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HTTP Requests

In many modern browsers, it’s possible to initiate HTTP requests directly from JavaScript and get the

result back in JavaScript, completely eliminating the need for hidden frames and other such trickery.

At the center of this exciting new capability is an object Microsoft created called the XML HTTP request.

This object came along with MSXML but wasn’t fully explored until recently. Essentially, an XML HTTP

request is a regular HTTP request with added functionality for sending and receiving XML code.

To create a new XML HTTP request in Internet Explorer, you must once again use an

ActiveXObject

var oRequest = new ActiveXObject(“Microsoft.XMLHTTP”);

Like the XML DOM in IE, the XML HTTP request object has multiple versions, so a function is necessary

to make sure you’re using the most recent one:

function createXMLHTTP() {

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.”);

}

After you have created it, you can use the

open()

method to specify the request to send. This method

takes three arguments: the type of request to send (GET, POST, or any other HTTP method supported by

the server); the URL of the request; and a Boolean indicating whether the request should be sent asyn-

chronously or not (the same as you do with the XML DOM

load()

method). For example:

oRequest.open(“get”, “example.txt”, false);

After opening the request, you must send it by using the

send()

method. This method always requires

an argument, which can be

null

most of the time:

oRequest.send(null);

If you choose to make the request synchronously (setting the third argument to

false

), the JavaScript

interpreter waits for the request to return. When the response comes back, it fills the

status

property

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Client-Server Communication

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