调用后Ajax请求未设置全局变量

Question

I am trying to make a AJAX call in Javascript to get two values. Then I want to use these values globally to do some calculation and then print the result out. The following is my codes.

   // my calculation functions will go here

   var value1 = 0;
   var value2 = 0;
   MakeRequest(); //after makeRequest() value1 and value2 should be 10 and 20 respectively.
   var total = value1 + value2;
   console.log(total); // this is still zero. because value1 and value2 are still 0.


//request AJAX
    function createXMLHTTPObject(){
        var ajaxRequest;  // The variable that makes Ajax possible!

        try{
            // IE 7+, Opera 8.0+, Firefox, Safari
            ajaxRequest = new XMLHttpRequest();
            return ajaxRequest;
        } catch (e){
            // Internet Explorer
            try{
                ajaxRequest = new ActiveXObject("Msxml2.XMLHTTP");
                return ajaxRequest;
            } catch (e) {
                try{
                    // Internet Explorer 5, 6
                    ajaxRequest = new ActiveXObject("Microsoft.XMLHTTP");
                    return ajaxRequest;
                } catch (e){
                    // Something went wrong
                    alert("Your browser broke!");
                    return false;
                }
            }
        }
    }

    // Create a function that will receive data sent from the server
    function AjaxRequest(url,callback,method){
        var req = createXMLHTTPObject();
        req.onreadystatechange= function(){
                if(req.readyState != 4) return;
                if(req.status != 200) return;
                callback(req);
        }
        req.open(method,url,true);
        req.send(null);
    }

    function AjaxResponse(req){
        var respXML=req.responseXML;
        if(!respXML) return;
        value1=respXML.getElementsByTagName("value1")[0].childNodes[0].nodeValue;
        value2= respXML.getElementsByTagName("value2")[0].childNodes[0].nodeValue;
        console.log("the value1 is "+ value1);  //successfully print the values
        console.log("the value2 is "+ value2);
    } 

    function MakeRequest(){
         AjaxRequest("values.xml",AjaxResponse,"get");
    }

1. So my first question is that why total = value 1 + value2 are still 0. I have made them global variables and then updated value1 and value2 inside of makeRequest(), but it seems not affect the value. What could I do so that I can update value1 and value2 in order to use them outside of those functions.

2. Basically I copy the ajax request codes from a tutorial online. There is one thing I dont understand here. When I call MakeRequest() function, it calls AjaxRequest("values.xml",AjaxResponse,"get"); However, AjaxReponse(req) needs a parameter "req" here. But AjaxResponse inside of AjaxRequest("values.xml",AjaxResponse,"get") did not put a parameter. it still works. Why is that? I do really understand this part.

Answer 1

Because AJAX calls are asynchronous which means your code runs like this in realtime:

var value1 = 0;
var value2 = 0;
MakeRequest();           // AJAX REQUEST is made, that will happen on its on timeline
var total = value1 + value2;
console.log(total);     // still will be 0 at this point, since AJAX response has not returned


// MakeRequest will fire AJAX request and WHEN THE REQUEST IS SUCCESSFUL, it can modify value1 and value2, then calculate total  

The total = value1 + value2 should be calculated after your AJAX request returns successfully, if you want value1 and value2 to be dependent on the AJAX request's result.