server.js

const express = require("express");
const expressValidator = require("express-validator");
const mongoose = require("mongoose");
const bodyParser = require("body-parser");
const app = express();

const port = 5846;

/**
 * 
 */
const userApi = require("./router/api/user");

const db = require("./config/key").mongoURI;
mongoose.connect(db,{ useNewUrlParser: true }).then( () => console.log("")).catch((err) => console.log(err));

app.use(bodyParser.urlencoded({extended:false}))  
app.use(bodyParser.json());
app.use(expressValidator());

/**
 * 
 */
app.use("/api/user",userApi);



/**
 *  app
 */
app.listen(port,() => {
    //
    console.log("")
});

router / api / user.js is the interface file in server.js

 const express = require("express");
 const {checkBody} = require("express-validator");
 const bcrypt = require("bcryptjs");
 const router = express.Router();
 const UserModel = require("../../models/User");

/**
  * 
  * POST api/user/register
  */
router.post("/register",(req,res) => {
    req.checkBody("userName", "Enter a user name.").notEmpty().trim();
    req.checkBody("userEmail","Enter a valid email address.").notEmpty();
    req.checkBody("userPassword","Enter a user password.").notEmpty();
    
    UserModel.findOne({userName:req.body.userName})
        .then((user) => {
            if(user){
                return res.status(400).json({userName:""})
            }else{
                const newUser = new UserModel({
                    userName: req.body.userName,
                    userEmail:req.body.userEmail,
                    userPassword: req.body.userPassword,
                    userDate:req.body.userDate
                });

                //
                bcrypt.genSalt(10, function(err, salt) {
                    bcrypt.hash(newUser.userPassword, salt, (err, hash) => {
                        if(err){
                            console.log(err);
                            res.send({
                                message :"Post request error!"
                            });
                        }
                        newUser.userPassword = hash;

                        //
                        newUser.save()
                                 .then(user => res.json(user))
                                 .catch(err => console.log(err));
                    });
                });
            }
        });
}

use postman to test data and return error message:

:

I don"t know what"s wrong with this reason. Ask for advice.

does it have to be verified before the submission can be successful?