Simple AJAX helper (not checked)

ajax.js

export const ajax = (url, { 
  method = 'GET',
  contentType = 'application/x-www-form-urlencoded; charset=UTF-8';
  data,
  onDone,
} = {}) => {
  const xhr = new XMLHttpRequest();
  
  if (typeof onDone === 'function') {
    const handler = () => onDone(xhr);
    xhr.onload = handler;
    xhr.onerror = handler;
  }
  
  xhr.open(method, url, true);
  xhr.setRequestHeader('Content-Type', contentType);
  xhr.send(contentType ? data : objectToURL(data));
}

export const objectToURL = (value, key) => {
  const list = [];
  const isPrimitive = value !== Object(value);
  
  if (!isPrimitive) {
    for (const index in value) {
      const parameterName = key
      ? `${key}[${Array.isArray(value) ? '' : index}]`
      : index;
      list.push(`${objectToURL(value[index], parameterName)}`);
    }
  } else {
    value = encodeURIComponent(value);
    list.push(key ? `${encodeURIComponent(key)}=${value}` : value);
  }
  
  return list.join('&');
}