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Using headers

As in the synchronous calls, the

status

statusText

, and

responseText

properties are filled with data.

With asynchronous calls, it’s possible to cancel the request altogether by calling the

abort()

method

before the

readyState

reaches

var oRequest = createXMLHTTP();

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

oRequest.onreadystatechange = function () {

if (oRequest.readyState == 3) {

oRequest.abort();

} else if (oRequest.readyState == 4) {

alert(“Status is “ + oRequest.status + “ (“ + oRequest.statusText + “)”);

alert(“Response text is: “ + oRequest.responseText);

}

oRequest.send(null);

In this example, the alerts are never displayed because the request is aborted when

readyState

Using headers

Every HTTP request sends along with it a group of headers with additional information. In everyday

browser use, these headers are hidden because they aren’t needed by the end user. However, these head-

ers can be quite necessary to developers, and so the XML HTTP request object provides methods to get

and set them.

The first is a method called

getAllResponseHeaders()

, which returns a string containing all the

headers attached to the response. Here’s a sample of the type of information returned by

getAllResponseHeaders()

Date: Sun, 14 Nov 2004 18:04:03 GMT

Server: Apache/1.3.29 (Unix)

Vary: Accept

X-Powered-By: PHP/4.3.8

Connection: close

Content-Type: text/html; charset=utf-8

From this header information, you can tell that the server is running Apache on Unix with PHP support

and the file being returned is an HTML file. If you want to retrieve only one of the headers, you can use

the

getResponseHeader()

method with the name of header to retrieve. For example, to retrieve the

value of the

“Server”

header, you can do this:

var sValue = oRequest.getResponseHeader(“Server”);

Reading request headers is just part of the equation; the other part is setting your own headers on the

request before it’s sent.

Using the

setRequestHeader()

method, you can set headers on the XML HTTP request before it’s sent

out. For example:

oRequest.setRequestHeader(“myheader”, “yippee”);

oRequest.setRequestheader(“weather”, “warm”);

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Client-Server Communication

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