Cannot read property 'data' of undefined;at api showModal success callback function

二话不错上错误代码：

wx.showModal({
        title: '提示',
        content: '确定添加?',
        success(res){
          if (res.confirm) {
            //添加对方为自己的好友
            db.collection('users').doc(app.userInfo._id).update({
              data:{
                friendList:this.data.messageId
              },
            }).then((res)=>{});
          }else{}
        }
      })
              

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