ajax.js

function ajax(obj) {
  'use strict';
  
  if (!obj || typeof obj !== 'object') throw new Error('Object parameter is null or undefined.');
  
  obj = {
    url: obj.url || null,
    type: obj.type || 'json',
    method: obj.method || 'GET',
    success: obj.success || null
  };

  function translateType(response) {
    switch(obj.type) {
      case 'text':
        return response;
      case 'json':
        return JSON.parse(response);
      default:
        return response;
    }
  }

  let xhr = new XMLHttpRequest();
  xhr.open(obj.method, obj.url);
  xhr.onreadystatechange = () => {
    if (xhr.readyState === 4 && xhr.status === 200)
      obj.success(translateType(xhr.responseText));
  }
  xhr.send();
}