callback.js

function reqListener() {  
  var data = JSON.parse(this.responseText);  
  console.log(data);  
}

function reqError(err) {  
  console.log('Fetch Error :-S', err);  
}

var request = new XMLHttpRequest();  
request.onload = function() {
  var data = JSON.parse(this.responseText);  
  //do stuff with data
};  

request.onerror = function() {
    alert('There was a problem with the request');
}
request.open('get', '/api/foo/bar', true);  
request.send();

promise.js

fetch('/api/foo/bar').then(function(data) {
    return data.json();
}).then(function(jsonData) {
    //do stuff with the data
}).catch(function(e) {
    alert('There was a problem with the request');
})