how node js works as backend code example

Example 1: backend in nodejs

import React from 'react';
export default class AddBook extends React.Component {

constructor(){
        super();
        this.state = {
            bookname:'',
            author:'',

        };

    }

updateInfo = (event) =>{
        let fieldName = event.target.name;
        let fieldValue = event.target.value;
        if(fieldName === 'bookname') {
            this.setState({bookname: fieldValue});
        }
        else if(fieldName === 'author'){
            this.setState({author:fieldValue});
        }
};

addBook=(e)=>{

let {bookname,author}=this.state;
fetch('localhost:3000/add_book', {
      method: 'post',
      headers: {'Content-Type': 'application/json'},
      body: JSON.stringify({
        bookname:bookname,
        author:author,
      })
      }).then(response=>response.json()).then(data=>{
      window.alert(data)
      //Do anything else like Toast etc.
})
}

render(){
return(
<div className="add_book">

<div>
	<label>Book Name</label>
	<input onChange={this.updateInfo} name="bookname" value{this.state.bookname}/>
</div>
<div>
	<label >Author</label>
	<input onChange={this.updateInfo} name="author" value={this.state.author}/>
</div>

<button onClick={this.addBook}>Add</button>                                 
</div>
    )
}
}

Example 2: backend in nodejs

const express = require('express');
const bodyParser = require('body-parser');
var connection  = require('express-myconnection'); 
var mysql = require('mysql');

const app = express(); 
app.use(bodyParser.json());

app.use(

        connection(mysql,{

            host: 'localhost', //'localhost',
            user: 'userEHX',
            password : 'hMmx56FN4GHpMXOl',
            port : 3306, //port mysql
            database:'sampledb'

        },'pool')); //or single

       app.post('/add_book',(req,res)=>{

        let {book_name,author,} = req.body;


        if(!book_name) return res.status(400).json('Book Name cant be blank');
        if(!author) return res.status(400).json('Author cant be blank');

        var data={book_name:book_name,
                  author:author};


         var query = connection.query("INSERT INTO books set ? ",data, 
        function(err, rows)
        {

          if (err){
            //If error
              res.status(400).json('Sorry!!Unable To Add'));
              console.log("Error inserting : %s ",err );
             }
         else
          //If success
          res.status(200).json('Book Added Successfully!!')

        });


        });


         app.listen(3000, ()=> {
      console.log(`app is running on port 3000`);
});