微信小程序开发，实现上一题下一题并计总分功能，总分unfined?

微信小程序开发答题计分，并有上一题下一题功能。在四个单选选项中定义了加分功能。在data里定义总分但在方法里依旧显示unfined。导致无法实现上一题。在结果页面无法显示正确的总分。
点击上一题时警告：Setting data field "totalScore" to undefined is invalid.

 ClickA:function(){
    
    var trueValue = this.data.true[this.data.index]
    if(trueValue == "A"){
      
      this.setData({totalScore: this.data.totalScore + 10})

       }else{
      this.setData({totalScore:this.data.totalScore})
      
    }
  },

data: {
  chooseValue:[],
  index:1,
  realIndex : 0,
  totalScore:0,
  questionDetail:app.globalData.question[0].question,
  answerA:app.globalData.question[0].option.A,
  answerB:app.globalData.question[0].option.B,
  answerC:app.globalData.question[0].option.C,
  answerD:app.globalData.question[0].option.D,
  trueValue:app.globalData.question[0].option,
  listABC:['A','B','C','D'],
  list:[0,1,2,3,4,5,6,7,8,9],
  true:['','B','C','D','B','C','A','D','B','C','B'],
  
 },

next:function(){
  this.setData({
    index:this.data.index+1,
    realIndex:this.data.list[this.data.index],
  })
  this.setData({
    questionDetail:
    app.globalData.question[this.data.realIndex].question,
    answerA:
    app.globalData.question[this.data.realIndex].option[this.data.listABC[0]],
    answerB:
    app.globalData.question[this.data.realIndex].option[this.data.listABC[1]],
    answerC:
    app.globalData.question[this.data.realIndex].option[this.data.listABC[2]],
    answerD:
    app.globalData.question[this.data.realIndex].option[this.data.listABC[3]],})
 },
submit:function(){
   let chooseValue = JSON.stringify(this.data.chooseValue);
   wx.reLaunch({
     url: '/pages/result/result?totalScore='+this.data.totalScore + '&chooseValue=' + chooseValue,
   })
 },

last:function(){
    this.setData({
      totalScore : this.data.totalScore[this.data.index-2],})
    if(this.data.index>1){
    this.setData({
      index:this.data.index-1,
      realIndex:this.data.list[this.data.index-2],
    })
    this.setData({
      questionDetail:
      app.globalData.question[this.data.realIndex].question,
      answerA:
      app.globalData.question[this.data.realIndex].option[this.data.listABC[0]],
      answerB:
      app.globalData.question[this.data.realIndex].option[this.data.listABC[1]],
      answerC:
      app.globalData.question[this.data.realIndex].option[this.data.listABC[2]],
      answerD:
      app.globalData.question[this.data.realIndex].option[this.data.listABC[3]],})

  }else{
    wx.showModal({
      title: '提示',
      content: '已是第一题',
    })
  }
  },