code: function doRequest(reqOpts) {
      reqOpts = Object.assign({}, reqOpts);
      if (currRequestNum >= requestMaxNum) {
          return new Promise((resolve, reject) => {
              waitingQueues.push({
                  options: reqOpts,
                  resolve,
                  reject
              });
          });
      }
  
      currRequestNum++;
  
      let rawComplete = reqOpts.complete;
      reqOpts.complete = function (...args) {
          currRequestNum--;
  
          rawComplete && rawComplete(...args);
  
          if (!waitingQueues.length) {
              return;
          }
  
          let processReqInfo = waitingQueues.shift();
          doRequest(processReqInfo.options).then(
              res => processReqInfo.resolve(res),
              err => processReqInfo.reject(err)
          );
      };
  
      return rawRequest(reqOpts);
  }
docs: Do request
  
  @param {Object} reqOpts the request options
  @return {Promise}