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why slash URL runs with all of other URLs in the middleware of NodeJS?

Why I enter http://localhost:3000/product on the browser, the output will be both outputs of '/' and '/product'?
Please look at this snippet code.
const express = require('express');
const app = express();
// http://localhost:3000/product
app.use('/product', (req, res, next)=>{
console.log('In product page');
res.send('<h1>Product Page</h1>');
});
// http://localhost:3000/
app.use('/', (req, res, next)=>{
console.log('In main page');
res.send('<h1>Main Page</h1>');
});
app.listen(3000);
This image is my app's output.

It could have multiple reasons. One that I think of right now is that the browser requests http://localhost:3000/favicon.ico automatically after product.html, which triggers the use('/', ...) route.
Maybe you should use app.all(...) instead of app.use(...), to avoid this "wildcard" on every path that should rather be a 404 page.

The app.use() method is used to bind application level middleware. Not for accepting GET request as you are expecting.
You should use
// http://localhost:3000/product
app.get('/product', (req, res)=>{
console.log('In product page');
res.send('<h1>Product Page</h1>');
});
// http://localhost:3000/
app.get('/', (req, res)=>{
console.log('In main page');
res.send('<h1>Main Page</h1>');
});

Because in express whatever you define with app.use() is middleware and it always executes until and unless it has some path defined
app.use('/', (req, res, next)=>{
console.log('In main page');
res.send('<h1>Main Page</h1>');
});
above will always execute because it contains root path and every url has root path
Check this link for further info https://expressjs.com/en/4x/api.html#app.use
You should define your routing with express router using app.get("/") or app.post("/") and it will help for more info https://expressjs.com/en/guide/routing.html

Express.js Routing is not making sense

I'm working on an Express.js app and I think I am having trouble understanding the nuances of multi-layer routing. My app has the following segment of code in its app.js file:
//app.js
var createError = require('http-errors');
var express = require('express');
var path = require('path');
var cookieParser = require('cookie-parser');
var logger = require('morgan');
//new stuff
const routes = require('./routes');
var app = express();
// view engine setup
app.set('views', path.join(__dirname, 'views'));
app.set('view engine', 'pug');
// middleware functions required
app.use(logger('dev')); //handles logging
app.use(express.json()); //JSON payload parser
app.use(express.urlencoded({ extended: false })); //urlencoded payload parser
app.use(cookieParser());
//content routes
app.use('/jquery', express.static(__dirname + '/node_modules/jquery/dist/'));
app.use('/', express.static(path.join(__dirname, 'public'))); //static files (css and other js files)
app.use('/', routes); //everything else (see /routes/index.js)
// catch 404 and forward to error handler
app.use(function(req, res, next) {
next(createError(404));
});
.
.
.
As indicated in the code, after a bunch of static file and default middleware setup for logging and such, app.use('/', routes); is declared, handing off to an index.js file in the /routes folder as follows:
//index.js
//main router entry point, sets up all route modules
//instantiate the express.Router class
const express = require('express');
const router = express.Router();
//import our route modules
const indexRouter = require('./indexRouter');
const resetRouter = require('./resetRouter');
const enrollRouter = require('./enrollRouter');
//map individual route modules to their respective routes
router.use('/', indexRouter);
router.use('/reset', resetRouter);
router.use('/enroll(/*)?', enrollRouter);
module.exports = router;
The three router.use lines call individual route files in the same directory as shown. The first two are simple get routes. The third one (router.use('/enroll(/*)?', enrollRouter);) has both get and post components via the enrollRouter.js file as follows:
//enrollRouter.js
const express = require('express');
const router = express.Router();
const { getEnroller } = require('../controllers/enrollGetController');
const { postEnrollment } = require('../controllers/enrollPostController');
router.post(/^(\/enroll\/new)/i, postEnrollment);
//router.post('/', postEnrollment);
router.get('/', getEnroller);
module.exports = router;
Here's where things get interesting. I'm using postman to make a post request to "/route/new". If the route string for the post route in the code above is set to '/', everything works fine, and enrollPostController.js is called returning appropriate content. However, if I set the route string to '/enroll/new' instead (the path I'm calling... doesn't matter if it's a string or a regex), the 404 error code in app.js is called.
I'm not following what is going on. I'm under the impression that the string at the beginning of a router.get or router.post call represents the path to be matched for the callback defined as the next parameter. When the path is clearly the one specified, why am I getting a 404?
The way I'm thinking, when the /enroll/new post request comes in, app.js should hand off to index.js for all routes matching '/'. Then index.js should hand off to enrollRouter because the route matches '/enroll(/*)?'. And then finally, enrollRouter.js should call postEnrollment defined over in ../controllers/enrollPostController, because the route matches '/enroll/new'. But it doesn't work that way.
Can someone enlighten? or fill in the holes in my understanding?
Here's the code in enrollPostController.js
//enrollPostController.js
module.exports = {
postEnrollment(erq, res) {
//at this point we have the form submission data.
console.log(erq.body);
res.send("received form submission!");
},
};

Try the following:
routes/index.js
// Delegate "/enroll/*" to router
router.use('/enroll', enrollRouter);
routes/enrollRouter.js
// Handle POST requests to "/enroll/new"
router.post('/new', postEnrollment);
I hope this helps.

If you are following some pattern and only endpoints are getting changed for example
http://localhost:4000/mycode/testUrl/abc
http://localhost:4000/mycode/testUrl/Pqr/xyz
http://localhost:4000/mycode/testUrl/abc/xyz/pqs
const service = express.Router();
// **console** is just to check your endUrl. you can print originalUrl as well
service.get('/*', function(req, res) {console.log(req.url); }
service.post('/*', function(req, res) { console.log(req.url);}
app.use('/mycode/testUrl', service);

In your index.js
The express router defined as
router.use('/enroll(/*)?', enrollRouter);
passes the '/enroll/new' to your enrollRouter.js
so all you need to give the "/" in the post route, so the "/enroll/new" is right in front of it but due to nesting of routes we can separate them.
Hope I answered the question.

TypeError: Router.use() requires a middleware function but got a Object nodejs [duplicate]

I am getting this error when I run npm start to run my express app.
TypeError: Router.use() requires middleware function but got a Object
my app.js code
var express = require('express');
var path = require('path');
var favicon = require('serve-favicon');
var logger = require('morgan');
var cookieParser = require('cookie-parser');
var bodyParser = require('body-parser');
var routes = require('./routes/index');
var users = require('./routes/users');
var app = express();
// view engine setup
app.set('views', path.join(__dirname, 'views'));
app.set('view engine', 'ejs');
// uncomment after placing your favicon in /public
//app.use(favicon(__dirname + '/public/favicon.ico'));
app.use(logger('dev'));
app.use(bodyParser.json());
app.use(bodyParser.urlencoded({ extended: true }));
app.use(cookieParser());
app.use(express.static(path.join(__dirname, 'public')));
app.use('/', routes);
app.use('/users', users);
/// catch 404 and forwarding to error handler
app.use(function(req, res, next) {
var err = new Error('Not Found');
err.status = 404;
next(err);
});
/// error handlers
// development error handler
// will print stacktrace
if (app.get('env') === 'development') {
app.use(function(err, req, res, next) {
res.status(err.status || 500);
res.render('error', {
message: err.message,
error: err
});
});
}
// production error handler
// no stacktraces leaked to user
app.use(function(err, req, res, next) {
res.status(err.status || 500);
res.render('error', {
message: err.message,
error: {}
});
});
module.exports = app;
my index.js code
var express = require('express');
var router = express.Router();
/* GET home page. */
router.get('/', function(req, res) {
res.render('index', { title: 'Express' });
});
/* GET Hello World page. */
router.get('/helloworld', function(req, res) {
res.render('helloworld', { title: 'Hello, World!' })
});
module.exports = router;
I am quirte new to using Node and express. I cant see where I have gone wrong. Can anybody see what my problem is?

I found the answer in the comments from Kop4lyf:
check your users.js. It should also be exporting the router like
index.js, if you can try that.
However, this question was my top search result when I ran into this issue, so I am promoting to an answer.
The error is caused because one of your route modules is not being exported - meaning Express does not have access to it when it tries to identify all of your routes.
You can fix this by adding module.exports = router; to the end of each of your route files.
Example:
var express = require('express');
var router = express.Router();
router.get('/', function(req, res, next) {
//Do whatever...
});
module.exports = router;
More information about module.exports can be found on this question or the offcial Node.js documentation.

I have fixed this by adding which i am using somewhere. So please check your all exports.
module.exports = router;

If you use in routes
exports default router
Your solution can be
module.exports = router

I had the same error , fixed it by replacing app.use('view engine', 'ejs') with app.set('view engine', 'ejs').
For reference I used this webpage Migrating from 3.x to 4.x
I didn't have to make any changes to either index.js or application.js. For more information on EJS one could refer Using EJS with Express and Express 4.x API

Your index.js file is fine you just have to create users.js and export it.
let express = require('express');
let router = express.Router();
//Login Page - GET REQUEST
router.get('/login',(req,res)=> {
res.send('login page');
})
//Register Page - GET REQUEST
router.get('/register',(req,res)=> {
res.send('register page');
});
module.exports = router;

in every module **export the router** and **keep one handler for the default
path '/'**
// in index.js
const authRoute = require("./routes/authRoute");
app.use("/auth", authRoute);
// in authRoute.js
const express = require("express");
const router = express.Router();
router.get("/", (req, res) => {
// code
});
module.exports = router;

This error comes when you forgot to export the module which uses the Router.
Your mentioned code works perfectly with some tweaks.
if your app.js is main/starting point of the app.
it should have
const port = process.env.PORT || 3000;
app.listen(port, () => console.log(`Listening on port ${port}...`));
instead of
module.exports = app;
(optional)Generally index.js is used for starting point of app. Rename index.js as helloworld.js and change same at require statement
var routes = require('./routes/index');
to
var routes = require('./routes/helloworld');
run this app using the following command
node app.js

If you have checked all the solution than also having this error than check this one
Another cause of having this error is calling a method which is not exist or not not exported.
In my case i am calling login method but i forgot to define them
I was trying to call this method
app.post('/api/login', db.login);
but i had forgot to create login method so i got this error. also try to check spelling mistake may be you might have typed wrong spell

I had the same problem, and then I discovered that I was missing this line in one of my controllers !
return api; //it might be return routerfor your code !
I added this line to my code and it worked fine.

Whew, my problem was that i was doing module.exports = { router } instead of module.exports = router

I found it after lot of struggle! as everything syntactically correct, nothing wrong with code that was written, it was due to the code that was not written yet! This could happen if you have implemented index.js but not yet users.js. However, you have already defined both lines
app.use('/', routes);
app.use('/users', users);
If you are eager to test index.js right away without waiting for users.js to be implemented. That's exactly when it errors out.

if you are still facing this problem and try every solution then just replace router with routes and it worked fine

For anybody using EJS:
In my case I was receiving this error as I used
app.use("view engine","ejs")
instead of
app.get("view engine","ejs")

I fixed it by removing the app.use(/users, users);
I don't need this at the minute so maybe that is why it started breaking.

Override app.get('*', func) route in express

I have an Angular app running alongside a NodeJS server.
This code in my server.js file:
app.get('*', (req, res) => {
res.sendFile(path.join(__dirname, 'dist/index.html'));
});
require('./server/routes/routes')(app, passport);
Seems to be preventing any GET requests to my api, for example:
module.exports = function (app, passport) {
app.get('/api/profile', isLoggedIn, function (req, res) {
res.status(200).json(req.user);
});
When the exported /api/profile handler is changed to a post the request works.
Should the route not overwrite the initial handler? How to achieve this? Can I serve the app for all routes excluding beginning with '/api'?

Move the app.get('*') so it becomes the last route that gets declared:
require('./server/routes/routes')(app, passport);
app.get('*', (req, res) => {
res.sendFile(path.join(__dirname, 'dist/index.html'));
});
Express checks route handlers in order of their declaration, not in order of their specificity (how well they match a particular request).

Why am I getting "name undefined" when trying to post using Express and Body Parser

I'm attempting to build a MEAN app and trying to test POSTing with POSTMAN. When I do, I keep getting the dreaded "TypeError: Cannot read property 'name' of undefined". If I type in a simple string, the POST goes through fine. But when I use "req.body.name" I get the error. I've looked in every place and I'm not seeing my mistake. I even followed the suggestions on this thread with no luck. Any help or suggestions would be greatly appreciated.
Here's the code I am currently working with in my server.js file:
const express = require('express');
var bodyParser = require('body-parser');
var Bear = require('./models/bear')
var path = require('path');
var mongoose = require('mongoose');
var router = express.Router();
var app = express();
var staticAssets = __dirname + '/public';
app.use(express.static(staticAssets));
app.use('/api', router)
app.use(bodyParser.json());
app.use(bodyParser.urlencoded({extended: true}));
// Routes for my API
//===================================
// middleware to use for all requests
router.use(function(req,res,next){
// logging happens here
console.log('Something will happen.');
next(); // Head to the next router...don't stop here
});
// Test router to make sure everything is working (accessed at GET http://localhost:3000/api)
router.get('/', function(req, res){
res.json({message: 'hooray! welcome to our api!'})
})
//More routes will happen here with routes that end in "/bears"
router.route('/bears')
//Create a bear (accessed at POST http://localhost:3000/api/bears)
.post(function(req,res){
var bear = new Bear(); // Create a new instance of the bear model
console.log(req);
bear.name = req.body.name; // set the bears name (comes from the request)
//res.send(200, req.body);
bear.save(function(err){
if (err)
res.send(err);
res.json({message: 'Bear Created!!'});
});
});
//======================================
//var Products = require('./products.model.js');
var Product = require('./models/product.model');
var db = 'mongodb://localhost/27017';
mongoose.connect(db);
var server = app.listen(3000);
console.log("App is listening on port 3000");
Thanks.
Also, the url I'm trying to use inside of POSTMAN is http://localhost:3000/api/bears

Express processes requests Top-Down, meaning if you require a piece of functionality to be applied to all routes via middleware, than that middleware needs to be added to your app before any routes that require it. This is usually the case for middleware such as body-parser.
When using Router Middleware, you don't typically construct the router in the same file as the actual Express app that will use it as middleware. Instead, place it in a separate file and/or directory for organization purposes, this is considered a best practice.
Express Apps can be structured like so
/lib
/models
bear.js
product.js
/node_modules
/public
/css
/routes
api.js
package.json
server.js
The routes directory is where you would place any applicable Router Middleware files such as your api router. server.js is your main Express App and public is where your static assets are stored. lib is directory that contains any business logic files and models.
The actual Express app and Router files should look something like this
server.js
'use strict';
const express = require('express');
const bodyParser = require('body-parser');
const path = require('path');
const apiRouter = require('./routes/api');
const app = express();
app.use(bodyParser.json());
app.use(bodyParser.urlencoded({ extended: true }));
app.use(express.static(path.join(__dirname, public)));
app.use(/api, apiRouter);
app.listen(port, () => {
console.log(`Listening on port ${port});
});
module.exports = app;
routes/api.js
'use strict';
const router = require('express').Router();
const Bear = require('./lib/models/bear');
router.use((req, res, next) => {
// logging happens here
console.log('Something will happen.');
next(); // Head to the next router...don't stop here
});
router.get('/', (req, res) => {
return res.json({ message: 'hooray! welcome to our api!'})
});
router.route('/bears')
//Create a bear (accessed at POST http://localhost:3000/api/bears)
.post((req, res) => {
var bear = new Bear(); // Create a new instance of the bear model
console.log(req);
bear.name = req.body.name; // set the bears name (comes from the request)
//res.send(200, req.body);
bear.save((err) => {
if (err)
return res.send(err);
return res.json({message: 'Bear Created!!'});
});
});
module.exports = router;
To note, you could break up your API even further to increase the amount of decoupling. An example of this would be to move the /api/bear route to its own router middleware and into its own route file. Then simply add it to your routes/api.js router as a middleware like you would in server.js. If your app is going to have a decent sized API, then this would be the best approach because it would allow the most flexibility when it comes to applying middleware to only certain routes and would make maintaining the source much easier.