微信小程序实现答题计分功能时选择上一题导致结果数据显示NaN

微信小程序实现答题上一题下一题计分功能，但点击上一题时结果页的总分会显示NaN，该如何改正？

 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'],
  
 },
 


last:function(){
    if(this.data.index>1){
    this.setData({
      index:this.data.index-1,
      realIndex:this.data.list[this.data.index-2],
      totalScore : this.data.totalScore.index-1，
    })
    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',
    })
  }
  },
 



<view class="score">{{userName}}你的总分为：{{totalScore}}view>

img