[Ajax] onreadystatechange の代わりに loadend イベントを使う ref: http://qiita.com/MizuiroFolder/items/99913016b1485fdf7c36

file0.js

var xhr = new XMLHttpRequest();
xhr.onreadystatechange = function(){
  if(xhr.readyState === 4 && xhr.status === 200){
    console.log(xhr.response);
  }
}
xhr.open('GET', '/path/to/file', true);
xhr.send();

file1.js

var xhr = new XMLHttpRequest();
xhr.addEventListener('loadend', function(){
  if(xhr.status === 200){
    console.log(xhr.response);
  }else{
    console.error(xhr.status+' '+xhr.statusText);
  }
});
xhr.open('GET', '/path/to/file', true);
xhr.send();