/**

- 
- @param {*} options 
- url
- data
- method
- async
- timeout
*/
function ajax(options) {
  let url = options.url;
  const method = options.method.toLocaleLowerCase() || 'get';
  const async = options.async != false;  // default is true
  const data = options.data;

  const xhr = new XMLHttpRequest();

  if (options.timeout && options.timeout > 0) {
    xhr.timeout = options.timeout;
  }

  return new Promise((resolve, reject) => {
    xhr.ontimeout = () => {reject && reject('请求超时');}
    xhr.onreadystatechange = () => {
      if (xhr.readyState === 4) { // 请求完成
        if (xhr.status >= 200 && xhr.status < 300 || xhr.status === 304) {
          resolve && resolve(xhr.responseText);
        } else {
          reject && reject();
        }
      }
    }
    xhr.onerror = err => reject && reject(err);

    let paramList = [];
    let encodeData;
    if (data instanceof Object) {
      for (let key in data) {
        paramList.push(encodeURIComponent(key)   '='   encodeURIComponent(data[key]));
      }
      encodeData = paramList.join('&');
    }
    
    if (method === 'get') {
      const index = url.indexOf('?');
      if (index === -1) {
        url  = '?';
      } else if (index !== url.length - 1) {
        url  = '&';
      }
      url  = encodeData;
    }
    
    xhr.open(method, url, async);
    if (method === 'get') {
      xhr.send(null);
    } else {
      xhr.setRequestHeader('Content-Type', 'application/x-www-form-urlencoded;charset=UTF-8');
      xhr.send(encodeData);
    }

  })
}