
Or is there no established practice on this point?
I'm even wondering whether it might be common not to send any response in the get handler but to defer that to a dedicated response handler coming after... by which I mean an OK response handler rather than an error response handler (for which it seems to be common practice to defined a final error handler and call next(err)). So, in a non-error situation, you would call next() and in the following middleware you would do your res.status(200).send(req.mydata) where req.mydata is added in your get handler.
No. You should only call next() if you want something else to handle the request. Usually it's like saying that your route may match that request but you want to act like it didn't. For example you may have two handlers for the same route:
app.get('/test', (req, res, next) => {
if (something) {
return next();
}
// handle request one way (1)
});
app.get('/test', (req, res) => {
// handle request other way (2)
});
Always the first matching handler is called, so for the GET /test request the first handler will be called, but it can choose to pass the control to the second handler, as if the first didn't match the request.
Note that if the second handler doesn't intend to pass the request to the next handler, it doesn't even have next in its arguments.
If there was no second handler, then the standard 404 handler would be used if the first one called next().
If you pass an argument to next() then an error handling middleware will be called.
My rule of thumb is to handle the response in the handler if you're going to give a 20x (Success) response code, and in centralized error handling if not. That looks something like this in practice:
// ./routes/things.js
const express = require('express');
const Thing = require('../models/thing');
const Router = express.Router();
// note, the handlers might get pulled out into a controllers file, if they're getting more complex.
router.param('thingId', (req, res, next, id) => {
Thing.findById(id, (e, thing) => {
if (e) return next(e);
// let's say we have defined a NotFoundError that has 'statusCode' property which equals 404
if (!bot) return next(new NotFoundError(`Thing ${id} not found`));
req.thing = thing;
return next();
});
});
router.get('/', (req, res, next) => {
// possibly pull in some sort, limit, and filter stuff
Thing.find({}, (e, things) => {
if (e) return next(e);
res.send(things);
});
});
router.route('/:thingId')
.get((req, res) => {
// if you get here, you've already got a thing from the param fn
return res.send(req.thing);
})
.put((req, res, next) => {
const { name, description } = req.body; // pull whitelist of changes from body
let thing = req.thing;
thing = Object.assign(thing, { name, description }); // copy new stuff into the old thing
thing.save((e) => {
if (e) return next(e);
return res.send(thing); // return updated thing
});
});
Keeping each logical chunk in its own file can reduce repetition
// ./routes/index.js then mounts the subrouters to the main router
const thingsRoute = require('./things');
const express = require('express');
const router = express.Router();
/* .... other routes **/
router.use('/things', thingsRoute);
Error handling is then centralized, and can be mounted either in its own file or right on the app:
// in ./index.js (main app entry point)
const express = require('express');
// this will require by default ./routes/index.js
const routes = require('./routes');
const app = express();
const log = require('./log');// I prefer debug.js to console.log, and ./log.js is my default config file for it
/* ... other app setup stuff */
app.use(routes);
// you can mount several of these, passing next(e) if you don't handle the error and want the next error handler to do so.
app.use((err, req, res, next) => {
// you can tune log verbosity, this is just an example
if (err.statusCode === 404) {
return res.status(404).send(err.message);
}
log.error(err.message);
log.verbose(err.stack); // don't do stack traces unless log levels are set to verbose
return res.status(500).send(err.message);
});
There are a few posts on this question but none that answers the issue directly, head-on. Let me clarify that I understand (or so I think) the use of next(), next('route'), return next(), return and their impact on control flow.
My entire middleware for the app consists of a series of app.use, as in:
app.use(f1);
app.use(f2);
app.use(f3);
app.use(f4);
...
In each of these middlewares, I have possibility of sending the response and be done without any need for further processing. My problem is that I am unable to stop the processing from going to the next middleware.
I have a clumsy work around. I just set a res.locals.completed flag after sending a response. In all the middlewares, at the very start, I check this flag and skip processing in the middleware if the flag is set. In the very first middleware, this flag is unset.
Surely, there must be a better solution, what is it? I would think that Express implicitly would do this checking and skip the middlewares through some express-specific method?
According to the express documentation on http://expressjs.com/guide/using-middleware.html
If the current middleware does not end the request-response cycle,
it must call next() to pass control to the next middleware,
otherwise the request will be left hanging.
so if a middleware needs to end the request-response early, simply do not call next() but make sure that the middleware really ends the request-response by calling res.end, res.send, res.render or any method that implicitely calls res.end
app.use(function (req, res, next) {
if (/* stop here */) {
res.end();
} else {
next();
}
});
Here is an example server showing that it works
var express = require('express');
var app = express();
var count = 0;
app.use(function(req, res, next) {
console.log('f1');
next();
})
app.use(function(req, res, next) {
console.log('f2');
if (count > 1) {
res.send('Bye');
} else {
next();
}
})
app.use(function(req, res, next) {
console.log('f3');
count++;
next();
})
app.get('/', function (req, res) {
res.send('Hello World: ' + count);
});
var server = app.listen(3000);
you will see the after 3 requests, the server shows "Bye" and f3 is not reached
Is there a canonical way to remove middleware added with app.use from the stack? It seems that it should be possible to just modify the app.stack array directly, but I am wondering if there is a documented method I should be considering first.
use actually comes from Connect (not Express), and all it really does is push the middleware function onto the app's stack.
So you should be just fine splicing the function out of the array.
However, keep in mind there is no documentation around app.stack nor is there a function to remove middleware. You run the risk of a future version of Connect making changes incompatible with your code.
This is a useful functionality if you are inheriting some unwanted middleware from a framework built on express.
Building on some of the answers that came before me: In express 4.x the middleware can be found in app._router.stack. Note that the middleware are invoked in order.
// app is your express service
console.log(app._router.stack)
// [Layer, Layer, Layer, ...]
Tip: You can search the individual layers for the one you want to remove/move
const middlewareIndex = app._router.stack.findIndex(layer => {
// logic to id the specific middleware
});
Then you can just move/remove them with standard array methods like splice/unshift/etc
// Remove the matched middleware
app._router.stack.splice(middlewareIndex, 1);
There seems to be no built in way to do that, but you can manage to get the same result with a small trick. Create your own array of middleware (let's call it dynamicMiddleware) but don't push that into express, instead push just 1 middleware that will execute all the handlers in dynamicMiddleware asynchronously and in order.
const async = require('async')
// Middleware
const m1 = (req, res, next) => {
// do something here
next();
}
const m2 = (req, res, next) => {
// do something here
next();
}
const m3 = (req, res, next) => {
// do something here
next();
}
let dynamicMiddleware = [m1, m2, m3]
app.use((req, res, next) => {
// execute async handlers one by one
async.eachSeries(
// array to iterate over
dynamicMiddleware,
// iteration function
(handler, callback) => {
// call handler with req, res, and callback as next
handler(req, res, callback)
},
// final callback
(err) => {
if( err ) {
// handle error as needed
} else {
// call next middleware
next()
}
}
);
})
The code is a bit rough as I don't have a chance to test it right now, but the idea should be clear: wrap all dynamic handlers array in 1 middleware, that will loop through the array. And as you add or remove handlers to the array, only the ones left in the array will be called.
You can use the express-dynamic-middleware to make this.
https://github.com/lanbomo/express-dynamic-middleware
Use it like this
const express = require('express');
// import express-dynamic-middleware
const dynamicMiddleware = require('express-dynamic-middleware');
// create auth middleware
const auth = function(req, res, next) {
if (req.get('Authorization') === 'Basic') {
next();
} else {
res.status(401).end('Unauthorization');
}
};
// create dynamic middleware
const dynamic = dynamicMiddleware.create(auth);
// create express app
const app = express();
// use the dynamic middleware
app.use(dynamic.handle());
// unuse auth middleware
dynamic.unuse(auth);
No way of removing a middleware as far as I know. however, you can assign a boolean flag to 'deactivate' a middleware at anytime you want.
let middlewareA_isActivate = true;
// Your middleware code
function(req, res, next) {
if (!middlewareA_isActivate) next();
// .........
}
// Deactivate middleware
middlewareA_isActivate = false;
EDIT :
After reading through ExpressJs (4.x) code, I notice that you can access the middlewares stack via app._router.stack, manipulation goes from there I guess. Still, I think this 'trick' might not be able to work in future Express
P/s: Not tested how Express behaves when manipulate the middlewares stack directly though
Following from the hints above, I've add success with the following on express 4.x. My use case was logging what was coming in with Slack Bolt, so I could capture and then mock it:
// Define a handy function for re-ordering arrays
Array.prototype.move = function(from, to) {
this.splice(to, 0, this.splice(from, 1)[0]);
};
// Use the normal use mechanism, so that 'extra' stuff can be done
// For example, to log further up the order, use app.use(morgan("combined"))
app.use([my-middleware]);
// Now adjust the position of what I just added forward
const numElements = app._router.stack.length;
app._router.stack.move(numElements - 1, 1);
You can use
console.log("Stack after adjustment", app._router.stack)
to confirm the new order is what you want. (For Slack Bolt, I had to use app.receiver.app because the Bolt app wraps the express app.)
We can write like this.
// route outside middleware
route.get("/list", (req, res)=>{
res.send("from listing route");
});
//use middleware
router.use(Middlewares.AuthMiddleware.isValidToken);
//routes inside the middleware
route.post("/create", (req, res)=>{
res.send("from create route");
});
route.delete("/delete", (req, res)=>{
res.send("from delete route");
});
So basically, write routes before injecting middleware into your route.
Have been trying to find a good description of what the next() method does. In the Express documentation it says that next('route') can be used to jump to that route and skip all routes in between, but sometimes next is called without arguments. Anybody knows of a good tutorial etc that describes the next function?
next() with no arguments says "just kidding, I don't actual want to handle this". It goes back in and tries to find the next route that would match.
This is useful, say if you want to have some kind of page manager with url slugs, as well as lots of other things, but here's an example.
app.get('/:pageslug', function(req, res, next){
var page = db.findPage(req.params.pageslug);
if (page) {
res.send(page.body);
} else {
next();
}
});
app.get('/other_routes', function() {
//...
});
That made up code should check a database for a page with a certain id slug. If it finds one render it! if it doesn't find one then ignore this route handler and check for other ones.
So next() with no arguments allows to pretend you didn't handle the route so that something else can pick it up instead.
Or a hit counter with app.all('*'). Which allows you to execute some shared setup code and then move on to other routes to do something more specific.
app.all('*', function(req, res, next){
myHitCounter.count += 1;
next();
});
app.get('/other_routes', function() {
//...
});
In most frameworks you get a request and you want to return a response. Because of the async nature of Node.js you run into problems with nested call backs if you are doing non trivial stuff. To keep this from happening Connect.js (prior to v4.0, Express.js was a layer on top of connect.js) has something that is called middleware which is a function with 2, 3 or 4 parameters.
function (<err>, req, res, next) {}
Your Express.js app is a stack of these functions.
The router is special, it's middleware that lets you execute one or more middleware for a certain url. So it's a stack inside a stack.
So what does next do? Simple, it tells your app to run the next middleware. But what happens when you pass something to next? Express will abort the current stack and will run all the middleware that has 4 parameters.
function (err, req, res, next) {}
This middleware is used to process any errors. I like to do the following:
next({ type: 'database', error: 'datacenter blew up' });
With this error I would probably tell the user something went wrong and log the real error.
function (err, req, res, next) {
if (err.type === 'database') {
res.send('Something went wrong user');
console.log(err.error);
}
};
If you picture your Express.js application as a stack you probably will be able to fix a lot of weirdness yourself. For example when you add your Cookie middleware after you router it makes sense that your routes wont have cookies.
Docs
How do I setup an error handler?
Error Handling
You define error-handling middleware in the same way as other middleware, except with four arguments instead of three; specifically with the signature (err, req, res, next):
app.use(function (err, req, res, next) {
console.error(err.stack)
res.status(500).send('Something broke!')
})
IMHO, the accepted answer to this question is not really accurate. As others have stated, it's really about controlling when next handler in the chain is run. But I wanted to provide a little more code to make it more concrete. Say you have this simple express app:
var express = require('express');
var app = express();
app.get('/user/:id', function (req, res, next) {
console.log('before request handler');
next();
});
app.get('/user/:id', function (req, res, next) {
console.log('handling request');
res.sendStatus(200);
next();
});
app.get('/user/:id', function (req, res, next) {
console.log('after request handler');
next();
});
app.listen(3000, function () {
console.log('Example app listening on port 3000!')
});
If you do
curl http://localhost:3000/user/123
you will see this printed to console:
before request handler
handling request
after request handler
Now if you comment out the call to next() in the middle handler like this:
app.get('/user/:id', function (req, res, next) {
console.log('handling request');
res.sendStatus(200);
//next();
});
You will see this on the console:
before request handler
handling request
Notice that the last handler (the one that prints after request handler) does not run. That's because you are no longer telling express to run the next handler.
So it doesn't really matter if your "main" handler (the one that returns 200) was successful or not, if you want the rest of the middlewares to run, you have to call next().
When would this come in handy? Let's say you want to log all requests that came in to some database regardless of whether or not the request succeeded.
app.get('/user/:id', function (req, res, next) {
try {
// ...
}
catch (ex) {
// ...
}
finally {
// go to the next handler regardless of what happened in this one
next();
}
});
app.get('/user/:id', function (req, res, next) {
logToDatabase(req);
next();
});
If you want the second handler to run, you have to call next() in the first handler.
Remember that node is async so it can't know when the first handler's callback has finished. You have to tell it by calling next().
next() without parameter invokes the next route handler OR next middleware in framework.
Summarizing rightly mentioned answers in one place,
next() : move control to next function in same route. case of
multiple functions in single route.
next('route') :move control to next route by skipping all remaining
function in current route.
next(err) : move control to error middleware
app.get('/testroute/:id', function (req, res, next) {
if (req.params.id === '0') next() // Take me to the next function in current route
else if (req.params.id === '1') next('route') //Take me to next routes/middleware by skipping all other functions in current router
else next(new Error('Take me directly to error handler middleware by skipping all other routers/middlewares'))
}, function (req, res, next) {
// render a regular page
console.log('Next function in current route')
res.status(200).send('Next function in current route');
})
// handler for the /testroute/:id path, which renders a special page
app.get('/testroute/:id', function (req, res, next) {
console.log('Next routes/middleware by skipping all other functions in current router')
res.status(200).send('Next routes/middleware by skipping all other functions in current router');
})
//error middleware
app.use(function (err, req, res, next) {
console.log('take me to next routes/middleware by skipping all other functions in current router')
res.status(err.status || 500).send(err.message);
});
Question also asked about use of next('route') which seems to be covered week in provided answers so far:
USAGE OF next():
In short: next middleware function.
Extract from this official Express JS documentation - 'writing-middleware' page:
"The middleware function myLogger simply prints a message, then passes on the request to the next middleware function in the stack by calling the next() function."
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)
This page of Express JS documentation states "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."
USAGE OF next('route') :
In short: next route (vs. next middleware function in case of next() )
Extract from this Express JS documentation - 'using-middleware' page:
"To skip the rest of the middleware functions from a router middleware stack, call next('route') to pass control to the next route. NOTE: next('route') will work only in middleware functions that were loaded by using the app.METHOD() or router.METHOD() functions.
This example shows a middleware sub-stack that handles GET requests to the /user/:id path."
app.get('/user/:id', function (req, res, next) {
// if the user ID is 0, skip to the next route
if (req.params.id === '0') next('route')
// otherwise pass the control to the next middleware function in this stack
else next()
}, function (req, res, next) {
// render a regular page
res.render('regular')
})
// handler for the /user/:id path, which renders a special page
app.get('/user/:id', function (req, res, next) {
res.render('special')
})
Its simply means pass control to the next handler.
Cheers
Notice the call above to next(). 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 could be named anything, but by convention, it is always named “next”. To avoid confusion, always use this convention.
next() is the callback argument to the middleware function with req, and res being the http request and response arguments to next in the below code.
app.get('/', (req, res, next) => { next() });
So next() calls the passed in middleware function. If current middleware function does not end the request-response cycle, it should call next(), else the request will be left hanging and will timeout.
next() fn needs to be called within each middleware function when multiple middleware functions are passed to app.use or app.METHOD, else the next middleware function won’t be called (incase more than 1 middleware functions are passed). To skip calling the remaining middleware functions, call next(‘route’) within the middleware function after which no other middleware functions should be called. In the below code, fn1 will be called and fn2 will also be called, since next() is called within fn1. However, fn3 won’t be called, since next(‘route’) is called within fn2.
app.get('/fetch', function fn1(req, res, next) {
console.log("First middleware function called");
next();
},
function fn2(req, res, next) {
console.log("Second middleware function called");
next("route");
},
function fn3(req, res, next) {
console.log("Third middleware function will not be called");
next();
})
Almost every Express app I see has an app.use statement for middleware but I haven't found a clear, concise explanation of what middleware actually is and what the app.use statement is doing. Even the express docs themselves are a bit vague on this. Can you explain these concepts for me please?
middleware
I'm halfway through separating the concept of middleware in a new project.
Middleware allows you to define a stack of actions that you should flow through. Express servers themselves are a stack of middlewares.
// express
var app = express();
// middleware
var stack = middleware();
Then you can add layers to the middleware stack by calling .use
// express
app.use(express.static(..));
// middleware
stack.use(function(data, next) {
next();
});
A layer in the middleware stack is a function, which takes n parameters (2 for express, req & res) and a next function.
Middleware expects the layer to do some computation, augment the parameters and then call next.
A stack doesn't do anything unless you handle it. Express will handle the stack every time an incoming HTTP request is caught on the server. With middleware you handle the stack manually.
// express, you need to do nothing
// middleware
stack.handle(someData);
A more complete example :
var middleware = require("../src/middleware.js");
var stack = middleware(function(data, next) {
data.foo = data.data*2;
next();
}, function(data, next) {
setTimeout(function() {
data.async = true;
next();
}, 100)
}, function(data) {
console.log(data);
});
stack.handle({
"data": 42
})
In express terms you just define a stack of operations you want express to handle for every incoming HTTP request.
In terms of express (rather than connect) you have global middleware and route specific middleware. This means you can attach a middleware stack to every incoming HTTP requests or only attach it to HTTP requests that interact with a certain route.
Advanced examples of express & middleware :
// middleware
var stack = middleware(function(req, res, next) {
users.getAll(function(err, users) {
if (err) next(err);
req.users = users;
next();
});
}, function(req, res, next) {
posts.getAll(function(err, posts) {
if (err) next(err);
req.posts = posts;
next();
})
}, function(req, res, next) {
req.posts.forEach(function(post) {
post.user = req.users[post.userId];
});
res.render("blog/posts", {
"posts": req.posts
});
});
var app = express.createServer();
app.get("/posts", function(req, res) {
stack.handle(req, res);
});
// express
var app = express.createServer();
app.get("/posts", [
function(req, res, next) {
users.getAll(function(err, users) {
if (err) next(err);
req.users = users;
next();
});
}, function(req, res, next) {
posts.getAll(function(err, posts) {
if (err) next(err);
req.posts = posts;
next();
})
}, function(req, res, next) {
req.posts.forEach(function(post) {
post.user = req.users[post.userId];
});
res.render("blog/posts", {
"posts": req.posts
});
}
], function(req, res) {
stack.handle(req, res);
});
After simplifying things, a web server can be seen as a function that takes in a request and outputs a response. So if you view a web server as a function, you could organize it into several pieces and separate them into smaller functions so that the composition of them will be the original function.
Middlewares are the smaller functions that you can compose with others and the obvious benefit is that you can reuse them.
I add a late answer to add something not mentioned in the previous answers.
By now it should be clear that middleware is/are function(s) run between the client request and the server answer. The most common middleware functionality needed are error managing, database interaction, getting info from static files or other resources. To move on the middleware stack the next callback must be called, you can see it in the end of middleware function to move to the next step in the flow.
You can use the app.use approach and have a flow like this:
var express = require('express'),
app = express.createServer(),
port = 1337;
function middleHandler(req, res, next) {
console.log("execute middle ware");
next();
}
app.use(function (req, res, next) {
console.log("first middle ware");
next();
});
app.use(function (req, res, next) {
console.log("second middle ware");
next();
});
app.get('/', middleHandler, function (req, res) {
console.log("end middleware function");
res.send("page render finished");
});
app.listen(port);
console.log('start server');
but you can also use another approach and pass each middleware as function arguments. Here is a example from the MooTools Nodejs website where midleware gets the Twitter, Github and Blog flow before the response is sent back to the client. Note how the functions are passed as arguments in app.get('/', githubEvents, twitter, getLatestBlog, function(req, res){. Using app.get will only be called for GET requests, app.use will be called for all requests.
// github, twitter & blog feeds
var githubEvents = require('./middleware/githubEvents')({
org: 'mootools'
});
var twitter = require('./middleware/twitter')();
var blogData = require('./blog/data');
function getLatestBlog(req, res, next){
blogData.get(function(err, blog) {
if (err) next(err);
res.locals.lastBlogPost = blog.posts[0];
next();
});
}
// home
app.get('/', githubEvents, twitter, getLatestBlog, function(req, res){
res.render('index', {
title: 'MooTools',
site: 'mootools',
lastBlogPost: res.locals.lastBlogPost,
tweetFeed: res.locals.twitter
});
});
expressjs guide has pretty neat answer to your question, I highly recommend you to read that, I am posting a short snippet of the guide, the guide is quite good.
Writing middleware for use in Express apps
Overview
Middleware functions are functions that have access to the request object (req), the response object (res), and the next function in the application’s request-response cycle. The next function is a function in the Express router which, when invoked, executes the middleware succeeding the current middleware.
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 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.
Example
Here is an example of a simple “Hello World” Express application. The remainder of this article will define and add two middleware functions to the application: one called myLogger that prints a simple log message and another called requestTime1 that displays the timestamp of the HTTP request.
var express = require('express')
var app = express()
app.get('/', function (req, res) {
res.send('Hello World!')
})
app.listen(3000)
Middleware function myLogger
Here is a simple example of a middleware function called “myLogger”. This function just prints “LOGGED” when a request to the app passes through it. The middleware function is assigned to a variable named myLogger.
var myLogger = function (req, res, next) {
console.log('LOGGED')
next()
}
Notice the call above to next(). 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 could be named anything, but by convention it is always named “next”. To avoid confusion, always use this convention.
To load the middleware function, call app.use(), specifying the middleware function. For example, the following code loads the myLogger middleware function before the route to the root path (/).
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)
Every time the app receives a request, it prints the message “LOGGED” to the terminal.
The order of middleware loading is important: middleware functions that are loaded first are also executed first.
If myLogger is loaded after the route to the root path, the request never reaches it and the app doesn’t print “LOGGED”, because the route handler of the root path terminates the request-response cycle.
The middleware function myLogger simply prints a message, then passes on the request to the next middleware function in the stack by calling the next() function.
This post will only contain myLogger middleware, for further post you could go to the original expressjs guide here.
=====Very very simple explanation=====
Middlewares are often used in the context of Express.js framework and are a fundamental concept for node.js . In a nutshell, Its basically a function that has access to the request and response objects of your application. The way I'd like to think about it, is a series of 'checks/pre-screens' that the request goes through before the it is handled by the application. For e.g, Middlewares would be a good fit to determine if the request is authenticated before it proceeds to the application and return the login page if the request is not authenticated or for logging each request. A lot of third-party middlewares are available that enables a variety of functionality.
Simple Middleware example:
var app = express();
app.use(function(req,res,next)){
console.log("Request URL - "req.url);
next();
}
The above code would be executed for each request that comes in and would log the request url, the next() method essentially allows the program to continue. If the next() function is not invoked, the program would not proceed further and would halt at the execution of the middleware.
A couple of Middleware Gotchas:
The order of middlewares in your application matters, as the request would go through each one in a sequential order.
Forgetting to call the next() method in your middleware function can halt the processing of your request.
Any change the req and res objects in the middleware function, would make the change available to other parts of the application that uses req and res
Middlewares are functions executed in the middle after the input/source then produces an output which could be the final output or could be used by the next middleware until the cycle is complete.
It is like a product that goes through an assembly line where it gets modified as it moves along until it gets completed, evaluated or gets rejected.
A middleware expects some value to work on (i.e. parameter values) and based on some logic the middleware will call or not call the next middleware or send a response back to the client.
If you can't still grasp the middleware concept, it is in a way similar to the Decorator or Chain of command patterns.
Middleware is a subset of chained functions called by the Express js routing layer before the user-defined handler is invoked. Middleware functions have full access to the request and response objects and can modify either of them.
The middleware chain is always called in the exact order in which it has been defined, so it is vital for you to know exactly what a specific piece of middleware is doing. Once a middleware function finishes, it calls the next function in the chain by invoking its next argument as function. After the complete chain gets executed,the user request handler is called.
Keep things simple, man!
Note: the answer is related to the ExpressJS builtin middlware cases, however there are different definitions and use cases of middlewares.
From my point of view, middleware acts as utility or helper functions but its activation and use is fully optional by using the app.use('path', /* define or use builtin middleware */) which don't wants from us to write some code for doing very common tasks which are needed for each HTTP request of our client like processing cookies, CSRF tokens and ..., which are very common in most applications so middleware can help us do these all for each HTTP request of our client in some stack, sequence or order of operations then provide the result of the process as a single unit of client request.
Example:
Accepting clients requests and providing back responses to them according to their requests is the nature of web server technology.
Imagine if we are providing a response with just "Hello, world!" text for a GET HTTP request to our webserver's root URI is very simple scenario and don't needs anything else, but instead if we are checking the currently logged-in user and then responding with "Hello, Username!" needs something more than usual in this case we need a middleware to process all the client request metadata and provide us the identification info grabbed from the client request then according to that info we can uniquely identify our current user and it is possible to response to him/her with some related data.
Hope it to help someone!
In very basic term if i want to explain it like this i learn this from traversymedia youtube channel express crash course.
ok so middle ware is a function who execute after you make a call to your route like this.
var logger = function(req, res, next){
console.log('logging...');
next();
}
app.use(logger);
This logger function execute every time you refresh your page that means you can write anything in it that you required to do after your page get rendered any operation api call, reset things basically anything. and put this middleware before your route function order of middleware is really important or it dons't work