NodeJS Express what is app use

Question

In the docs for the NodeJS express module  the example code has app use         What is the use function and where is it defined

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Each app use middleware  is called every time a request is sent to the server

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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

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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

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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

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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

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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

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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

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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  lt specific middleware layer here gt   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

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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

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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

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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

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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

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You can use app use   apis test        gt         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

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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    gt        console log  quot middleware ran quot        next         app get  quot   quot    req  res    gt        console log  quot Home route quot         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

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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

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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

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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

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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

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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

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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

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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

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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

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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

[node.js] NodeJS / Express: what is "app.use"?

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