Middleware doesn't work if i change endpoint

Question

I have a strange problem with my NodeJs - Express server which serves as a back-end for my mobile application. The thing is that i send post requests to some endpoints like checkmail, checkusername from front-end using axios and it works, but the problem is it doesn't work for any other middleware function. I literally copied the same checkmail and just used different route and I get status 404 while with /checkmail it works! Also, the /login does not work, im using express. Router in there. Here is my app.js code:

const express = require("express");
const mongoose = require("mongoose");
const bodyParser = require("body-parser");
var cors = require("cors");
const User = require("./models/user");
var AuthController = require('./auth/authController');
const app = express();
let server = require("http").Server(app);

app.use(cors());
app.use(
    bodyParser.urlencoded({
        extended: true
    })
);
app.use(bodyParser.json());

//Check if e-mail is aready in use, return "success" if not
app.use("/signup", (req, res) => {
    User.findOne({
        email: req.body.email
    },
        function (err, user) {
            if (user) {
                res.send("error");
            } else {
                res.send("success");
            }
        }
    );
});

//Check if e-mail is aready in use, return "success" if not
app.use("/checkmail", (req, res) => {
    User.findOne({
        email: req.body.email
    },
        function (err, user) {
            if (user) {
                res.send("error");
            } else {
                res.send("success");
            }
        }
    );
});

app.use('/login', AuthController);

const port = process.env.PORT || 8500;

server.listen(port, () => { });

Answer 1

Middleware should have third parameter next.

app.use("/checkmail", (req,res,next)=>{
//do something
})

Answer 2

You must have third parameter in middleware, it's callback to tell application go to next route.

app.use('/signup', (req, res, next) => { // Normal middleware, it must have 3 parameters
  User.findOne({
     email: req.body.email
   },
   function (err, user) {
   if (user) {
     next(new Error('Some thing wrong')); // Failed, we will go to error-handling middleware functions
   } else {
     next(); // Success and we will go to next route
   }
  }
 );
});

app.get('/signup', (req, res, next) => { // This is "next route"
  res.send('Yay, welcome to signup')
});

app.use((err, req, res, next) => { //  error-handling middleware function, It must have 4 parameters
  res.status(500).send(err)
});

You can find document in here: https://expressjs.com/en/guide/error-handling.html