JavaScript AJAX request

gistfile1.js

function ajax(url, opts){
  var xhr = new XMLHttpRequest();
 
  xhr.onreadystatechange = function(){
    var completed = 4;
    if(xhr.readyState === completed){
      if(xhr.status === 200){
        opts.success(xhr.responseText, xhr);
      }else{
        opts.error(xhr.responseText, xhr);
      }
    }
  }
 
  xhr.open(opts.method, url, true);
  xhr.send(opts.data);
 
}
 
ajax('/foo', {
  method: 'GET',
  success: function(response){
    console.log(response);
  },
  error: function(response){
    console.log(response);
  }
});