问关于 小程序 this 变量的问题，自定义 function 中无法获取this 变量？

var app = getApp();

Page({

  /**

   * 页面的初始数据

   */

  data: {

    askNO: 1,

    ask: {},

    progress: 8.333,

    askLength: 0,

    answerIng: false,

    finish: false

  },

  /**

   * 生命周期函数--监听页面加载

   */

  onLoad: function onLoad(options) {

    var questionnaire = app.globalData.questionnaire;

    var lastIndex = app.globalData.lastAnswerAskIndex - 1;

    var ask = questionnaire.askList[lastIndex == null ? 0 : lastIndex];

    console.log(this);

    // let that = this;

    this.setData({

      askLength: questionnaire.askList.length,

      askNO: lastIndex + 1

    });

    // this.fullAsk(that, ask, lastIndex);

  },

  prevAnswer: function prevAnswer(e) {

    console.log(e);

    console.log(undefined);

    if (undefined.data.answerIng) {

      return;

    }

    // let questionnaire = app.globalData.questionnaire;

    // let lastIndex = --app.globalData.lastAnswerAskIndex - 1;

    // let ask = questionnaire.askList[lastIndex];

    // this.fullAsk(this, ask, lastIndex);

  }）｝