es6的疑惑

Question

get(url, data = {}, success = {}) {

    axios({

      url: url,

      params: data,

      method: 'GET',

      headers: {

        'X-SESSIONID': localStorage.sessionid

      }

    }).then(res => success(res.data)).catch(() => alert('网络请求失败'))

  }

函数中传data = {}和success = {}是什么意思啊；