The problem (ignoring the cross-domain issue that slebetman covered) is that when your ready state change callback is fired it is using the i variable from the containing scope which will be 3 after the loop completes. One way to fix that is as follows:
for (var i=0; i<3; i++){
(function(i) {
nRequest[i] = new XMLHttpRequest();
nRequest[i].open("GET", URL[i], true);
nRequest[i].onreadystatechange = function (oEvent) {
if (nRequest[i].readyState === 4) {
if (nRequest[i].status === 200) {
console.log(nRequest[i].responseText);
alert(nRequest[i].responseText);
} else {
console.log("Error", nRequest[i].statusText);
}
}
};
nRequest[i].send(null);
})(i);
}
This introduces an immediately invoked function expression for each loop iteration such that the code inside the function has its own i - the magic of JS closures means that when the onreadystatechange function is called it will access the parameter i of the anonymous function (even though that function has completed), not the i of the outer scope, so the right nRequest element will be processed each time.
Also you had a typo on the .open() line where you said wURL[i] but should've had URL[i].
Depending on what you plan to do with the response text I'm not sure that you need an array of requests at all: you could encapsulate the Ajax code into a function that takes a URL and a callback function as parameters, and then call that function in the loop...
