what is ajax

Ajax 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 without interfering with the display and behaviour of the 
existing page. 

function loadDoc() {

  var xhttp = new   XMLHttpRequest();

  xhttp.onreadystatechange = function() {
    
if (this.readyState == 4 && this.status == 200) {
     
document.getElementById("demo").innerHTML = this.responseText;
    
}
  };
  xhttp.open("GET", "ajax_info.txt", true);
  
xhttp.send();

}