Check out example codes for "js read mysql database". It will help you in understanding the concepts better.

Code Example 1

// Use https://github.com/JackChilds/jsmysqldb

// Then once it's setup you can just do this:

// Config that will be sent to server
const config = {
  "table" : "customers",
  "limit" : "50",
  "offset" : "0"
};

// Use the _POST_REQUEST function to send data to the server and get a response
_POST_REQUEST("link-to-server/database.php", 'config=' + JSON.stringify(config), (response) => {
  // Convert the response into an array of rows:
  var data = JSON.parse(response);
  // Now do what you want with data
  // data[row index][column name]
});

Code Example 2

con.connect(function(err) {
  if (err) throw err;
  console.log("Connected!");

   
  con.query(sql, function (err, result) {
    if (err) throw err;
    console.log("Result: 
  " + result);
  });
});

Code Example 3

//put these lines in a seperate file
const mysql = require('mysql2');

const connection = mysql.createPool({
    host: "localhost",
    user: "",
    password: "",
    database: ""
    // here you can set connection limits and so on
});

module.exports = connection;

//put these on destination page
const connection = require('../util/connection');

async function getAll() {
    const sql = "SELECT * FROM tableName";
    const [rows] = await connection.promise().query(sql);
    return rows;
} 
exports.getAll = getAll;

Learn ReactJs, React Native from akashmittal.com