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For a good example of how AJAX works see the following Project Code

function makeRequest(url) {

var http_request = false;

// All major browser except IE will pass this line.
if (window.XMLHttpRequest) { // Mozilla, Safari,...

http_request = new XMLHttpRequest();
if (http_request.overrideMimeType) {

// This line will cause an error in FireFox if the page called is not valid XML
// If you see 'Not well formed' error check your XML request. http_request.overrideMimeType('text/xml');

}

// This line of code is especially for IE
} else if (window.ActiveXObject) { // IE

// There is also a slightly different way that IE invokes XMLHttpRequest Object.
// And depending on the version of IE the implementation can be slightly different.
try {

http_request = new ActiveXObject("Msxml2.XMLHTTP");

} catch (e) {

try {

http_request = new ActiveXObject("Microsoft.XMLHTTP");

} catch (e) {}

}

}

if (!http_request) {

alert('Giving up :( Cannot create an XMLHTTP instance');
return false;

}
// Make the function call that will wait for and validate the response.
http_request.onreadystatechange = function() { alertContents(http_request); };
http_request.open('GET', url, true);
http_request.send(null);

}

function alertContents(http_request) {

// The 'ready.state' of 4 means the request is completed.
if (http_request.readyState == 4) {

// A server response of 200 means everything is OK.
if (http_request.status == 200) {

alert(http_request.responseText);

} else {

alert('There was a problem with the request.');

}

</script>

<span

style="cursor: pointer; text-decoration: underline"
onclick="makeRequest('test.html')">

Make a request

</span>