My first experience using Ajax with pure JavaScript.

main.js

var httpRequest;

if (window.XMLHttpRequest){

    httpRequest = new XMLHttpRequest();

} else if (window.ActiveXObject) {

    httpRequest = new ActiveXObject();

}

httpRequest.onreadystatechange = ajaxResponse;

httpRequest.open("POST", "localhost/ajax", true);
httpRequest.send();

function ajaxResponse () {
    
    //Checking request status

    /* ReadyState is equivalent to the state of the request, it is who informs
     to our code if the server's response is ready to be
     consumed. */

    if (httpRequest.readyState === 4) {

        //Checking the status of the HTTP request

        /* status returns a code that is equivalent to the status list of the
         HTTP request, this list refers to the behavior of that
         communication with the server, and indicates whether it was successful,
         or if there was any impediment, change of protocol, etc ... */
        
        if (httpRequest.status === 200) {
        
            //SUCESSFUL REQUEST
        
        }else{

            /*There is something wrong with the request, maybe it has
             returned a 404, which says "Not found". */

            /*I ask the javascript to print the HTTP status on the console,
              as a log. */
            
             console.log(httpRequest.status);
        
        }

    }else{
        //Request is not yet ready or was unsuccessful.
    }

};