[Fixed] How can I validate and handle errors in proper way?

Issue

Im totally noob with node. I want to validate if user exist in db and validate if passwords matches to each other. This is what I managed so far :

this is my router

const router = require('express').Router();
let User = require('../models/user.model');
const { check, validationResult } = require('express-validator')

router.route('/').get((req, res) => {
  User.find()
    .then(users => res.json(users))
    .catch(err => res.status(400).json('Error: ' + err));
});

router.route('/add').post([
  check('username', "min 5").isLength({ min: 5 }).custom(value => {
    return User.findOne({
        where: {
            username: value
        }
    }).then(user => {
        if (user) {
          throw new Error('user exist');
        }
    });
  }),
  check('password', "cant be empty").isLength({ min: 8 }),
],(req, res) => {

  const errors = validationResult(req)
  if (!errors.isEmpty()) {
    res.json("fields cant be empty")
  }

  if(req.body.password2 !== req.body.password) {
      res.json("passwords dont match")
  }

  const user = {
      username: req.body.username,
      password: req.body.password,
  }

  const newUser = new User(user);
  newUser.save()
    .then(() => res.json('User added!'))
    .catch(err => res.status(400).json('Error: ' + err));
});

module.exports = router;

and this is my submit function on client side

  onSubmit(e) {
    e.preventDefault();
    const user = {
      username: this.state.username,
      password: this.state.password,
      password2: this.state.password2
    }
    
    axios.post('http://localhost:5000/users/add', user)
      .then(res => console.log(res.data))

    this.setState({
      username: ''
    })
  }

This works somehow but I’d like to do in proper way. I appreciate any tips how can I achieve this and how can I handle errors in proper way.

Solution

You can use mongoose buit-in validators for that.all schemaType have some in built validators.

const userSchema=new mongoose.Schema({
name:{
     type:String,
     required:[true,'Please provide user name'],
     minlength:5  
},
email:{
     type:String,
     required:[true,'Please provide email'],
     unique:true,
     validate:[validator.isEmail,'Please provide a valid email']
},
password:{
     type:String,
     required:[true,'Please provide password'],
     minlength:8, 
},
confirmPassword:{
     type:String,
     validate:{
           validator:function(el){
              return el===this.password;
             },
           message:'Passwords are not same!'
      }
}

});