异常：thirdScriptError totalCost is not defined;at api request success callback function ReferenceError

异常提示

VM789:1 thirdScriptError
totalCost is not defined;at api request success callback function
ReferenceError: totalCost is not defined
    at success (http://127.0.0.1:12446/appservice/pages/home/home.js:92:25)
    at Function.o.(anonymous function) (http://127.0.0.1:12446/appservice/__dev__/WAService.js:1:1296511)
    at http://127.0.0.1:12446/appservice/__dev__/WAService.js:1:125690
    at k (http://127.0.0.1:12446/appservice/__dev__/WAService.js:1:526119)
    at i.<anonymous> (http://127.0.0.1:12446/appservice/__dev__/WAService.js:1:832038)
    at i.emit (http://127.0.0.1:12446/appservice/__dev__/WAService.js:1:574350)
    at Xi (http://127.0.0.1:12446/appservice/__dev__/WAService.js:1:832676)
    at http://127.0.0.1:12446/appservice/__dev__/WAService.js:1:833896
    at http://127.0.0.1:12446/appservice/__dev__/WAService.js:1:597101
    at n (http://127.0.0.1:12446/appservice/__dev__/asdebug.js:1:3175)

出现场景及部分相关代码

getUserInfo: function (e) {
    **********
    **********
    success:function(res){
        if(res.data.message == "success"){
          console.log("调用homeAction/getTotalCostAndCalories成功，且获取数据成功");
          this.setData({
            totalCost : res.data.totalCost,
            calories : res.data.calories
          })
          
        }
        if(res.data.message == "fail"){
          console.log("调用homeAction/getTotalCostAndCalories成功，获取数据失败");
        }
      }
    **********
    **********

从后台（Java）向前台（微信小程序home.js）返回两个值（totalCost，calories ）。
在将这两个值分别赋值给页面参数（totalCost，calories ）的时候，出现如上错误。

分析原因

根据提示，应该是与回调函数callback 有关。
网上的说法：作用域问题——回调函数中的作用域已经脱离了调用函数了，因此需要在回调函数外边把this赋给一个新的变量才可以了。

参考：

1. 微信小程序Cannot read property ‘setData’ of null;
2. 逍遥云天微信小程序开发——使用回调函数出现异常：TypeError: Cannot read property ‘setData’ of undefined

解决方法

```javascript
getUserInfo: function (e) {
	**var myThis = this;**
    **********
    **********
    success:function(res){
        if(res.data.message == "success"){
          console.log("调用homeAction/getTotalCostAndCalories成功，且获取数据成功");
          **myThis**.setData({
            totalCost : res.data.totalCost,
            calories : res.data.calories
          })
          
        }
        if(res.data.message == "fail"){
          console.log("调用homeAction/getTotalCostAndCalories成功，获取数据失败");
        }
      }
    **********
    **********

以上。