callback function wont run when its called - ajax

Question

ajax : function(typ,url,callback) {
if(window.XMLHttpRequest) {
var xml = new XMLHttpRequest();
}
if(window.ActiveXObject) {
var xml = new ActiveXObject("Microsoft.XMLHTTP");
}
xml.onreadystatechange = function(callback) {
if(xml.readyState == 4 && xml.status == 200) {
callback();
}
}
xml.open(typ,url,true);
xml.send();
}
}
//Function being called
window.onload = function() {
JS.ajax("GET","/server/chkEmail.php?email=email#email.com",function() {
alert(xml.responseText);
});
}
It throws the error saying:
Uncaught TypeError: callback is not a functionxml.onreadystatechange #
global.js:30
Any ideas?
I fixed it by passing the data variable through the onreadystatechange function and calling it in the callback function.
ajax : function(typ,url,callback) {
if(window.XMLHttpRequest) {
var xml = new XMLHttpRequest();
}
if(window.ActiveXObject) {
var xml = new ActiveXObject("Microsoft.XMLHTTP");
}
xml.onreadystatechange = function(data) {
if(xml.readyState == 4 && xml.status == 200) {
var data = xml.responseText;
callback(data);
}
};
xml.open(typ,url,true);
xml.send();
}
}
window.onload = function() {
JS.ajax("GET","/server/chkEmail.php? email=jonwcode#gmail.com",function(data){
alert(data);
});
}

Answer 1

Try like this:
xml.onreadystatechange = function() {
if (xml.readyState == 4 && xml.status == 200) {
callback();
}
};
Notice that the onreadystatechange function doesn't take any parameters whereas in your code you have passed it a parameter with the name callback which will override the callback variable from the outer scope.
UPDATE:
It looks like you have improperly scoped the xml variable and it isn't available in your AJAX callback. I strongly recommend you reading more about javascript variables scope. Here's how you could make the xml variable visible:
var xml = null;
if (window.XMLHttpRequest) {
xml = new XMLHttpRequest();
} else if (window.ActiveXObject) {
xml = new ActiveXObject("Microsoft.XMLHTTP");
}
if (xml == null) {
alert('Sorry, your browser doesn\'t seem to support AJAX - please upgrade to a modern browser');
} else {
xml.onreadystatechange = function() {
if(xml.readyState == 4 && xml.status == 200) {
var data = xml.responseText;
callback(data);
}
};
xml.open(typ,url,true);
xml.send();
}