how to handle xhr request code example

Example 1: xhr post send

var xhr = new XMLHttpRequest();
var params = 'field1='+postfield1+'&field2='+postfield2;
xhr.open('POST', 'http://exmaple.com', true);

//Send the proper header information along with the request
xhr.setRequestHeader('Content-type', 'application/x-www-form-urlencoded');

xhr.onload = function() {//Call a function when the state changes.
    if(xhr.status == 200) {
        alert(this.responseText);
    }
}
xhr.send(params);

Example 2: xmlhttprequest javascript

function reqListener () {
  console.log(this.responseText);
}

var oReq = new XMLHttpRequest();
oReq.onload = reqListener;
oReq.open("GET", "http://www.example.org/example.txt");
oReq.send();