ajax请求成功后返回值如何赋值给js变量

var msg = ""
$.ajax({
            type:"POST",
            url :"ajax/checkArray.php",
            data:"nns_config_item_value="+nns_config_item_value,
            success:function(data){
                         if(data!==‘‘){
                            msg = data;
                         }
                }
         });
if(msg){
   alert(msg);
   return false;
}else{
   return true;
}

上面一段代码，运行发现：msg始终是空。

我们需要在ajax中添加async : false,将ajax异步请求关闭。（默认是异步请求）

$.ajax({
            type:"POST",
            url :"ajax/checkArray.php",
            data:"nns_config_item_value="+nns_config_item_value,
            async : false,
            success:function(data){
                         if(data!==‘‘){
                            msg = data;
                         }
                }
         });