javascript - 尽管使用有效参数调用，但设置方法未设置成员

Question

我因为这段代码而感到沮丧:

    function Model(){
       this.GetAjaxData = function(){ 
          //private Member to be returned
          var res;

          function setRes(newVal){
             res = newVal;
             alert(newVal); // to verify its really being called
          }

          // calls a Ajax-Service(1st param) with the given arguments(2nd param),
          // the 3rd param is a function with gets called back, commiting the 
          // output of the Ajax-Service as arguments for the called function
          tw.coach.callService(
             "GetServerTime", 
             "<inputs><variable name='input1' type='String'>lala</variable></inputs>", 
             function(arg){ setRes(arg['Result']); }
          );

          return res;
       };
    }

现在，一旦模型的实例被初始化并且方法被调用如下:

    var _model = new Model();
    document.getElementById("myDiv").innerHTML = _model.GetAjaxData();

警报弹出并显示预期数据(Ajax 服务仅返回 {Result: this came via Ajax.})但 myDiv 包含 undefined。这告诉我 setRes() 被正确调用，但它只是没有设置 res 的值。

我也不知道为什么。

Answer 1

考虑到 AJAX 请求的异步性质，改变您的方法:

function Model() {
    this.GetAjaxData = function(callback) {
        var data = "<inputs><variable name='input1' type='String'>lala</variable></inputs>";
        tw.coach.callService("GetServerTime", data, function(arg) {
            callback(arg['Result']);
        });
    };
}​

var _model = new Model();
_model.GetAjaxData(function(res) {
    document.getElementById("myDiv").innerHTML = res;
});