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Copycat implementations

Assuming that you have some server-side logic designed to take these headers into account, you can

provide some additional functionality and/or evaluation of requests.

This object proved to be so popular among Web developers that other browser makers copied the imple-

mentation. Mozilla was the first of the copycats, creating a JavaScript object called

XMLHttpRequest

that

behaves exactly the same as Microsoft’s version. Both Safari (as of 1.2) and Opera (as of 7.6) copied Mozilla’s

implementation, creating their own

XMLHttpRequest

objects.

To allow creation of an XML HTTP request in a common way, just add this simple wrapper class to your

pages:

if (typeof XMLHttpRequest == “undefined” && window.ActiveXObject) {

function XMLHttpRequest() {

var arrSignatures = [“MSXML2.XMLHTTP.5.0”, “MSXML2.XMLHTTP.4.0”,

“MSXML2.XMLHTTP.3.0”, “MSXML2.XMLHTTP”,

“Microsoft.XMLHTTP”];

for (var i=0; i < arrSignatures.length; i++) {

try {

var oRequest = new ActiveXObject(arrSignatures[i]);

return oRequest;

} catch (oError) {

//ignore

}

throw new Error(“MSXML is not installed on your system.”);

}

This code allows you to use the following line to create an XML HTTP request in all browsers that support it:

var oRequest = new XMLHttpRequest();

After this point, the XML HTTP request can be used in all supporting browsers as described in the previ-

ous sections.

Performing a GET request

The most common type of request on the Web is a GET request. Every time you enter a URL into your

browser and click Go, you are sending a GET request to a server.

Parameters to a GET request are attached to the end of the URL with a question mark, followed by

name/value pairs separated by an ampersand. For example:

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