jQuery.ajax() inside a loop [duplicate] - ajax

Question

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JavaScript closure inside loops – simple practical example
(44 answers)
Closed 6 years ago.
If I call jQuery.ajax() inside a loop, would it cause the call in current iteration overwrite the last call or a new XHR object is assigned for the new request?
I have a loop that do this, while from console log I can see requests done 200 ok but just the result data of the last request in the loop is stored by the request success callback as supposed .
the code:
var Ajax = {
pages: {},
current_request: null,
prefetch: function () {
currentPath = location.pathname.substr(1);
if(this.pages[currentPath])
{
var current = this.pages[currentPath];
delete this.pages[currentPath];
current['name']=currentPath;
current['title']=$("title").text().replace(' - '.SITE_NAME, '');
current['meta_description']=$("meta[name=description]").attr('content');
current['meta_keywords']=$("meta[name=keywords]").attr('content');
}
var _Ajax = this;
//the loop in question *****
for(var key in this.pages)
{
$.ajax({
method: 'get',
url:'http://'+location.hostname+'/'+key,
success: function(data) {
_Ajax.pages[key] = data;
}
});
console.debug(this.pages);
}
if(current)
{
this.pages[currentPath] = current;
}
}
};//Ajax Obj
for(var i in pages)
{
Ajax.pages[pages[i]]={};
}
$(function() {
Ajax.prefetch();
});//doc ready

Answer 1

You'll need a closure for key:
for(var k in this.pages){
(function(key){
$.ajax({
method: 'get',
url:'http://'+location.hostname+'/'+key,
success: function(data) {
_Ajax.pages[key] = data;
}
});
console.debug(this.pages);
})(k);
}
that way you make sure that key is always the correct on in each ajax success callback.
but other than that it should work
i made a small closure demonstration using timeout instead of ajax but the principle is the same:
http://jsfiddle.net/KS6q5/

Answer 2

You need to use async:false in you ajax request. It will send the ajax request synchronously waiting for the previous request to finish and then sending the next request.
$.ajax({
type: 'POST',
url: 'http://stackoverflow.com',
data: data,
async: false,
success: function(data) {
//do something
},
error: function(jqXHR) {
//do something
}
});

Answer 3

I believe what's happening here has to do with closure. In this loop:
for(var key in this.pages)
{
$.ajax({
method: 'get',
url:'http://'+location.hostname+'/'+key,
success: function(data) {
_Ajax.pages[key] = data;
}
});
console.debug(this.pages);
}
The variable key is actually defined outside the for loop. So by the time you get to the callbacks, the value has probably changed. Try something like this instead:
http://jsfiddle.net/VHWvs/
var pages = ["a", "b", "c"];
for (var key in pages) {
console.log('before: ' + key);
(function (thisKey) {
setTimeout(function () {
console.log('after: ' + thisKey);
}, 1000);
})(key);
}

Answer 4

I was facing the same situation, I solved using the ajax call inside a new function then invoke the function into the loop.
It would looks like:
function a(){
for(var key in this.pages)
{
var paramsOut [] = ...
myAjaxCall(key,paramsOut);
.......
}
}
function myAjaxCall(paramsIn,paramsOut)
{
$.ajax({
method: 'get',
url:'http://'+location.hostname+'/'+paramsIn[0],
success: function(data) {
paramsOut[key] = data;
}
});
}

Answer 5

This is how I always do a ajax loop..
I use a recursive function that gets called after the xhr.readyState == 4
i = 0
process()
function process() {
if (i < 10) {
url = "http://some.." + i
var xhr = new XMLHttpRequest();
xhr.open("GET", url, true);
xhr.onreadystatechange = function () {
if (xhr.readyState == 4) {
alert(xhr.responseText)
i++
process()
}
}
xhr.send();
} else {
alert("done")
}
}