Develop Reference

xmlHttpRequest returns status 0

I can't get xmlHttpRequest to return any status but 0. The responseText is always empty. No matter, which url I am opening, this happens. Could someone tell me what I am doing wrong?
var url = "http://stackoverflow.com/";
var xmlHttp = new XMLHttpRequest();
var response = document.getElementById("response");
xmlHttp.open("GET", url, true); // true for asynchronous
xmlHttp.send(null);
xmlHttp.onreadystatechange = function() {
response.innerHTML = xmlHttp.status;
};
https://jsfiddle.net/6ub7ct6j/
Thanks in advance

Check the console. You're trying to load an http resource from an https page, and the browser is blocking it. However, if you try https://stackoverflow.com, you'll get a CORS error.
stackoverflow.com doesn't allow access via Ajax:
VM94:1 XMLHttpRequest cannot load https://stackoverflow.com/. No 'Access-Control-Allow-Origin' header is present on the requested resource. Origin 'https://fiddle.jshell.net' is therefore not allowed access.

Ajax request with JavaScript responds with 302 (Found)

I am trying to send data by Ajax with the Post method, but the answer gives me 302 (Found) and I do not know what it can be. This is my code:
function sendData(){
if (window.XMLHttpRequest) {
xmlhttp = new XMLHttpRequest();
} else {
xmlhttp = new ActiveXObject("Microsoft.XMLHTTP");
}
xmlhttp.onreadystatechange = function() {
if (xmlhttp.readyState == 4 && xmlhttp.status == 200) {
console.log("Response" + xmlhttp.responseText);
}
}
xmlhttp.open("POST", "request.php", true);
xmlhttp.setRequestHeader("Content-type", "application/x-www-form-urlencoded");
xmlhttp.send("name="+document.getElementById('username').value);
}
and my .php file only print a message
<?php echo 'hello : ' . $_POST["name"] ?>

The 302 status code indicates that the resource you are requesting has redirected to another resource. If this is behind some authentication, or requiring a session to be active then yes, it would follow that the session timing out is responsible for the ajax resource being called to redirect to possibly a login screen maybe.
Try to use jQuery request, as it much simpler - also recommended in the above comments!

One way to get such a message is, to forget to add a / to the end of a URL, which is a "sub-directory" of the document root. As the URL your.domain/whatever and your.domain/whatever/ might not be the same. (Depending on server configuration, and the actual application serving those URLs.)
I can see, in this case you are actually POSTing to /request.php, so this might not apply, but just in case, I'd try to make that request 'by hand' and see what happens.
There are many browser plugins, that let you generate AJAX queries.

Ajax - "Access-Control-Allow-Origin" Error

I'm trying to work with the Livestream API to see if a certain channel is live but keep getting this error:
XMLHttpRequest cannot load http://channel.api.livestream.com/1.0/livestatus?channel=huskystarcraft. Origin http://www.webdevstl.com is not allowed by Access-Control-Allow-Origin.
Do I need to run it through PHP or am I doing something wrong in my ajax call? It's pretty straight forward code:
function getActive(){
if(window.XMLHttpRequest)
{
xmlhttp = new XMLHttpRequest();
}
else
{
xmlhttp = new ActiveXObject("Microsoft.XMLHTTP");
}
xmlhttp.onreadystatechange = function()
{
if(xmlhttp.readyState == 4 && xmlhttp.status == 200)
{
var json = JSON.parse(xmlhttp.responseText);
console.log(json);
}
}
xmlhttp.open("GET", "http://channel.api.livestream.com/1.0/livestatus?channel=huskystarcraft", true);
xmlhttp.setRequestHeader("Content-type", "application/x-www-form-urlencoded");
xmlhttp.send();
}
getActive();

You're running into restrictions imposed by the Same Origin Policy. In short, AJAX calls to a different domain are prohibited and will fail - unless explicitly permitted by the remote host.
You need to either use JSONP (mostly applicable to data returned by APIs) or proxy the request through your own server/domain.
CORS would also be an option, but that assumes you having access to the remote server's config.

XMLHttpRequest cannot load "url". Origin null is not allowed by Access-Control-Allow-Origin

I am trying to call a web service from javascript. In Internet Explorer 9 works properly, while chrome does not work. The error is as follows:
"OPTIONS http://www.restfulwebservices.net/wcf/CurrencyService.svc?wsdl 400 (Bad Request)
XMLHttpRequest cannot load http://www.restfulwebservices.net/wcf/CurrencyService.svc?wsdl. Origin null is not allowed by Access-Control-Allow-Origin."
I leave the code for help me. Thanks.
`
function SOAPClient() {
this.wsdl = '';
this.async = true;
this.action = '';
this.xml = '';
SOAPClient.prototype.invoke = function(){
var xhr;
if (window.XMLHttpRequest) {// code for IE7+, Firefox, Chrome, Opera, Safari
xhr = new XMLHttpRequest();
} else {// code for IE6, IE5
xhr=new ActiveXObject("Microsoft.XMLHTTP");
}
xhr.onreadystatechange=function() {
if (xhr.readyState >= 3){
alert ('ReadyState '+xhr.readyState+' - - Status '+xhr.status);
if(xhr.status == 200)
document.getElementById("txtResult").innerHTML=xhr.responseText;
else
document.getElementById("txtResult").innerHTML='Error';
}
}
xhr.open("POST", this.wsdl,this.async);
xhr.setRequestHeader("SOAPAction", this.action);
xhr.setRequestHeader("Content-Type", "text/xml");
xhr.setRequestHeader("Connection", "close");
xhr.send(this.xml);
return false;
}
}
</script>
<script type="text/javascript">
var wsdl = 'http://www.restfulwebservices.net/wcf/CurrencyService.svc?wsdl';
var action = 'GetConversionRate';
var xml = '';
var async = true;
var response = '';
function prueba(){
var client = new SOAPClient();
client.wsdl = wsdl;
client.action = action;
client.xml = '<soapenv:Envelope xmlns:soapenv="http://schemas.xmlsoap.org/soap/envelope/" xmlns:ns="http://www.restfulwebservices.net/ServiceContracts/2008/01"><soapenv:Header/><soapenv:Body><ns:GetConversionRate><ns:FromCurrency>EUR</ns:FromCurrency><ns:ToCurrency>GBP</ns:ToCurrency></ns:GetConversionRate></soapenv:Body></soapenv:Envelope>';
client.invoke();
return false;
}
</script>
</head>
<body>
<p>Versión 2.5</p>
<form name="form" action="#">
Term: <input type="text" name="inputValue" method="post"/>
<button onclick="prueba()">Search</button>
<p id="txtResult"></p>
</form>
</body>
`

First, your request must either:
abide by the same-origin policy (i.e., requesting domain == receiving domain), or
be exclipity permitted to access the service's pages by a Access-Control-Allow-Origin header that lists your domain as a domain allowed to access that server in a cross-domain way.
Furthermore, you are making the request from a file:// document, and Chrome might disallow it from performing any cross-domain XHR, even if the server gives back an all-permissive Access-Control-Allow-Origin: *. You should run a local server to access your files through HTTP, or simply test in a different browser.

If you're on a different domain you must, as stated above, set the Access-Control-Allow-Origin header.
It also sounds like your not handling the OPTIONS request that is made at all by your application. This is just an additional request made by the browser when the type of request isn't of GET or POST. All you'll be doing is returning headers with the proper Access-Control-Allow-Origin permissions to let the browser know it's permitted to make the cross domain request.
I'm not sure what you're using in the background, but see this post to see how it was done in Rails.
How to be aple to POST, PUT and DELETE from Backbone app to Rails app on different subdomains?

Access is denied. JavaScript error on request to secured page

On page SomePage.aspx, by JavaScript code (XMLHttpRequest) I call SecuredPage.aspx used next code:
var httpRequest = GetXmlHttp();
var url = "https://myhost.com/SecuredPage.aspx";
var params = "param1=" + document.getElementById('param1').value +
"&param2=" + document.getElementById('param2').value;
httpRequest.open("POST", url, true);
httpRequest.setRequestHeader("Content-Type", "application/x-www-form-urlencoded");
httpRequest.onreadystatechange = function() {
//Call a function when the state changes.
if (httpRequest.readyState == 4 && httpRequest.status == 200) {
alert(httpRequest.responseText);
}
}
httpRequest.send(params); // HERE ACCESS IS DENIED.
//---------------------------------------------
function GetXmlHttp() {
var xmlhttp = false;
if (window.XMLHttpRequest) {
xmlhttp = new XMLHttpRequest();
}
else if (window.ActiveXObject)
// Code for Internet Explorer.
{
try {
xmlhttp = new ActiveXObject("Msxml2.XMLHTTP");
}
catch (e) {
try {
xmlhttp = new ActiveXObject("Microsoft.XMLHTTP");
} catch (E) {
xmlhttp = false;
}
}
}
return xmlhttp;
}
It throws an Access is denied error. If send to http (http://myhost.com/SecuredPage.aspx), it works fine.
How is it possible to resolve this problem?

If you wish to fetch an HTTPS page via Ajax you need to do it from an HTTPS page on the same domain, there is no other way, as long as you use Ajax. This is because of the same origin policy.
That said, there are plenty of ways to do this not using Ajax, for instance you can use frames.
Another way is to use JSONP, but this requires that you are fetching, well, JSON :)
A third way, that tends not to be very useful for production websites, but still can be fun to tinker around with, is to use YQL as a proxy.
Lastly you can always set up a serverside proxy of your own, so that you call an HTTP address that fetches the HTTPS page and sends it on, but this is rarely a good solution if it can be avoided.

This is because the browser considers http and https as 2 different sites/domains, and therefore you have to adhere to the same origin policy.
Due to browser security restrictions, most "Ajax" requests are subject to the same origin policy; the request can not successfully retrieve data from a different domain, subdomain, or protocol.
One way to solve it is using jsonp.

As it's been said, your problem is that your browser sees this as a cross domain request. Another way to accommodate this is to set up a crossdomain.xml file like this:
<?xml version="1.0"?>
<!DOCTYPE cross-domain-policy SYSTEM "http://www.macromedia.com/xml/dtds/cross-domain-policy.dtd">
<cross-domain-policy>
<allow-access-from domain="myhost.com" />
<allow-access-from domain="ourhost.com" />
<site-control permitted-cross-domain-policies="master-only" />
</cross-domain-policy>
I'm not an expert on this method, but I have used it successfully. Other domains can be added by adding more allow-access-from tags. You may need to do some fiddling. YMMV.