循环中的onreadystatechange，readystate改变太晚了
我在循环中向 Google 地图服务器发出总共 8 个 xmlhttp 请求，并处理服务器返回的 json 对象以检索邮政编码。如果 xmlhttprequests 是同步的，则代码可以正常工作。由于我应该使用异步请求，因此我尝试将代码转换为异步。这不起作用。
我使用两个警报来监控 myZip。运行代码时，返回正上方的第二个警报运行八次，并将 myZip 显示为 null 或未定义，这就是返回的内容。然后第一个警报运行八次并给出所需的邮政编码，不幸的是，为时已晚。在我看来，就绪状态不会改变，直到为时已晚。
我应该如何修改代码，以便它返回邮政编码，而不是空？任何帮助将不胜感激。
var url = "http://maps.googleapis.com/maps/api/geocode/json?address="+address+city+state+"&sensor=false";
req.open("GET", url,true);
var myZip;
req.onreadystatechange = function()
{
if(req.readyState == 4 && req.status == 200) {
(function(data){
var myObj = eval( '(' + data + ')' );
if(myObj.status=="OK"){
for(i=0; i <myObj.results[0].address_components.length; i++){
if(myObj.results[0].address_components[i].types=="postal_code"){
myZip=myObj.results[0].address_components[i].long_name;
alert('zip is '+myZip);
}
}
}
else
{
alert("Error: returned status code "+req.status+" "+req.statusText);
}
})(req.responseText);
}
}
req.send();
alert(myZip);
return myZip;
I issue in a loop a total of eight xmlhttprequests to a Google map server and process the json objects the server returns to retrieve the zip codes. The code works fine if the xmlhttprequests are synchronous. Since I'm supposed to use asynchronous requests, I'm trying to convert the code to asynchronous. It doesn't work.
I use two alerts to monitor myZip. When the code is run, the second alert, right above the return, runs eight times and shows myZip as null or undefined and that is what is returned. Then the first alert runs eight times and gives the desired zip code, too late, unfortunately. It seems to me the readystate doesn't change until too late.
How should I modify the code so it will return the zip code, not null? Any help will be greatly appreciated.
var url = "http://maps.googleapis.com/maps/api/geocode/json?address="+address+city+state+"&sensor=false";
req.open("GET", url,true);
var myZip;
req.onreadystatechange = function()
{
if(req.readyState == 4 && req.status == 200) {
(function(data){
var myObj = eval( '(' + data + ')' );
if(myObj.status=="OK"){
for(i=0; i <myObj.results[0].address_components.length; i++){
if(myObj.results[0].address_components[i].types=="postal_code"){
myZip=myObj.results[0].address_components[i].long_name;
alert('zip is '+myZip);
}
}
}
else
{
alert("Error: returned status code "+req.status+" "+req.statusText);
}
})(req.responseText);
}
}
req.send();
alert(myZip);
return myZip;
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我猜你多次使用同一个对象（req），并且会在每个循环中被覆盖。
因此，对每个请求使用单独的对象，或者在最后一个请求完成后启动一个新请求。
如果不知道 req 是什么，我无法确切地说你如何做到这一点。
I guess you use the same object(req) a couple of times and will be overwritten on each loop.
So use separate objects for every requests or start a new request if the last one is finished.
How exactly you can do this I cannot say without knowing what req is.
您应该使用回调而不是 return 语句，或者您应该将其作为分层 JavaScript 运行：http://stratifiedjs.org 。
然后您可以以同步方式编写它，就像您所做的那样，即使它不会阻止您的浏览器。
You should either use a callback instead of return statement, or you should run this as Stratified JavaScript: http://stratifiedjs.org.
Then you can write it in a synchronous fashion, like you kind of did, even though it will NOT block your browser.