兼容Promise的callback函数

createPromiseCallback.js

function createPromiseCallback() {
  const callback = (err, ret) => {
    if (err) {
      callback.reject(err);
    } else {
      callback.resolve(ret);
    }
  };
  callback.promise = new Promise((resolve, reject) => {
    callback.resolve = resolve;
    callback.reject = reject;
  });
  return callback;
}

testCreatePromiseCallback.js

function method(callback) {
  callback = callback || createPromiseCallback();
  this._request(params, callback);
  return callback.promise;
}