139.js

// BASIC THEORY

// AJAX : asynchronous javascript and XML

// HTTP request

// is a set of "web development techniques" 
// using many web technologies on the "client-side "
// to create asynchronous web applications. 


// With Ajax, web applications can send and retrieve 
// data from a server asynchronously (in the background) 
// without interfering with the display and 
// behaviour of the existing page

// We don't use data in XML format anymore. 
// we use JSON now. 


// we have 3 most common ways to create and send request to server
// 1.) xmlHTTPRequest (old way of doing)
// 2.) fetch API (new way of doing)
// 3.) axios (this is third party library)