In the docs for the NodeJS express module, the example code has app.use(...).
What is the use function and where is it defined?
The app object is instantiated on creation of the Express server. It has a middleware stack that can be customized in app.configure()(this is now deprecated in version 4.x).
To setup your middleware, you can invoke app.use(<specific_middleware_layer_here>) for every middleware layer that you want to add (it can be generic to all paths, or triggered only on specific path(s) your server handles), and it will add onto your Express middleware stack. Middleware layers can be added one by one in multiple invocations of use, or even all at once in series with one invocation.
See use documentation for more details.
To give an example for conceptual understanding of Express Middleware, here is what my app middleware stack (app.stack) looks like when logging my app object to the console as JSON:
stack:
[ { route: '', handle: [Function] },
{ route: '', handle: [Function: static] },
{ route: '', handle: [Function: bodyParser] },
{ route: '', handle: [Function: cookieParser] },
{ route: '', handle: [Function: session] },
{ route: '', handle: [Function: methodOverride] },
{ route: '', handle: [Function] },
{ route: '', handle: [Function] } ]
As you might be able to deduce, I called app.use(express.bodyParser()), app.use(express.cookieParser()), etc, which added these express middleware 'layers' to the middleware stack. Notice that the routes are blank, meaning that when I added those middleware layers I specified that they be triggered on any route. If I added a custom middleware layer that only triggered on the path /user/:id that would be reflected as a string in the route field of that middleware layer object in the stack printout above.
Each layer is essentially adding a function that specifically handles something to your flow through the middleware.
E.g. by adding bodyParser, you're ensuring your server handles incoming requests through the express middleware. So, now parsing the body of incoming requests is part of the procedure that your middleware takes when handling incoming requests -- all because you called app.use(bodyParser).
Each app.use(middleware) is called every time a request is sent to the server.
use is a method to configure the middleware used by the routes of the Express HTTP server object. The method is defined as part of Connect that Express is based upon.
Update Starting with version 4.x, Express no longer depends on Connect.
The middleware functions that were previously included with Express are now in separate modules; see the list of middleware functions.
app.use() acts as a middleware in express apps. Unlike app.get() and app.post() or so, you actually can use app.use() without specifying the request URL. In such a case what it does is, it gets executed every time no matter what URL's been hit.
app.use() used to Mounts the middleware function or mount to a specified path,the middleware function is executed when the base path matches.
For example:
if you are using app.use() in indexRouter.js , like this:
//indexRouter.js
var adsRouter = require('./adsRouter.js');
module.exports = function(app) {
app.use('/ads', adsRouter);
}
In the above code app.use() mount the path on '/ads' to adsRouter.js.
Now in adsRouter.js
// adsRouter.js
var router = require('express').Router();
var controllerIndex = require('../controller/index');
router.post('/show', controllerIndex.ads.showAd);
module.exports = router;
in adsRouter.js, the path will be like this for ads- '/ads/show', and then it will work according to controllerIndex.ads.showAd().
app.use([path],callback,[callback]) :
we can add a callback on the same.
app.use('/test', function(req, res, next) {
// write your callback code here.
});
app.use() handles all the middleware functions.
What is middleware?
Middlewares are the functions which work like a door between two all the routes.
For instance:
app.use((req, res, next) => {
console.log("middleware ran");
next();
});
app.get("/", (req, res) => {
console.log("Home route");
});
When you visit / route in your console the two message will be printed. The first message will be from middleware function. If there is no next() function passed then only middleware function runs and other routes are blocked.
app.use(function middleware1(req, res, next){
// middleware1 logic
}, function middleware2(req, res, next){
// middleware2 logic
}, ... middlewareN);
app.use is a way to register middleware or chain of middlewares (or multiple middlewares) before executing any end route logic or intermediary route logic depending upon order of middleware registration sequence.
Middleware: forms chain of functions/middleware-functions with 3 parameters req, res, and next. next is callback which refer to next middleware-function in chain and in case of last middleware-function of chain next points to first-middleware-function of next registered middlerare-chain.
app.use() works like that:
Request event trigered on node http server instance.
express
does some of its inner manipulation with req object.
This is when
express starts doing things you specified with app.use
which very simple.
And only then express will do the rest of the stuff like routing.
The .use() method in express is a *middleware handler. An Express application is essentially a series of middleware function calls.
An Express application can use 5 different types of middleware, of which these two are majorly used:
Application-level middleware
Router-level middleware
App.use() is used to bind *application-level middleware to an instance of the app object which is instantiated on the creation of the Express server (router.use() for router-level middleware).
Syntax : app.use(path, middleware function/s)
Here, the path is optional. When no path is specified the function gets executed every time the app receives a request, irrespective of which URL has been hit.
*Example:
Auth middleware - In a To-Do app, once an already created user logs in, he is provided with a JWT token, which must be verified every time the user makes a GET, PUT, PATCH, POST or DELETE request.
app.use("/api/*", verifyToken(req, res, next): void {
const jwt: string = req.headers['x-token-header'];
if (!jwt) {
res.status(403).send({ message: 'No token provided!' });
} else {
jsonwebtoken.verify(jwt, config.get('secretString'), (err) => {
if (err) {
res.status(403).send(err);
} else {
next();
}
});
});
Here, the path /api has been added to differentiate from requests that do not need a JWT authentication such as sign up and log in (since we don't want the middleware to be executed when there's no need for authentication).
*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.
Syntax of a middleware: function(req, res, next)
In express if we
import express from "express"
and use app = express();
then app having all functionality of express
if we use app.use()
with any module/middleware function to use in whole express project
app.use is woks as middleware for app request.
syntax
app.use('pass request format',function which contain request response onject)
example
app.use('/',funtion(req,res){
console.log(all request pass through it);
// here u can check your authentication and other activities.
})
also you can use it in case of routing your request.
app.use('/', roting_object);
app.use is a function requires middleware. For example:
app.use('/user/:id', function (req, res, next) {
console.log('Request Type:', req.method);
next();
});
This example shows the middleware function installed in the /user/:id path. This function is executed for any type of HTTP request in the /user/:id path.
It is similar to the REST Web Server, just use different /xx to represent different actions.
app.use() is a method that allows us to register a middleware.
The middleware method is like an interceptor in java, this method always executes for all requests.
Purpose and use of middleware:-
To check if the session expired or not
for user authentication and authorization
check for cookie (expiry date)
parse data before the response
Middleware is a general term for software that serves to "glue together" so
app.use is a method to configure the middleware, for example: to parse and handle the body of request:
app.use(bodyParser.urlencoded({ extended: true }));
app.use(bodyParser.json());
there are many middlewares you can use in your express application
just read the doc :
http://expressjs.com/en/guide/using-middleware.html
app.use applies the specified middleware to the main app middleware stack. When attaching middleware to the main app stack, the order of attachment matters; if you attach middleware A before middleware B, middleware A will always execute first. You can specify a path for which a particular middleware is applicable. In the below example, “hello world” will always be logged before “happy holidays.”
const express = require('express')
const app = express()
app.use(function(req, res, next) {
console.log('hello world')
next()
})
app.use(function(req, res, next) {
console.log('happy holidays')
next()
})
It enables you to use any middleware (read more) like body_parser,CORS etc. Middleware can make changes to request and response objects. It can also execute a piece of code.
You can also create your own middleware function like
app.use( function(req, res, next) {
// your code
next();
})
It contains three parameters req, res, next
You can also use it for authentication and validation of input params to keep your
controller clean.
next() is used for go to next middleware or route.
You can send the response from middleware
app.use is Application level middleware
Bind application-level middleware to an instance of the app object by using the app.use() and app.METHOD() functions, where METHOD is the HTTP method of the request that the middleware function handles (such as GET, PUT, or POST) in lowercase.
you can use to check all requests, for example, you want to check token/access token you need to write a middleware by using app.use to check the token in the request.
This example shows a middleware function with no mount path. The function is executed every time the app receives a request.
var app = express()
app.use(function (req, res, next) {
console.log('Time:', Date.now())
next()
})
reference from https://expressjs.com/en/guide/using-middleware.html
app.use(path, middleware) is used to call middleware function that needs to be called before the route is hit for the corresponding path. Multiple middleware functions can be invoked via an app.use.
app.use(‘/fetch’, enforceAuthentication) -> enforceAuthentication middleware fn will be called when a request starting with ‘/fetch’ is received. It can be /fetch/users, /fetch/ids/{id}, etc
Some middleware functions might have to be called irrespective of the request. For such cases, a path is not specified, and since the the path defaults to / and every request starts with /, this middleware function will be called for all requests.
app.use(() => { // Initialize a common service })
next() fn needs to be called within each middleware function when multiple middleware functions are passed to app.use, else the next middleware function won’t be called.
reference : http://expressjs.com/en/api.html#app.use
Note: The documentation says we can bypass middleware functions following the current one by calling next('route') within the current middleware function, but this technique didn't work for me within app.use but did work with app.METHOD like below. So, fn1 and fn2 were called but not fn3.
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();
})
app.use(req, res, next) is an API that allows us to add one or more middlewares to the request pipeline of express. A middleware is a function that has a defined signature, and through that, you can modify or end the request, returning a response according to a condition that you program. For example, I can call res.end() and finish the request to the client. Middlewares are executed in the order they're added. I can simply decorate the req object, adding or removing properties, for example, authenticating an user and setting req.user = 'any user of database', and calling next(), the next middleware will begin its execution, receiving the same instance of req, res, next.
Bind application-level middleware to an instance of the app object by using the app.use() and app.METHOD() functions, where METHOD is the HTTP method of the request that the middleware function handles (such as GET, PUT, or POST) in lowercase.
In short app.use() supports all type of requests [eg:get,post,...]
so its mostly used to setup middelware.
or can be used for when the routes and functions seperated
example:
app.use("/test",functionName)
and functionName is located in different file
app.use
is created by express(nodejs middleware framework )
app.use is use to execute any specific query at intilization process
server.js(node)
var app = require('express');
app.use(bodyparser.json())
so the basically app.use function called every time when server up
You can use app.use('/apis/test', () => {...}) for writing middleware for your api, to handle one or some action (authentication, validation data, validation tokens, etc) before it can go any further or response with specific status code when the condition that you gave was not qualified.
Example:
var express = require('express')
var app = express()
app.use(function (req, res, next) {
// Your code to handle data here
next()
})
More detail is, this part actually an anonymous function for you to write the logic on runtime
function (req, res, next) {
// Your code to handle data here
next()
}
You can split it into another function from another file and using module.export to use
next() here for the logic that if you handle everything is fine then you can use then for the program to continue the logic that its used to.
The app.use() function is used to mount the specified middleware function(s) at the path which is being specified. It is mostly used to set up middleware for your application.
Syntax
app.use(path, callback)
Parameters:
path: It is the path for which the middleware function is being called. It can be a string representing a path or path pattern or regular expression pattern to match the paths.
callback: It is a middleware function or a series/array of middleware functions.
In simple words app.use() is a function that takes another function (callback) as a parameter and runs every time, when the request is sent to the express app/server.
The function passed inside app.use is also called middleware, middleware is just a fancy name for a function that exists in express app and has three parameters request, response and next. You can read more about middleware.
Middleware are called between request and response cycle. If you want a middleware to be applied on all the routes then you can use app.use() or do some validation, error checking and other things.
app.use() will be called for every request: GET, POST, PUT, PATCH, DELETE
Let's say we have a set of routes that our site can handle
app.get('/1/', function(req, res) {
res.send('page1');
});
app.get('/2/', function(req, res) {
res.send('page2');
});
Obviously, if an address is requested that we do not process, then a 404 error should be returned. Express, however, does not do this by default. But it's easy to implement.
The special method app.use will help us with this. It allows you to intercept all raw addresses
Let's use this method to return a 404 error
app.use(function(req, res) {
res.status(404).send('not found');
});
Now let's place our construction after all app.get
app.get('/1/', function(req, res) {
res.send('page1');
});
app.get('/2/', function(req, res) {
res.send('page2');
});
app.use(function(req, res) {
res.status(404).send('not found');
});
As the name suggests, it acts as a middleware in your routing.
Let's say for any single route, you want to call multiple url or perform multiple functions internally before sending the response.
you can use this middleware and pass your route and perform all internal operations.
syntax:
app.use( function(req, res, next) {
// code
next();
})
next is optional, you can use to pass the result using this parameter to the next function.
app.use() is the application middleware.
Bind application-level middleware to an instance of the app object by using the app. use() and app. METHOD() functions, where METHOD is the HTTP method of the request that the middleware function handles (such as GET, PUT, or POST)
var express = require('express');
var app = express();
var PORT = 3000;
// This middleware will not allow the
// request to go beyond it
app.use(function (req, res, next) {
console.log("Middleware called")
next();
});
// Requests will never reach this route
app.get('/user', function (req, res) {
console.log("/user request called");
res.send('Hello test');
});
app.listen(PORT, function(err){
if (err) console.log(err);
console.log("Server listening on PORT", PORT);
});
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