js 函数中怎么获取另一个函数的返回值

Question

function getAllDeptData(){
var str = "全院";
var data = getDataByDept(str);
alert(data);
}

//获取json中的数据
function getDataByDept(obj){
var dataset = [];
$.getJSON("tourism.json",function(data){
if((obj.indexOf("全院")==0 )&& ("全院".indexOf(obj) == 0)){
$.each(data.provinces,function(i,info){
var temp = {};
temp.name = info["name"];
temp.dept = info["dept"];
temp.value = info["value"];

dataset.push(temp);
});
return dataset;
}else{
$.each(data.provinces,function(i,info){
if(info["dept"] == obj){
var temp = {};
temp.name = info["name"];
temp.dept = info["dept"];
temp.value = info["value"];

dataset.push(temp);
}
});
return dataset;
}
});
}

为什么最后打印出来的是undefined？请问我应该怎么修改呢？